linux-kernel - Re: [RFC PATCH] dma: Add barrierless dma mapping/unmapping api

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Message-Id: <20100126134207T.fujita.tomonori@lab.ntt.co.jp>
Date:	Tue, 26 Jan 2010 13:43:01 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	benh@...nel.crashing.org
Cc:	adharmap@...eaurora.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, dwalker@...eaurora.org,
	arnd@...db.de, fujita.tomonori@....ntt.co.jp,
	akpm@...ux-foundation.org, mingo@...e.hu, joerg.roedel@....com,
	maciej.sosnowski@...el.com, dan.j.williams@...el.com,
	beckyb@...nel.crashing.org, yanghy@...fujitsu.com,
	linux-arch@...r.kernel.org, adharmap@...cinc.com
Subject: Re: [RFC PATCH] dma: Add barrierless dma mapping/unmapping api

On Tue, 26 Jan 2010 15:30:10 +1100
Benjamin Herrenschmidt <benh@...nel.crashing.org> wrote:

> On Mon, 2010-01-25 at 18:35 -0800, adharmap@...eaurora.org wrote:
> > From: Abhijeet Dharmapurikar <adharmap@...cinc.com>
> > 
> > Tests have shown that there is a significant performance gain
> > when a driver uses mapping/unmapping functions without a barrier
> > multiple times and calls dsb() only after the last buffer. This
> > api adds support for barrierless dma operations on ARMv6
> > and ARMv7.
> 
> Gotta love when each architecture has subtely different barriers
> semantics ...
> 
> This is ARM stuff at this stage and as such not a huge deal to me but I
> don't like seeing new arch specific API extending generic ones for
> drivers, it's just going to make it harder when those drivers start
> using them and those devices are use on other archs.

Completely agreed. Adding architecture specific DMA API is a really
bad idea. We need to keep the DMA API that works for all the
architectures.


> There are people nowadays putting AXI bridges and the whole ARM
> paraphernalia of IP cores behind them on PowerPC cores for example and I
> can see that happening with x86 as well.
> 
> In your case, I believe your are fixing the wrong problem anyways. The
> right approach would be instead to put all your buffer into an sglist
> and use dma_map/unmap_sg().

Agreed again. That's exactly what I suggested before:

http://marc.info/?l=linux-kernel&m=126294076917753&w=2
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