FreeBSD/Linux Kernel Cross Reference
sys/block/blk-settings.c

     /*
      * Functions related to setting various queue properties from drivers
      */
     #include <linux/kernel.h>
     #include <linux/module.h>
     #include <linux/init.h>
     #include <linux/bio.h>
     #include <linux/blkdev.h>
     #include <linux/bootmem.h>      /* for max_pfn/max_low_pfn */
    #include <linux/gcd.h>
    #include <linux/lcm.h>
    #include <linux/jiffies.h>
    #include <linux/gfp.h>
    
    #include "blk.h"
    
    unsigned long blk_max_low_pfn;
    EXPORT_SYMBOL(blk_max_low_pfn);
    
    unsigned long blk_max_pfn;
    
    /**
     * blk_queue_prep_rq - set a prepare_request function for queue
     * @q:          queue
     * @pfn:        prepare_request function
     *
     * It's possible for a queue to register a prepare_request callback which
     * is invoked before the request is handed to the request_fn. The goal of
     * the function is to prepare a request for I/O, it can be used to build a
     * cdb from the request data for instance.
     *
     */
    void blk_queue_prep_rq(struct request_queue *q, prep_rq_fn *pfn)
    {
            q->prep_rq_fn = pfn;
    }
    EXPORT_SYMBOL(blk_queue_prep_rq);
    
    /**
     * blk_queue_unprep_rq - set an unprepare_request function for queue
     * @q:          queue
     * @ufn:        unprepare_request function
     *
     * It's possible for a queue to register an unprepare_request callback
     * which is invoked before the request is finally completed. The goal
     * of the function is to deallocate any data that was allocated in the
     * prepare_request callback.
     *
     */
    void blk_queue_unprep_rq(struct request_queue *q, unprep_rq_fn *ufn)
    {
            q->unprep_rq_fn = ufn;
    }
    EXPORT_SYMBOL(blk_queue_unprep_rq);
    
    /**
     * blk_queue_merge_bvec - set a merge_bvec function for queue
     * @q:          queue
     * @mbfn:       merge_bvec_fn
     *
     * Usually queues have static limitations on the max sectors or segments that
     * we can put in a request. Stacking drivers may have some settings that
     * are dynamic, and thus we have to query the queue whether it is ok to
     * add a new bio_vec to a bio at a given offset or not. If the block device
     * has such limitations, it needs to register a merge_bvec_fn to control
     * the size of bio's sent to it. Note that a block device *must* allow a
     * single page to be added to an empty bio. The block device driver may want
     * to use the bio_split() function to deal with these bio's. By default
     * no merge_bvec_fn is defined for a queue, and only the fixed limits are
     * honored.
     */
    void blk_queue_merge_bvec(struct request_queue *q, merge_bvec_fn *mbfn)
    {
            q->merge_bvec_fn = mbfn;
    }
    EXPORT_SYMBOL(blk_queue_merge_bvec);
    
    void blk_queue_softirq_done(struct request_queue *q, softirq_done_fn *fn)
    {
            q->softirq_done_fn = fn;
    }
    EXPORT_SYMBOL(blk_queue_softirq_done);
    
    void blk_queue_rq_timeout(struct request_queue *q, unsigned int timeout)
    {
            q->rq_timeout = timeout;
    }
    EXPORT_SYMBOL_GPL(blk_queue_rq_timeout);
    
    void blk_queue_rq_timed_out(struct request_queue *q, rq_timed_out_fn *fn)
    {
            q->rq_timed_out_fn = fn;
    }
    EXPORT_SYMBOL_GPL(blk_queue_rq_timed_out);
    
    void blk_queue_lld_busy(struct request_queue *q, lld_busy_fn *fn)
    {
            q->lld_busy_fn = fn;
    }
   EXPORT_SYMBOL_GPL(blk_queue_lld_busy);
   
