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

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Date:	Sat, 14 Nov 2015 11:02:58 +0100
From:	Anton Bondarenko <anton.bondarenko.sama@...il.com>
To:	Sascha Hauer <s.hauer@...gutronix.de>
Cc:	broonie@...nel.org, b38343@...escale.com,
	linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	vladimir_zapolskiy@...tor.com, jiada_wang@...tor.com
Subject: Re: [PATCH v3 1/7] spi: imx: Fix DMA transfer


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@...tor.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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