atmel_spi: chain DMA transfers
authorSilvester Erdeg <slipszi@gmail.com>
Wed, 6 Feb 2008 09:38:12 +0000 (01:38 -0800)
committerLinus Torvalds <torvalds@woody.linux-foundation.org>
Wed, 6 Feb 2008 18:41:10 +0000 (10:41 -0800)
commit154443c72f47169ebcb3a7befbff0e934c49bff3
treec0567d5aa3c46ce4056da07ddacead626f933c73
parent1eed29df472a33bba013d5a2ea2f9e32f4414397
atmel_spi: chain DMA transfers

Add support for chained transfers in the atmel_spi driver, letting the DMA
controller switch to the next buffer pair without CPU intervention.  This
reduced I/O latencies by about 2% in one bulk I/O test.  It should also help
work around several interrelated errata affecting chipselect 0 on at91rm9200
chips.

Almost all of the changes are in the reworked atmel_spi_next_xfer() function.
That's now called with the driver in one of three states:

 1. It isn't transferring anything (in which case the first transfer
    of the current message is going to be sent)
 2. It has finished transfering a non-chainable transfer (in which
    case it will go to the next transfer in the message)
 3. It has finished transfering a chained transfer (in which case the
    next transfer is already queued)

After that it will queue the next transfer if it can be chained.

Signed-off-by: Szilveszter Ordog <slipszi@gmail.com>
Acked-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
drivers/spi/atmel_spi.c