   /**
    * blk_set_default_limits - reset limits to default values
    * @lim:  the queue_limits structure to reset
    *
    * Description:
    *   Returns a queue_limit struct to its default state.
    */
   void blk_set_default_limits(struct queue_limits *lim)
   {
           lim->max_segments = BLK_MAX_SEGMENTS;
           lim->max_integrity_segments = 0;
           lim->seg_boundary_mask = BLK_SEG_BOUNDARY_MASK;
           lim->max_segment_size = BLK_MAX_SEGMENT_SIZE;
           lim->max_sectors = lim->max_hw_sectors = BLK_SAFE_MAX_SECTORS;
           lim->max_write_same_sectors = 0;
           lim->max_discard_sectors = 0;
           lim->discard_granularity = 0;
           lim->discard_alignment = 0;
           lim->discard_misaligned = 0;
           lim->discard_zeroes_data = 0;
           lim->logical_block_size = lim->physical_block_size = lim->io_min = 512;
           lim->bounce_pfn = (unsigned long)(BLK_BOUNCE_ANY >> PAGE_SHIFT);
           lim->alignment_offset = 0;
           lim->io_opt = 0;
           lim->misaligned = 0;
           lim->cluster = 1;
   }
   EXPORT_SYMBOL(blk_set_default_limits);
   
   /**
    * blk_set_stacking_limits - set default limits for stacking devices
    * @lim:  the queue_limits structure to reset
    *
    * Description:
    *   Returns a queue_limit struct to its default state. Should be used
    *   by stacking drivers like DM that have no internal limits.
    */
   void blk_set_stacking_limits(struct queue_limits *lim)
   {
           blk_set_default_limits(lim);
   
           /* Inherit limits from component devices */
           lim->discard_zeroes_data = 1;
           lim->max_segments = USHRT_MAX;
           lim->max_hw_sectors = UINT_MAX;
           lim->max_sectors = UINT_MAX;
           lim->max_write_same_sectors = UINT_MAX;
   }
   EXPORT_SYMBOL(blk_set_stacking_limits);
   
   /**
    * blk_queue_make_request - define an alternate make_request function for a device
    * @q:  the request queue for the device to be affected
    * @mfn: the alternate make_request function
    *
    * Description:
    *    The normal way for &struct bios to be passed to a device
    *    driver is for them to be collected into requests on a request
    *    queue, and then to allow the device driver to select requests
    *    off that queue when it is ready.  This works well for many block
    *    devices. However some block devices (typically virtual devices
    *    such as md or lvm) do not benefit from the processing on the
    *    request queue, and are served best by having the requests passed
    *    directly to them.  This can be achieved by providing a function
    *    to blk_queue_make_request().
    *
    * Caveat:
    *    The driver that does this *must* be able to deal appropriately
    *    with buffers in "highmemory". This can be accomplished by either calling
    *    __bio_kmap_atomic() to get a temporary kernel mapping, or by calling
    *    blk_queue_bounce() to create a buffer in normal memory.
    **/
   void blk_queue_make_request(struct request_queue *q, make_request_fn *mfn)
   {
           /*
            * set defaults
            */
           q->nr_requests = BLKDEV_MAX_RQ;
   
           q->make_request_fn = mfn;
           blk_queue_dma_alignment(q, 511);
           blk_queue_congestion_threshold(q);
           q->nr_batching = BLK_BATCH_REQ;
   
           blk_set_default_limits(&q->limits);
   
           /*
            * by default assume old behaviour and bounce for any highmem page
            */
           blk_queue_bounce_limit(q, BLK_BOUNCE_HIGH);
   }
   EXPORT_SYMBOL(blk_queue_make_request);
   
