linux-kernel - Re: [PATCH 06/13] DMAENGINE: driver for the ARM PL080/PL081 PrimeCells

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Message-ID: <20101222122240.GB14693@n2100.arm.linux.org.uk>
Date:	Wed, 22 Dec 2010 12:22:40 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Linus Walleij <linus.walleij@...ricsson.com>
Cc:	Viresh Kumar <viresh.kumar@...com>,
	Kukjin Kim <kgene.kim@...sung.com>, yuanyabin1978@...a.com,
	linux-kernel@...r.kernel.org, Ben Dooks <ben-linux@...ff.org>,
	Peter Pearse <peter.pearse@....com>,
	Dan Williams <dan.j.williams@...el.com>,
	linux-arm-kernel@...ts.infradead.org,
	Alessandro Rubini <rubini@...pv.it>
Subject: Re: [PATCH 06/13] DMAENGINE: driver for the ARM PL080/PL081
	PrimeCells

On Tue, Dec 21, 2010 at 06:20:37PM +0000, Russell King - ARM Linux wrote:
> Having just looked at this while trying to undo the DMA API abuses
> in the PL011 UART driver, I'm getting rather frustrated with this
> code.

Right, just tried this on the Versatile PB/926, which has a PL080.
The result is DMA errors.  This turns out to be the hard-coding of
which AHB bus is used.

You can't hard-code this information into the driver - it's part of
the bus matrix configuration.  On Versatile PB/926, the two AHB
buses have different memory maps - see DUI0224 page 3-13:

* DMA0 (which is DMA AHB M1) has access to the APB peripherals
  but not the system memory.
* DMA1 (which is DMA AHB M2) has access to the system memory but
  none of the APB peripherals.

Throwing in #ifdef's to sort this out resolves the problem on the PB/926.

Looking at the driver code too, I'm having a hard time understanding it,
probably because of the use of "master" and "slave" - I don't think this
has anything to do with which AHB master is used.  The comments against
pl08x_choose_master_bus() seem to imply that it controls which AHB master
is used, but it has no apparant effect on that.  I don't think this
function does anything like that anymore.
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