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

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Date:	Fri, 29 Jul 2011 18:02:57 +0530
From:	"Koul, Vinod" <vinod.koul@...el.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Viresh Kumar <viresh.kumar@...com>, linus.walleij@...aro.org,
	pratyush.anand@...com, rajeev-dlh.kumar@...com,
	bhupesh.sharma@...com, shiraz.hashim@...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, 2011-07-29 at 12:05 +0100, Russell King - ARM Linux wrote:
> 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.
Typically in few dmac IPs i know of:
There is max limitation of what a burst length can be for dma, and what
a max transfer length can be.
The dma will push/pull data into peripheral FIFO based on
max_burst_length parameter (note length in dma terminology is always
items not bytes, and one item is usually defined as
transfer_length/src_width). For a transfer (single block or in
multi-block) the dmac will break the transaction into multiple bursts
and based on max_burst_length it will push/pull data to/from FIFO. Also
matching FIFO thresholds with max_burst_length is also important and can
be tuned based on perf/power requirements. 

Sharing my knowledge, hopefully it helps :)

-- 
~Vinod

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