   /**
    * blk_queue_bounce_limit - set bounce buffer limit for queue
    * @q: the request queue for the device
    * @dma_mask: the maximum address the device can handle
    *
    * Description:
    *    Different hardware can have different requirements as to what pages
    *    it can do I/O directly to. A low level driver can call
    *    blk_queue_bounce_limit to have lower memory pages allocated as bounce
    *    buffers for doing I/O to pages residing above @dma_mask.
    **/
   void blk_queue_bounce_limit(struct request_queue *q, u64 dma_mask)
   {
           unsigned long b_pfn = dma_mask >> PAGE_SHIFT;
           int dma = 0;
   
           q->bounce_gfp = GFP_NOIO;
   #if BITS_PER_LONG == 64
           /*
            * Assume anything <= 4GB can be handled by IOMMU.  Actually
            * some IOMMUs can handle everything, but I don't know of a
            * way to test this here.
            */
           if (b_pfn < (min_t(u64, 0xffffffffUL, BLK_BOUNCE_HIGH) >> PAGE_SHIFT))
                   dma = 1;
           q->limits.bounce_pfn = max(max_low_pfn, b_pfn);
   #else
           if (b_pfn < blk_max_low_pfn)
                   dma = 1;
           q->limits.bounce_pfn = b_pfn;
   #endif
           if (dma) {
                   init_emergency_isa_pool();
                   q->bounce_gfp = GFP_NOIO | GFP_DMA;
                   q->limits.bounce_pfn = b_pfn;
           }
   }
   EXPORT_SYMBOL(blk_queue_bounce_limit);
   
   /**
    * blk_limits_max_hw_sectors - set hard and soft limit of max sectors for request
    * @limits: the queue limits
    * @max_hw_sectors:  max hardware sectors in the usual 512b unit
    *
    * Description:
    *    Enables a low level driver to set a hard upper limit,
    *    max_hw_sectors, on the size of requests.  max_hw_sectors is set by
    *    the device driver based upon the combined capabilities of I/O
    *    controller and storage device.
    *
    *    max_sectors is a soft limit imposed by the block layer for
    *    filesystem type requests.  This value can be overridden on a
    *    per-device basis in /sys/block/<device>/queue/max_sectors_kb.
    *    The soft limit can not exceed max_hw_sectors.
    **/
   void blk_limits_max_hw_sectors(struct queue_limits *limits, unsigned int max_hw_sectors)
   {
           if ((max_hw_sectors << 9) < PAGE_CACHE_SIZE) {
                   max_hw_sectors = 1 << (PAGE_CACHE_SHIFT - 9);
                   printk(KERN_INFO "%s: set to minimum %d\n",
                          __func__, max_hw_sectors);
           }
   
           limits->max_hw_sectors = max_hw_sectors;
           limits->max_sectors = min_t(unsigned int, max_hw_sectors,
                                       BLK_DEF_MAX_SECTORS);
   }
   EXPORT_SYMBOL(blk_limits_max_hw_sectors);
   
   /**
    * blk_queue_max_hw_sectors - set max sectors for a request for this queue
    * @q:  the request queue for the device
    * @max_hw_sectors:  max hardware sectors in the usual 512b unit
    *
    * Description:
    *    See description for blk_limits_max_hw_sectors().
    **/
   void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int max_hw_sectors)
   {
           blk_limits_max_hw_sectors(&q->limits, max_hw_sectors);
   }
   EXPORT_SYMBOL(blk_queue_max_hw_sectors);
   
   /**
    * blk_queue_max_discard_sectors - set max sectors for a single discard
    * @q:  the request queue for the device
    * @max_discard_sectors: maximum number of sectors to discard
    **/
   void blk_queue_max_discard_sectors(struct request_queue *q,
                   unsigned int max_discard_sectors)
   {
           q->limits.max_discard_sectors = max_discard_sectors;
   }
   EXPORT_SYMBOL(blk_queue_max_discard_sectors);
   
   /**
    * blk_queue_max_write_same_sectors - set max sectors for a single write same
    * @q:  the request queue for the device
    * @max_write_same_sectors: maximum number of sectors to write per command
    **/
   void blk_queue_max_write_same_sectors(struct request_queue *q,
                                         unsigned int max_write_same_sectors)
   {
           q->limits.max_write_same_sectors = max_write_same_sectors;
   }
   EXPORT_SYMBOL(blk_queue_max_write_same_sectors);
   
