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

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Date:	Fri, 21 Sep 2012 15:03:12 -0400
From:	Matt Porter <mporter@...com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	"S, Venkatraman" <svenkatr@...com>,
	Linux DaVinci Kernel List 
	<davinci-linux-open-source@...ux.davincidsp.com>,
	Linux OMAP List <linux-omap@...r.kernel.org>,
	Benoit Cousson <b-cousson@...com>,
	Arnd Bergmann <arnd@...db.de>,
	Linux Documentation List <linux-doc@...r.kernel.org>,
	Tony Lindgren <tony@...mide.com>,
	Linux MMC List <linux-mmc@...r.kernel.org>,
	Devicetree Discuss <devicetree-discuss@...ts.ozlabs.org>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Vinod Koul <vinod.koul@...el.com>,
	Rob Landley <rob@...dley.net>, Dan Williams <djbw@...com>,
	Linux SPI Devel List 
	<spi-devel-general@...ts.sourceforge.net>,
	Chris Ball <cjb@...top.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 07:47:21PM +0100, Russell King wrote:
> On Fri, Sep 21, 2012 at 10:45:29PM +0530, S, Venkatraman 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 ?
> 
> Why not have a generic way that DMA engine can export these kinds of
> properties?

That's exactly what my note above is suggesting...

Something along the lines of:

	struct slave_sg_caps
	{
		int max_segs;	/* <0 is no limit */
	}

	struct slave_sg_cap *
	dmaengine_get_slave_sg_caps(struct dma_chan *chan);

I'm sure there are or will be other characteristics worth providing to
slave drivers.

-Matt
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