linux-kernel - Re: [PATCH v3 8/9] spi: rockchip: modify DMA max burst to 1

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Date:	Thu, 3 Sep 2015 18:45:02 +0100
From:	Mark Brown <broonie@...nel.org>
To:	Shawn Lin <shawn.lin@...k-chips.com>
Cc:	Vinod Koul <vinod.koul@...el.com>,
	Heiko Stuebner <heiko@...ech.de>,
	Jaroslav Kysela <perex@...ex.cz>,
	Takashi Iwai <tiwai@...e.com>,
	Doug Anderson <dianders@...omium.org>,
	Olof Johansson <olofj@...gle.com>,
	Sonny Rao <sonnyrao@...omium.org>,
	Addy Ke <addy.ke@...k-chips.com>, dmaengine@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-rockchip@...ts.infradead.org, alsa-devel@...a-project.org,
	linux-spi@...r.kernel.org, Olof Johansson <olof@...om.net>
Subject: Re: [PATCH v3 8/9] spi: rockchip: modify DMA max burst to 1

On Thu, Sep 03, 2015 at 09:41:43PM +0800, Shawn Lin wrote:
> On 2015/9/3 20:54, Mark Brown wrote:

> >If this is a quirk of the DMA controller why is it being fixed with a
> >property specific to the SPI controller?  Can't the DMA controller just
> >override the burst size?

> Cool...that's a good question. Actually DMA controller can't do that since
> block peripheral controller(e.g sd/emmc, they always access data by
> block-512Bytes.) can't meet the case. So It's hard for dma controller know
> who is the caller, and what should be done.

You could take a guess based on request size, but that's going to be a
bit error prone I imagine.

> The only thing dma controller can do is avoid to execute DMAFLUSHP on its
> own. But how about peripheral controller trigger non-multiple access? So
> that's what I mentioned on covery letter that we should do "broken things"
> for both dma controller and some peripherals.

The other thing it could do is provide the information out via some
internal Linux API which the drivers could query instead of directly
having a DT property in each client driver.  That way at least the DT
only needs the quirk in one place even if the clients need to handle it
individually.

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