   /**
    * blk_queue_max_segments - set max hw segments for a request for this queue
    * @q:  the request queue for the device
    * @max_segments:  max number of segments
    *
    * Description:
    *    Enables a low level driver to set an upper limit on the number of
    *    hw data segments in a request.
    **/
   void blk_queue_max_segments(struct request_queue *q, unsigned short max_segments)
   {
           if (!max_segments) {
                   max_segments = 1;
                   printk(KERN_INFO "%s: set to minimum %d\n",
                          __func__, max_segments);
           }
   
           q->limits.max_segments = max_segments;
   }
   EXPORT_SYMBOL(blk_queue_max_segments);
   
   /**
    * blk_queue_max_segment_size - set max segment size for blk_rq_map_sg
    * @q:  the request queue for the device
    * @max_size:  max size of segment in bytes
    *
    * Description:
    *    Enables a low level driver to set an upper limit on the size of a
    *    coalesced segment
    **/
   void blk_queue_max_segment_size(struct request_queue *q, unsigned int max_size)
   {
           if (max_size < PAGE_CACHE_SIZE) {
                   max_size = PAGE_CACHE_SIZE;
                   printk(KERN_INFO "%s: set to minimum %d\n",
                          __func__, max_size);
           }
   
           q->limits.max_segment_size = max_size;
   }
   EXPORT_SYMBOL(blk_queue_max_segment_size);
   
   /**
    * blk_queue_logical_block_size - set logical block size for the queue
    * @q:  the request queue for the device
    * @size:  the logical block size, in bytes
    *
    * Description:
    *   This should be set to the lowest possible block size that the
    *   storage device can address.  The default of 512 covers most
    *   hardware.
    **/
   void blk_queue_logical_block_size(struct request_queue *q, unsigned short size)
   {
           q->limits.logical_block_size = size;
   
           if (q->limits.physical_block_size < size)
                   q->limits.physical_block_size = size;
   
           if (q->limits.io_min < q->limits.physical_block_size)
                   q->limits.io_min = q->limits.physical_block_size;
   }
   EXPORT_SYMBOL(blk_queue_logical_block_size);
   
   /**
    * blk_queue_physical_block_size - set physical block size for the queue
    * @q:  the request queue for the device
    * @size:  the physical block size, in bytes
    *
    * Description:
    *   This should be set to the lowest possible sector size that the
    *   hardware can operate on without reverting to read-modify-write
    *   operations.
    */
   void blk_queue_physical_block_size(struct request_queue *q, unsigned int size)
   {
           q->limits.physical_block_size = size;
   
           if (q->limits.physical_block_size < q->limits.logical_block_size)
                   q->limits.physical_block_size = q->limits.logical_block_size;
   
           if (q->limits.io_min < q->limits.physical_block_size)
                   q->limits.io_min = q->limits.physical_block_size;
   }
   EXPORT_SYMBOL(blk_queue_physical_block_size);
   
   /**
    * blk_queue_alignment_offset - set physical block alignment offset
    * @q:  the request queue for the device
    * @offset: alignment offset in bytes
    *
    * Description:
    *   Some devices are naturally misaligned to compensate for things like
    *   the legacy DOS partition table 63-sector offset.  Low-level drivers
    *   should call this function for devices whose first sector is not
    *   naturally aligned.
    */
   void blk_queue_alignment_offset(struct request_queue *q, unsigned int offset)
   {
           q->limits.alignment_offset =
                   offset & (q->limits.physical_block_size - 1);
           q->limits.misaligned = 0;
   }
   EXPORT_SYMBOL(blk_queue_alignment_offset);
   
