FreeBSD/Linux Kernel Cross Reference
sys/Documentation/SM501.txt

                             SM501 Driver
                             ============
     
     Copyright 2006, 2007 Simtec Electronics
     
     The Silicon Motion SM501 multimedia companion chip is a multifunction device
     which may provide numerous interfaces including USB host controller USB gadget,
     asynchronous serial ports, audio functions, and a dual display video interface.
     The device may be connected by PCI or local bus with varying functions enabled.
    
    Core
    ----
    
    The core driver in drivers/mfd provides common services for the
    drivers which manage the specific hardware blocks. These services
    include locking for common registers, clock control and resource
    management.
    
    The core registers drivers for both PCI and generic bus based
    chips via the platform device and driver system.
    
    On detection of a device, the core initialises the chip (which may
    be specified by the platform data) and then exports the selected
    peripheral set as platform devices for the specific drivers.
    
    The core re-uses the platform device system as the platform device
    system provides enough features to support the drivers without the
    need to create a new bus-type and the associated code to go with it.
    
    
    Resources
    ---------
    
    Each peripheral has a view of the device which is implicitly narrowed to
    the specific set of resources that peripheral requires in order to
    function correctly.
    
    The centralised memory allocation allows the driver to ensure that the
    maximum possible resource allocation can be made to the video subsystem
    as this is by-far the most resource-sensitive of the on-chip functions.
    
    The primary issue with memory allocation is that of moving the video
    buffers once a display mode is chosen. Indeed when a video mode change
    occurs the memory footprint of the video subsystem changes.
    
    Since video memory is difficult to move without changing the display
    (unless sufficient contiguous memory can be provided for the old and new
    modes simultaneously) the video driver fully utilises the memory area
    given to it by aligning fb0 to the start of the area and fb1 to the end
    of it. Any memory left over in the middle is used for the acceleration
    functions, which are transient and thus their location is less critical
    as it can be moved.
    
    
    Configuration
    -------------
    
    The platform device driver uses a set of platform data to pass
    configurations through to the core and the subsidiary drivers
    so that there can be support for more than one system carrying
    an SM501 built into a single kernel image.
    
    The PCI driver assumes that the PCI card behaves as per the Silicon
    Motion reference design.
    
    There is an errata (AB-5) affecting the selection of the
    of the M1XCLK and M1CLK frequencies. These two clocks
    must be sourced from the same PLL, although they can then
    be divided down individually. If this is not set, then SM501 may
    lock and hang the whole system. The driver will refuse to
    attach if the PLL selection is different.

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