linux-kernel - Re: [RFC PATCH 08/13] mmc: omap_hsmmc: limit max

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Message-ID: <CANfBPZ-q68qqpF0TvrY9HXH4mOjVE2j1T7+J0-aKKpg2ZaR+4g@mail.gmail.com>
Date:	Fri, 21 Sep 2012 23:03:42 +0530
From:	"S, Venkatraman" <svenkatr@...com>
To:	balbi@...com
Cc:	Matt Porter <mporter@...com>, Tony Lindgren <tony@...mide.com>,
	Sekhar Nori <nsekhar@...com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Benoit Cousson <b-cousson@...com>,
	Russell King <linux@....linux.org.uk>,
	Vinod Koul <vinod.koul@...el.com>,
	Rob Landley <rob@...dley.net>, Chris Ball <cjb@...top.org>,
	Linux DaVinci Kernel List 
	<davinci-linux-open-source@...ux.davincidsp.com>,
	Arnd Bergmann <arnd@...db.de>,
	Linux Documentation List <linux-doc@...r.kernel.org>,
	Devicetree Discuss <devicetree-discuss@...ts.ozlabs.org>,
	Linux MMC List <linux-mmc@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Dan Williams <djbw@...com>,
	Linux SPI Devel List 
	<spi-devel-general@...ts.sourceforge.net>,
	Linux OMAP List <linux-omap@...r.kernel.org>,
	Linux ARM Kernel List <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 08/13] mmc: omap_hsmmc: limit max_segs with the EDMA DMAC

On Fri, Sep 21, 2012 at 10:48 PM, Felipe Balbi <balbi@...com> wrote:
> On Fri, Sep 21, 2012 at 10:47:30PM +0530, S, Venkatraman wrote:
>> On Fri, Sep 21, 2012 at 10:45 PM, S, Venkatraman <svenkatr@...com> wrote:
>> > On Thu, Sep 20, 2012 at 8:13 PM, Matt Porter <mporter@...com> wrote:
>> >> The EDMA DMAC has a hardware limitation that prevents supporting
>> >> scatter gather lists with any number of segments. Since the EDMA
>> >> DMA Engine driver sets the maximum segments to 16, we do the
>> >> same.
>> >>
>> >> Note: this can be removed once the DMA Engine API supports an
>> >> API to query the DMAC's segment limitations.
>> >>
>> >
>> > I wouldn't want to bind the properties of EDMA to omap_hsmmc as this patch
>> > suggests. Why don't we have a max_segs property, which when explicitly specified
>> > in DT, will override the default ?
>>
>> If you are adventurous, this can be a generic mmc DT binding instead
>> of restricting it to OMAP.
>
> I say if it's a limitation in the DMAC, then DMAC's driver should handle
> it, no ? Meaning that in this case you would copy from one multi-segment
> sg into a one-segment sg and when transfer is complete, before calling
> user's callback, copy data the other way around (?)
>

Right ! So even if the property is defined for MMC, Matt will end up coding the
limitation into every peripheral driver that uses EMAC, which doesn't scale.
Your solution is better.
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