   /**
    * blk_limits_io_min - set minimum request size for a device
    * @limits: the queue limits
    * @min:  smallest I/O size in bytes
    *
    * Description:
    *   Some devices have an internal block size bigger than the reported
    *   hardware sector size.  This function can be used to signal the
    *   smallest I/O the device can perform without incurring a performance
    *   penalty.
    */
   void blk_limits_io_min(struct queue_limits *limits, unsigned int min)
   {
           limits->io_min = min;
   
           if (limits->io_min < limits->logical_block_size)
                   limits->io_min = limits->logical_block_size;
   
           if (limits->io_min < limits->physical_block_size)
                   limits->io_min = limits->physical_block_size;
   }
   EXPORT_SYMBOL(blk_limits_io_min);
   
   /**
    * blk_queue_io_min - set minimum request size for the queue
    * @q:  the request queue for the device
    * @min:  smallest I/O size in bytes
    *
    * Description:
    *   Storage devices may report a granularity or preferred minimum I/O
    *   size which is the smallest request the device can perform without
    *   incurring a performance penalty.  For disk drives this is often the
    *   physical block size.  For RAID arrays it is often the stripe chunk
    *   size.  A properly aligned multiple of minimum_io_size is the
    *   preferred request size for workloads where a high number of I/O
    *   operations is desired.
    */
   void blk_queue_io_min(struct request_queue *q, unsigned int min)
   {
           blk_limits_io_min(&q->limits, min);
   }
   EXPORT_SYMBOL(blk_queue_io_min);
   
   /**
    * blk_limits_io_opt - set optimal request size for a device
    * @limits: the queue limits
    * @opt:  smallest I/O size in bytes
    *
    * Description:
    *   Storage devices may report an optimal I/O size, which is the
    *   device's preferred unit for sustained I/O.  This is rarely reported
    *   for disk drives.  For RAID arrays it is usually the stripe width or
    *   the internal track size.  A properly aligned multiple of
    *   optimal_io_size is the preferred request size for workloads where
    *   sustained throughput is desired.
    */
   void blk_limits_io_opt(struct queue_limits *limits, unsigned int opt)
   {
           limits->io_opt = opt;
   }
   EXPORT_SYMBOL(blk_limits_io_opt);
   
   /**
    * blk_queue_io_opt - set optimal request size for the queue
    * @q:  the request queue for the device
    * @opt:  optimal request size in bytes
    *
    * Description:
    *   Storage devices may report an optimal I/O size, which is the
    *   device's preferred unit for sustained I/O.  This is rarely reported
    *   for disk drives.  For RAID arrays it is usually the stripe width or
    *   the internal track size.  A properly aligned multiple of
    *   optimal_io_size is the preferred request size for workloads where
    *   sustained throughput is desired.
    */
   void blk_queue_io_opt(struct request_queue *q, unsigned int opt)
   {
           blk_limits_io_opt(&q->limits, opt);
   }
   EXPORT_SYMBOL(blk_queue_io_opt);
   
   /**
    * blk_queue_stack_limits - inherit underlying queue limits for stacked drivers
    * @t:  the stacking driver (top)
    * @b:  the underlying device (bottom)
    **/
   void blk_queue_stack_limits(struct request_queue *t, struct request_queue *b)
   {
           blk_stack_limits(&t->limits, &b->limits, 0);
   }
   EXPORT_SYMBOL(blk_queue_stack_limits);
   
   /**
    * blk_stack_limits - adjust queue_limits for stacked devices
    * @t:  the stacking driver limits (top device)
    * @b:  the underlying queue limits (bottom, component device)
    * @start:  first data sector within component device
    *
    * Description:
    *    This function is used by stacking drivers like MD and DM to ensure
    *    that all component devices have compatible block sizes and
    *    alignments.  The stacking driver must provide a queue_limits
    *    struct (top) and then iteratively call the stacking function for
    *    all component (bottom) devices.  The stacking function will
    *    attempt to combine the values and ensure proper alignment.
    *
    *    Returns 0 if the top and bottom queue_limits are compatible.  The
    *    top device's block sizes and alignment offsets may be adjusted to
    *    ensure alignment with the bottom device. If no compatible sizes
    *    and alignments exist, -1 is returned and the resulting top
    *    queue_limits will have the misaligned flag set to indicate that
    *    the alignment_offset is undefined.
    */
   int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
                        sector_t start)
   {
           unsigned int top, bottom, alignment, ret = 0;
   
