linux-kernel - Re: [PATCH 10/18] dmaengine/amba-pl08x: Get rid of pl08x_pre

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Message-ID: <20110729110525.GG25640@n2100.arm.linux.org.uk>
Date:	Fri, 29 Jul 2011 12:05:25 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Viresh Kumar <viresh.kumar@...com>
Cc:	linus.walleij@...aro.org, pratyush.anand@...com,
	rajeev-dlh.kumar@...com, bhupesh.sharma@...com,
	shiraz.hashim@...com, vinod.koul@...el.com,
	linux-kernel@...r.kernel.org, vipin.kumar@...com,
	armando.visconti@...com, amit.virdi@...com,
	vipulkumar.samar@...com, viresh.linux@...il.com,
	deepak.sikri@...com, dan.j.williams@...el.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 10/18] dmaengine/amba-pl08x: Get rid of
	pl08x_pre_boundary()

On Fri, Jul 29, 2011 at 04:19:20PM +0530, Viresh Kumar wrote:
> Pl080 Manual says: "Bursts do not cross the 1KB address boundary"
> 
> We can program the controller to cross 1 KB boundary on a burst and controller
> can take care of this boundary condition by itself.
> 
> Following is the discussion with ARM Technical Support Guys (David):
> [Viresh] Manual says: "Bursts do not cross the 1KB address boundary"
> 
> What does that actually mean? As, Maximum size transferable with a single LLI is
> 4095 * 4 =16380 ~ 16KB. So, if we don't have src/dest address aligned to burst
> size, we can't use this big of an LLI.
> 
> [David] There is a difference between bursts describing the total data
> transferred by the DMA controller and AHB bursts. Bursts described by the
> programmable parameters in the PL080 have no direct connection with the bursts
> that are seen on the AHB bus.
> 
> The statement that "Bursts do not cross the 1KB address boundary" in the TRM is
> referring to AHB bursts, where this limitation is a requirement of the AHB spec.
> You can still issue bursts within the PL080 that are in excess o f 1KB. The
> PL080 will make sure that its bursts are broken down into legal AHB bursts which
> will be formatted to ensure that no AHB burst crosses a 1KB boundary.
> 
> Based on above discussion, this patch removes all code related to 1 KB boundary
> as we are not required to handle this in driver.

Good.  I'm glad someone's finally getting to the bottom of what's
actually required and what isn't...  This driver has been a mess for
quite a long time and is in desperate need of these kinds of cleanups.
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