linux-kernel - Re: [PATCH v5] spi: spi-imx: add DMA support

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Message-ID: <20140911013737.GA366@Robin-OptiPlex-780>
Date:	Thu, 11 Sep 2014 09:37:38 +0800
From:	Robin Gong <b38343@...escale.com>
To:	Mark Brown <broonie@...nel.org>
CC:	<linux-arm-kernel@...ts.infradead.org>,
	<linux-spi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<marex@...x.de>, <shawn.guo@...aro.org>, <Frank.Li@...escale.com>
Subject: Re: [PATCH v5] spi: spi-imx: add DMA support

On Wed, Sep 10, 2014 at 04:52:16PM +0100, Mark Brown wrote:
> On Wed, Sep 10, 2014 at 11:18:01PM +0800, Robin Gong wrote:
> > On Wed, Sep 10, 2014 at 12:45:30PM +0100, Mark Brown wrote:
> 
> > > > Yes, you're right. I should use "transfer->tx_sg->sgl == NULL" or
> > > > "transfer->rx_sg->sgl == NULL" instead of usedma flag in driver, right?
> 
> > > Right.
> 
> > But looks spi core framework can make sure every rx or tx transfer in sequence.
> > If so, can_dma will never be called until this transfer finished
> > (spi_pump_messages), and my usedma flag also never be modified until next
> > transfer start. Please correct me if I am wrong, thanks.
> 
> Even if that were true currently this would still not be good code since
> it would break in the face of SPI core changes.  In any case the
> assumption isn't true for the current SPI core, the DMA mapping is done
> for all transfers in a message before we start running them and it's
> entirely likely that we will have patterns like a short PIO transfer
> followed by a big data block.
Understood, will improve it in v6.Thanks.

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