           t->max_sectors = min_not_zero(t->max_sectors, b->max_sectors);
           t->max_hw_sectors = min_not_zero(t->max_hw_sectors, b->max_hw_sectors);
           t->max_write_same_sectors = min(t->max_write_same_sectors,
                                           b->max_write_same_sectors);
           t->bounce_pfn = min_not_zero(t->bounce_pfn, b->bounce_pfn);
   
           t->seg_boundary_mask = min_not_zero(t->seg_boundary_mask,
                                               b->seg_boundary_mask);
   
           t->max_segments = min_not_zero(t->max_segments, b->max_segments);
           t->max_integrity_segments = min_not_zero(t->max_integrity_segments,
                                                    b->max_integrity_segments);
   
           t->max_segment_size = min_not_zero(t->max_segment_size,
                                              b->max_segment_size);
   
           t->misaligned |= b->misaligned;
   
           alignment = queue_limit_alignment_offset(b, start);
   
           /* Bottom device has different alignment.  Check that it is
            * compatible with the current top alignment.
            */
           if (t->alignment_offset != alignment) {
   
                   top = max(t->physical_block_size, t->io_min)
                           + t->alignment_offset;
                   bottom = max(b->physical_block_size, b->io_min) + alignment;
   
                   /* Verify that top and bottom intervals line up */
                   if (max(top, bottom) & (min(top, bottom) - 1)) {
                           t->misaligned = 1;
                           ret = -1;
                   }
           }
   
           t->logical_block_size = max(t->logical_block_size,
                                       b->logical_block_size);
   
           t->physical_block_size = max(t->physical_block_size,
                                        b->physical_block_size);
   
           t->io_min = max(t->io_min, b->io_min);
           t->io_opt = lcm(t->io_opt, b->io_opt);
   
           t->cluster &= b->cluster;
           t->discard_zeroes_data &= b->discard_zeroes_data;
   
           /* Physical block size a multiple of the logical block size? */
           if (t->physical_block_size & (t->logical_block_size - 1)) {
                   t->physical_block_size = t->logical_block_size;
                   t->misaligned = 1;
                   ret = -1;
           }
   
           /* Minimum I/O a multiple of the physical block size? */
           if (t->io_min & (t->physical_block_size - 1)) {
                   t->io_min = t->physical_block_size;
                   t->misaligned = 1;
                   ret = -1;
           }
   
           /* Optimal I/O a multiple of the physical block size? */
           if (t->io_opt & (t->physical_block_size - 1)) {
                   t->io_opt = 0;
                   t->misaligned = 1;
                   ret = -1;
           }
   
           /* Find lowest common alignment_offset */
           t->alignment_offset = lcm(t->alignment_offset, alignment)
                   & (max(t->physical_block_size, t->io_min) - 1);
   
           /* Verify that new alignment_offset is on a logical block boundary */
           if (t->alignment_offset & (t->logical_block_size - 1)) {
                   t->misaligned = 1;
                   ret = -1;
           }
   
           /* Discard alignment and granularity */
           if (b->discard_granularity) {
                   alignment = queue_limit_discard_alignment(b, start);
   
                   if (t->discard_granularity != 0 &&
                       t->discard_alignment != alignment) {
                           top = t->discard_granularity + t->discard_alignment;
                           bottom = b->discard_granularity + alignment;
   
                           /* Verify that top and bottom intervals line up */
                           if ((max(top, bottom) % min(top, bottom)) != 0)
                                   t->discard_misaligned = 1;
                   }
   
                   t->max_discard_sectors = min_not_zero(t->max_discard_sectors,
                                                         b->max_discard_sectors);
                   t->discard_granularity = max(t->discard_granularity,
                                                b->discard_granularity);
                   t->discard_alignment = lcm(t->discard_alignment, alignment) %
                           t->discard_granularity;
           }
   
           return ret;
   }
   EXPORT_SYMBOL(blk_stack_limits);
   
