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

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Message-ID: <20151105083422.GH8526@pengutronix.de>
Date:	Thu, 5 Nov 2015 09:34:22 +0100
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Anton Bondarenko <anton.bondarenko.sama@...il.com>
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 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

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