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

From: Anton Bondarenko
Date: Sat Nov 14 2015 - 05:03:11 EST

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@xxxxxxxxxx>

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
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.


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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