   /**
    * bdev_stack_limits - adjust queue limits for stacked drivers
    * @t:  the stacking driver limits (top device)
    * @bdev:  the component block_device (bottom)
    * @start:  first data sector within component device
    *
    * Description:
    *    Merges queue limits for a top device and a block_device.  Returns
    *    0 if alignment didn't change.  Returns -1 if adding the bottom
    *    device caused misalignment.
    */
   int bdev_stack_limits(struct queue_limits *t, struct block_device *bdev,
                         sector_t start)
   {
           struct request_queue *bq = bdev_get_queue(bdev);
   
           start += get_start_sect(bdev);
   
           return blk_stack_limits(t, &bq->limits, start);
   }
   EXPORT_SYMBOL(bdev_stack_limits);
   
   /**
    * disk_stack_limits - adjust queue limits for stacked drivers
    * @disk:  MD/DM gendisk (top)
    * @bdev:  the underlying block device (bottom)
    * @offset:  offset to beginning of data within component device
    *
    * Description:
    *    Merges the limits for a top level gendisk and a bottom level
    *    block_device.
    */
   void disk_stack_limits(struct gendisk *disk, struct block_device *bdev,
                          sector_t offset)
   {
           struct request_queue *t = disk->queue;
   
           if (bdev_stack_limits(&t->limits, bdev, offset >> 9) < 0) {
                   char top[BDEVNAME_SIZE], bottom[BDEVNAME_SIZE];
   
                   disk_name(disk, 0, top);
                   bdevname(bdev, bottom);
   
                   printk(KERN_NOTICE "%s: Warning: Device %s is misaligned\n",
                          top, bottom);
           }
   }
   EXPORT_SYMBOL(disk_stack_limits);
   
   /**
    * blk_queue_dma_pad - set pad mask
    * @q:     the request queue for the device
    * @mask:  pad mask
    *
    * Set dma pad mask.
    *
    * Appending pad buffer to a request modifies the last entry of a
    * scatter list such that it includes the pad buffer.
    **/
   void blk_queue_dma_pad(struct request_queue *q, unsigned int mask)
   {
           q->dma_pad_mask = mask;
   }
   EXPORT_SYMBOL(blk_queue_dma_pad);
   
   /**
    * blk_queue_update_dma_pad - update pad mask
    * @q:     the request queue for the device
    * @mask:  pad mask
    *
    * Update dma pad mask.
    *
    * Appending pad buffer to a request modifies the last entry of a
    * scatter list such that it includes the pad buffer.
    **/
   void blk_queue_update_dma_pad(struct request_queue *q, unsigned int mask)
   {
           if (mask > q->dma_pad_mask)
                   q->dma_pad_mask = mask;
   }
   EXPORT_SYMBOL(blk_queue_update_dma_pad);
   
   /**
    * blk_queue_dma_drain - Set up a drain buffer for excess dma.
    * @q:  the request queue for the device
    * @dma_drain_needed: fn which returns non-zero if drain is necessary
    * @buf:        physically contiguous buffer
    * @size:       size of the buffer in bytes
    *
    * Some devices have excess DMA problems and can't simply discard (or
    * zero fill) the unwanted piece of the transfer.  They have to have a
    * real area of memory to transfer it into.  The use case for this is
    * ATAPI devices in DMA mode.  If the packet command causes a transfer
    * bigger than the transfer size some HBAs will lock up if there
    * aren't DMA elements to contain the excess transfer.  What this API
    * does is adjust the queue so that the buf is always appended
    * silently to the scatterlist.
    *
    * Note: This routine adjusts max_hw_segments to make room for appending
    * the drain buffer.  If you call blk_queue_max_segments() after calling
    * this routine, you must set the limit to one fewer than your device
    * can support otherwise there won't be room for the drain buffer.
    */
   int blk_queue_dma_drain(struct request_queue *q,
                                  dma_drain_needed_fn *dma_drain_needed,
                                  void *buf, unsigned int size)
   {
           if (queue_max_segments(q) < 2)
                   return -EINVAL;
           /* make room for appending the drain */
           blk_queue_max_segments(q, queue_max_segments(q) - 1);
           q->dma_drain_needed = dma_drain_needed;
           q->dma_drain_buffer = buf;
           q->dma_drain_size = size;
   
