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SM501: Add Documentation/SM501.txt
Add documentation for the SM501 in Documentation/SM501.txt outlining the SM501
driver.
Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
authored by Ben Dooks and committed by Linus Torvalds ffd65af0 81906221
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Documentation/SM501.txt
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Documentation/SM501.txt
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SM501 Driver
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============
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Copyright 2006, 2007 Simtec Electronics
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Core
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----
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The core driver in drivers/mfd provides common services for the
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drivers which manage the specific hardware blocks. These services
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include locking for common registers, clock control and resource
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management.
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The core registers drivers for both PCI and generic bus based
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chips via the platform device and driver system.
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On detection of a device, the core initialises the chip (which may
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be specified by the platform data) and then exports the selected
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peripheral set as platform devices for the specific drivers.
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The core re-uses the platform device system as the platform device
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system provides enough features to support the drivers without the
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need to create a new bus-type and the associated code to go with it.
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Resources
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---------
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Each peripheral has a view of the device which is implicitly narrowed to
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the specific set of resources that peripheral requires in order to
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function correctly.
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The centralised memory allocation allows the driver to ensure that the
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maximum possible resource allocation can be made to the video subsystem
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as this is by-far the most resource-sensitive of the on-chip functions.
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The primary issue with memory allocation is that of moving the video
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buffers once a display mode is chosen. Indeed when a video mode change
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occurs the memory footprint of the video subsystem changes.
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Since video memory is difficult to move without changing the display
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(unless sufficient contiguous memory can be provided for the old and new
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modes simultaneously) the video driver fully utilises the memory area
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given to it by aligning fb0 to the start of the area and fb1 to the end
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of it. Any memory left over in the middle is used for the acceleration
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functions, which are transient and thus their location is less critical
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as it can be moved.
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Configuration
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-------------
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The platform device driver uses a set of platform data to pass
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configurations through to the core and the subsidiary drivers
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so that there can be support for more than one system carrying
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an SM501 built into a single kernel image.
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The PCI driver assumes that the PCI card behaves as per the Silicon
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Motion reference design.
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There is an errata (AB-5) affecting the selection of the
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of the M1XCLK and M1CLK frequencies. These two clocks
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must be sourced from the same PLL, although they can then
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be divided down individually. If this is not set, then SM501 may
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lock and hang the whole system. The driver will refuse to
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attach if the PLL selection is different.