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

[Anton Bondarenko <anton.bondarenko.sama@gmail.com>]

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@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