Re: [PATCH v2 1/3] dmaengine: add new dma API for max_segment_number - Russell King

From: Russell King - ARM Linux <linux@arm.linux.org.uk>
To: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: shawn.guo@linaro.org, patches@linaro.org, vinod.koul@intel.com,
	gregkh@suse.de, linux-mmc@vger.kernel.org,
	linux-kernel@vger.kernel.org, dan.j.williams@intel.com,
	cjb@laptop.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 1/3] dmaengine: add new dma API for max_segment_number
Date: Mon, 6 Jun 2011 11:15:56 +0100	[thread overview]
Message-ID: <20110606101556.GE10508@n2100.arm.linux.org.uk> (raw)
In-Reply-To: <20110606191041U.fujita.tomonori@lab.ntt.co.jp>

On Mon, Jun 06, 2011 at 07:12:20PM +0900, FUJITA Tomonori wrote:
> On Mon, 6 Jun 2011 10:47:51 +0100
> Russell King - ARM Linux <linux@arm.linux.org.uk> wrote:
> 
> > On Mon, Jun 06, 2011 at 06:41:09PM +0900, FUJITA Tomonori wrote:
> > > On Mon, 6 Jun 2011 10:14:10 +0100
> > > Russell King - ARM Linux <linux@arm.linux.org.uk> wrote:
> > > 
> > > > On Mon, Jun 06, 2011 at 05:06:03PM +0900, FUJITA Tomonori wrote:
> > > > > max_segs isn't unrelated with the dma mapping API. I explained above,
> > > > > IOMMUs doesn't increase the number of segments (could decrease the
> > > > > number of segments by merging).
> > > > > 
> > > > > The limitation about the number of segment already lives elsewhere
> > > > > (e.g. queue's limits.max_segments).
> > > > 
> > > > I think you're missing the point entirely.
> > > > 
> > > > Lets take the problem at hand: you have two devices.  One of them is
> > > > handled by the DMA engine code.  One of them is a block device.
> > > > 
> > > > The block layer needs to know the various parameters of what is
> > > > allowable for DMA, including such things as the maximum size of a
> > > > segment, and the _number_ of segments that can be placed into any
> > > > one request.
> > > > 
> > > > As the DMA provider is _entirely_ separate and unknown to the block
> > > > device driver, the block device driver has no way to sanely provide
> > > > these parameters to the block layer - they are not a property of the
> > > > block device driver, but of the DMA provider.
> > > 
> > > struct device_dma_parameters is used for a property of the block
> > > device drivers (and scsi HBA drivers, etc). Not DMA provider. Right?
> > 
> > Wrong.  struct device_dma_parameters is a property of the _DMA_ _provider_.
> > It has to be.  Read what I said above and think about it.
> 
> I think that it's up to your definition of DMA provider.

I give up.