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author | 2019-09-11 12:15:30 +0200 | |
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committer | 2019-09-11 15:56:46 +0100 | |
commit | bf75703d0912da7b1af72f14392c805b758c3fbc (patch) | |
tree | 82fcb3f3b02e822551e02a6da940d8ab657a30ce /fs | |
parent | spi: bcm2835: Cache CS register value for ->prepare_message() (diff) | |
download | linux-bf75703d0912da7b1af72f14392c805b758c3fbc.tar.gz linux-bf75703d0912da7b1af72f14392c805b758c3fbc.tar.bz2 linux-bf75703d0912da7b1af72f14392c805b758c3fbc.zip |
dmaengine: bcm2835: Avoid accessing memory when copying zeroes
The BCM2835 DMA controller is capable of synthesizing zeroes instead of
copying them from a source address. The feature is enabled by setting
the SRC_IGNORE bit in the Transfer Information field of a Control Block:
"Do not perform source reads.
In addition, destination writes will zero all the write strobes.
This is used for fast cache fill operations."
https://www.raspberrypi.org/app/uploads/2012/02/BCM2835-ARM-Peripherals.pdf
The feature is only available on 8 of the 16 channels. The others are
so-called "lite" channels with a limited feature set and performance.
Enable the feature if a cyclic transaction copies from the zero page.
This reduces traffic on the memory bus.
A forthcoming use case is the BCM2835 SPI driver, which will cyclically
copy from the zero page to the TX FIFO. The idea to use SRC_IGNORE was
taken from an ancient GitHub conversation between Martin and Noralf:
https://github.com/msperl/spi-bcm2835/issues/13#issuecomment-98180451
Tested-by: Nuno Sá <nuno.sa@analog.com>
Tested-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Vinod Koul <vkoul@kernel.org>
Acked-by: Stefan Wahren <wahrenst@gmx.net>
Acked-by: Martin Sperl <kernel@martin.sperl.org>
Cc: Florian Kauer <florian.kauer@koalo.de>
Link: https://lore.kernel.org/r/b2286c904408745192e4beb3de3c88f73e4a7210.1568187525.git.lukas@wunner.de
Signed-off-by: Mark Brown <broonie@kernel.org>
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