linux-kernel - Re: [PATCH 2/3] define ARM-specific dma_coherent_write

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Message-Id: <C8392961-1EBF-410F-9214-05C060FD6D24@jonmasters.org>
Date:	Mon, 3 Oct 2011 05:24:27 -0400
From:	Jon Masters <jonathan@...masters.org>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Mark Salter <msalter@...hat.com>,
	"ming.lei@...onical.com" <ming.lei@...onical.com>,
	"stern@...land.harvard.edu" <stern@...land.harvard.edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/3] define ARM-specific dma_coherent_write_sync


On Oct 3, 2011, at 4:44 AM, Catalin Marinas wrote:

> On Mon, Oct 03, 2011 at 02:40:19AM +0100, Jon Masters wrote:
>> On Sep 6, 2011, at 11:02 AM, Mark Salter wrote:
>>> In any case, the current thinking is that the original problem with
>>> the USB performance seen on cortex A9 multicore is probably something
>>> more than just write buffer delays. Once the original problem is better
>>> understood, we can take another look at this patch if it is still
>>> needed.
>> 
>> Thanks again for looking into this Mark. My understanding is that this
>> is still being investigated. I'll followup with ARM to see how that's
>> going since I've heard nothing recently :) Meanwhile, we're continuing
>> to carry a hack based on these patches in Fedora ARM kernels.
> 
> Not talking about hardware specifics here, the architecture (though
> ARMv7 onwards) mandates that the write buffer is eventually drained. But
> doesn't state any upper limit, so it could even be half a second.

<snip mechanics of dma_coherent_write_sync, etc.>

I guess my main question is, do you think this is just a write buffer delay? If you do, then we should definitely get back to this question of defining DMA extensions. But are we sure that's what this is?

Mark: did you have any more insight into this recently?

Jon.

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