linux-kernel - Re: using DMA-API on ARM

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Date:	Fri, 5 Dec 2014 18:53:03 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Arend van Spriel <arend@...adcom.com>
Cc:	Russell King <linux@....linux.org.uk>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	"brcm80211-dev-list@...adcom.com" <brcm80211-dev-list@...adcom.com>,
	David Miller <davem@...emloft.net>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: using DMA-API on ARM

On Fri, Dec 05, 2014 at 06:39:45PM +0000, Catalin Marinas wrote:
> On Fri, Dec 05, 2014 at 09:22:22AM +0000, Arend van Spriel wrote:
> > For our brcm80211 development we are working on getting brcmfmac driver
> > up and running on a Broadcom ARM-based platform. The wireless device is
> > a PCIe device, which is hooked up to the system behind a PCIe host
> > bridge, and we transfer information between host and device using a
> > descriptor ring buffer allocated using dma_alloc_coherent(). We mostly
> > tested on x86 and seen no issue. However, on this ARM platform
> > (single-core A9) we detect occasionally that the descriptor content is
> > invalid. When this occurs we do a dma_sync_single_for_cpu() and this is
> > retried a number of times if the problem persists. Actually, found out
> > that someone made a mistake by using virt_to_dma(va) to get the
> > dma_handle parameter. So probably we only provided a delay in the retry
> > loop. After fixing that a single call to dma_sync_single_for_cpu() is
> > sufficient. The DMA-API-HOWTO clearly states that:
> 
> Does your system have an L2 cache? What's the SoC topology, can PCIe see
> such L2 cache (or snoop the L1 caches)?

BTW, if you really have a PL310-like L2 cache, have a look at some
patches (I've seen similar symptoms) and make sure your configuration is
correct:

http://www.arm.linux.org.uk/developer/patches/viewpatch.php?id=6395/1

http://www.arm.linux.org.uk/developer/patches/viewpatch.php?id=6529/1

The first one is vexpress specific. The second one was eventually
discarded by Russell (I don't remember the reason, I guess it's because
SoC code is supposed to set the right bits in there anyway). In your
case, such bits may be set up by firmware, so Linux cannot fix anything
up.

-- 
Catalin
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