linux-kernel - Re: [PATCH 08/11] MTD: m25p80: Add option to limit SPI transfer size.

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Date:	Wed, 22 Jul 2015 10:19:25 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Michal Suchanek <hramrach@...il.com>
Cc:	Brian Norris <computersforpeace@...il.com>,
	Marek Vasut <marex@...x.de>,
	Richard Cochran <richardcochran@...il.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Mark Rutland <mark.rutland@....com>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	MTD Maling List <linux-mtd@...ts.infradead.org>,
	Alison Chaiken <alison_chaiken@...tor.com>,
	Bean Huo 霍斌斌 (beanhuo) 
	<beanhuo@...ron.com>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>,
	Rafał Miłecki <zajec5@...il.com>,
	Kukjin Kim <kgene@...nel.org>,
	Ben Hutchings <ben@...adent.org.uk>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Mark Brown <broonie@...nel.org>,
	Dan Williams <dan.j.williams@...el.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"grmoore@...era.com" <grmoore@...era.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-spi <linux-spi@...r.kernel.org>,
	Huang Shijie <b32955@...escale.com>,
	Rob Herring <robh+dt@...nel.org>,
	Han Xu <han.xu@...escale.com>,
	Knut Wohlrab <knut.wohlrab@...bosch.com>,
	dmaengine <dmaengine@...r.kernel.org>,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH 08/11] MTD: m25p80: Add option to limit SPI transfer size.

On Tue, Jul 21, 2015 at 10:14:11AM +0200, Michal Suchanek wrote:
> > Or alternatively we could publish the limitations of the channel using
> > capabilities so SPI knows I have a dmaengine channel and it can transfer max N
> > length transfers so would be able to break rather than guessing it or coding
> > in DT. Yes it may come from DT but that should be dmaengine driver rather
> > than client driver :)
> >
> > This can be done by dma_get_slave_caps(chan, &caps)
> >
> > And we add max_length as one more parameter to existing set
> >
> > Also all this could be handled in generic SPI-dmaengine layer so that
> > individual drivers don't have to code it in
> >
> > Let me know if this idea is okay, I can push the dmaengine bits...
> 
> It would be ok if there was a fixed limit. However, the limit depends
> on SPI slave settings. Presumably for other buses using the dmaengine
> the limit would depend on the bus or slave settings as well. I do not
> see a sane way of passing this all the way to the dmaengine driver.
I don't see why this should be client (SPI) dependent. The max length
supported is a dmaengine constraint, typically flowing from max
blocks/length it can transfer. Know this limit can allow clients to split
transfers.

-- 
~Vinod
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