linux-kernel - Re: [PATCH 16/18] dmaengine/amba-pl08x: Add support for sg len greater than one for slave transfers

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Message-ID: <20110821083306.GA12028@n2100.arm.linux.org.uk>
Date:	Sun, 21 Aug 2011 09:33:06 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Viresh Kumar <viresh.kumar@...com>
Cc:	"linus.walleij@...aro.org" <linus.walleij@...aro.org>,
	Pratyush ANAND <pratyush.anand@...com>,
	Rajeev KUMAR <rajeev-dlh.kumar@...com>,
	Bhupesh SHARMA <bhupesh.sharma@...com>,
	Shiraz HASHIM <shiraz.hashim@...com>,
	"vinod.koul@...el.com" <vinod.koul@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Vipin KUMAR <vipin.kumar@...com>,
	Armando VISCONTI <armando.visconti@...com>,
	Amit VIRDI <Amit.VIRDI@...com>,
	Vipul Kumar SAMAR <vipulkumar.samar@...com>,
	"viresh.linux@...il.com" <viresh.linux@...il.com>,
	Deepak SIKRI <deepak.sikri@...com>,
	"dan.j.williams@...el.com" <dan.j.williams@...el.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 16/18] dmaengine/amba-pl08x: Add support for sg len
	greater than one for slave transfers

On Thu, Aug 18, 2011 at 02:08:19PM +0530, Viresh Kumar wrote:
> On 8/14/2011 2:06 PM, Russell King - ARM Linux wrote:
> > On Fri, Jul 29, 2011 at 04:19:26PM +0530, Viresh Kumar wrote:
> >> Untill now, sg_len greater than one is not supported. This patch adds support to
> >> do that.
> > 
> > I'm not sure that this is the correct approach.  memcpy()s can only be
> > used with one single buffer, so the sg stuff for that (and the unmapping
> > support) doesn't make sense at all.
> > 
> 
> I am not sure if i get this completely. In memcpy, we still don't support
> more than one sg. We have created a new member in txd, which keeps track of
> data (addresses, len).

Yes, but we shouldn't need to translate it into any kind of scatterlist.

> While unmapping, we are unmapping only single dsg. I think i don't need to
> use list_for_each_entry() there, as there is only one element present in list.
> But can still keep that.

Correct.

> > The only place where this makes sense is the slave sg stuff.  I wonder
> > whether we can better deal with that by having the LLI setup code deal
> > with one SG list entry at a time, and chain each together.
> > 
> > Something I've also been pondering which is related to this is linking
> > together DMA operations using the LLI chaining when interrupts and
> > callbacks aren't required.  It's a very similar problem.  Other DMA
> > engine drivers do this.
> 
> Are you talking about linking all sg's together or linking multiple calls to
> prep_dma_memcpy? I am handling the first.

I'm talking about letting the hardware process as many txds as possible
without calling back into the driver to have it setup the next txd.

> > Finally, note that some of the PL08x code assumes that for any TXD,
> > the LLI list is a contiguous array.  See the first part of
> > pl08x_getbytes_chan(). That needs fixing as soon as we start going to
> > more than one SG list entry.
> 
> We still have one contiguous array for LLI list. In pl08x_fill_llis_for_desc()
> i am using same txd->llis_va for all sg's.

That's ok then.
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