Date: Wed, 22 Dec 2010 12:22:40 +0000
From: Russell King - ARM Linux <linux@arm.linux.org.uk>
To: Linus Walleij <linus.walleij@stericsson.com>
Cc: Viresh Kumar <viresh.kumar@st.com>, Kukjin Kim <kgene.kim@samsung.com>,
        yuanyabin1978@sina.com, linux-kernel@vger.kernel.org,
        Ben Dooks <ben-linux@fluff.org>, Peter Pearse <peter.pearse@arm.com>,
        Dan Williams <dan.j.williams@intel.com>,
        linux-arm-kernel@lists.infradead.org,
        Alessandro Rubini <rubini@unipv.it>
Subject: Re: [PATCH 06/13] DMAENGINE: driver for the ARM PL080/PL081
	PrimeCells
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References: <1276270031-1607-1-git-send-email-linus.walleij@stericsson.com> <20101221182037.GA4783@n2100.arm.linux.org.uk>
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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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