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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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