           return 0;
   }
   EXPORT_SYMBOL_GPL(blk_queue_dma_drain);
   
   /**
    * blk_queue_segment_boundary - set boundary rules for segment merging
    * @q:  the request queue for the device
    * @mask:  the memory boundary mask
    **/
   void blk_queue_segment_boundary(struct request_queue *q, unsigned long mask)
   {
           if (mask < PAGE_CACHE_SIZE - 1) {
                   mask = PAGE_CACHE_SIZE - 1;
                   printk(KERN_INFO "%s: set to minimum %lx\n",
                          __func__, mask);
           }
   
           q->limits.seg_boundary_mask = mask;
   }
   EXPORT_SYMBOL(blk_queue_segment_boundary);
   
   /**
    * blk_queue_dma_alignment - set dma length and memory alignment
    * @q:     the request queue for the device
    * @mask:  alignment mask
    *
    * description:
    *    set required memory and length alignment for direct dma transactions.
    *    this is used when building direct io requests for the queue.
    *
    **/
   void blk_queue_dma_alignment(struct request_queue *q, int mask)
   {
           q->dma_alignment = mask;
   }
   EXPORT_SYMBOL(blk_queue_dma_alignment);
   
   /**
    * blk_queue_update_dma_alignment - update dma length and memory alignment
    * @q:     the request queue for the device
    * @mask:  alignment mask
    *
    * description:
    *    update required memory and length alignment for direct dma transactions.
    *    If the requested alignment is larger than the current alignment, then
    *    the current queue alignment is updated to the new value, otherwise it
    *    is left alone.  The design of this is to allow multiple objects
    *    (driver, device, transport etc) to set their respective
    *    alignments without having them interfere.
    *
    **/
   void blk_queue_update_dma_alignment(struct request_queue *q, int mask)
   {
           BUG_ON(mask > PAGE_SIZE);
   
           if (mask > q->dma_alignment)
                   q->dma_alignment = mask;
   }
   EXPORT_SYMBOL(blk_queue_update_dma_alignment);
   
   /**
    * blk_queue_flush - configure queue's cache flush capability
    * @q:          the request queue for the device
    * @flush:      0, REQ_FLUSH or REQ_FLUSH | REQ_FUA
    *
    * Tell block layer cache flush capability of @q.  If it supports
    * flushing, REQ_FLUSH should be set.  If it supports bypassing
    * write cache for individual writes, REQ_FUA should be set.
    */
   void blk_queue_flush(struct request_queue *q, unsigned int flush)
   {
           WARN_ON_ONCE(flush & ~(REQ_FLUSH | REQ_FUA));
   
           if (WARN_ON_ONCE(!(flush & REQ_FLUSH) && (flush & REQ_FUA)))
                   flush &= ~REQ_FUA;
   
           q->flush_flags = flush & (REQ_FLUSH | REQ_FUA);
   }
   EXPORT_SYMBOL_GPL(blk_queue_flush);
   
   void blk_queue_flush_queueable(struct request_queue *q, bool queueable)
   {
           q->flush_not_queueable = !queueable;
   }
   EXPORT_SYMBOL_GPL(blk_queue_flush_queueable);
   
   static int __init blk_settings_init(void)
   {
           blk_max_low_pfn = max_low_pfn - 1;
           blk_max_pfn = max_pfn - 1;
           return 0;
   }
   subsys_initcall(blk_settings_init);

Cache object: 131d0f1ce5eb52b8749db37e87b7681e