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

Anton Bondarenko anton.bondarenko.sama at gmail.com
Sat Nov 14 02:02:58 PST 2015

On 05.11.2015 17:51, Anton Bondarenko wrote:
> On 05.11.2015 09:34, Sascha Hauer wrote:
>> 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.
>> Sascha
> A lot of times. Current implementation is trying to use it, but works
> incorrectly if length % WML != 0 (which means RX_DMA_LENGTH == 0).
> Regards, Anton

Does anyone has other comments regarding this commit?

Regards, Anton

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