[PATCH v3 1/7] spi: imx: Fix DMA transfer

Sascha Hauer s.hauer at pengutronix.de
Thu Nov 5 00:34:22 PST 2015

On Sun, Nov 01, 2015 at 03:41:35PM +0100, Anton Bondarenko wrote:
> From: Anton Bondarenko <anton_bondarenko at mentor.com>
> RX DMA tail data handling doesn't work correctly in many cases with
> current implementation. It happens because SPI core was setup
> to generates both RX watermark level and RX DATA TAIL events
> incorrectly. SPI transfer triggering for DMA also done in wrong way.
> SPI client wants to transfer 70 words for example. The old DMA
> implementation setup RX DATA TAIL equal 6 words. In this case
> RX DMA event will be generated after 6 words read from RX FIFO.
> The garbage can be read out from RX FIFO because SPI HW does
> not receive all required words to trigger RX watermark event.
> New implementation change handling of RX data tail. DMA is used to process
> all TX data and only full chunks of RX data with size aligned to FIFO/2.
> Driver is waiting until both TX and RX DMA transaction done and all
> TX data are pushed out. At that moment there is only RX data tail in
> the RX FIFO. This data read out using PIO.

Have you looked at the RX_DMA_LENGTH and RXTDEN fields of the DMA
register? These seem to be for handling the remaining bytes of a DMA
transfer which do not reach the watermark level. From reading the
documentation I haven't really understood how it works though.


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