linux-kernel - Re: [PATCH] [RFC] gpu: host1x: shut up warning about DMA API misuse

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Message-ID: <20170420094932.GM17774@n2100.armlinux.org.uk>
Date:   Thu, 20 Apr 2017 10:49:32 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Mikko Perttunen <cyndis@...si.fi>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Mikko Perttunen <mperttunen@...dia.com>,
        Thierry Reding <thierry.reding@...il.com>,
        linux-tegra@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] [RFC] gpu: host1x: shut up warning about DMA API misuse

On Thu, Apr 20, 2017 at 10:25:01AM +0200, Arnd Bergmann wrote:
> On Thu, Apr 20, 2017 at 9:02 AM, Mikko Perttunen <cyndis@...si.fi> wrote:
> > I think we have a "policy" on Tegra that the DMA API will never allocate
> > using the IOMMU (Thierry can elaborate on this), which is why I wrote the
> > code with that assumption. Essentially, we have made the DMA API into the
> > API that allocates CPU-visible memory.
> 
> I don't think this can be a per-platform policy.
> 
> > Considering that, I'm wondering if we can just have a temporary local
> > dma_addr_t and then cast that to phys_addr_t, combined with a good comment?
> 
> That was my first approach, and it does address the warning, but
> I did not send it because it still felt too wrong.

Sounds to me like the warning is justified - it's saying that there's
something not right here which could be a problem.  So I'd say, don't
fix the warning, it's doing its job, highlighting a potential problem
with the code.

(Consider hiding the warning and then running on a platform where the
assumptions are broken.)

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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