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

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Date:   Thu, 20 Apr 2017 12:02:42 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Mikko Perttunen <cyndis@...si.fi>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Mikko Perttunen <mperttunen@...dia.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-tegra@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [RFC] gpu: host1x: shut up warning about DMA API misuse

On Thu, Apr 20, 2017 at 11:44 AM, Mikko Perttunen <cyndis@...si.fi> wrote:
> On 20.04.2017 11:25, Arnd Bergmann wrote:
>> On Thu, Apr 20, 2017 at 9:02 AM, Mikko Perttunen <cyndis@...si.fi> wrote:
>>> On 19.04.2017 21:24, Arnd Bergmann wrote:
>>
>> I don't think this can be a per-platform policy.
>
>
> Yeah, now that we are using the ARM SMMU on T186 onwards it's more difficult
> than when we were using the Tegra SMMU, so we'll probably have to change
> that.
>
> The goal of the current code is to allocate memory from the CMA, so one
> option would be to change it to use dma_alloc_from_contiguous. That way we
> wouldn't get the double IOMMU mapping, which would be nice.

Right, also we shouldn't define what a particular API does based on
which platform you run on.

> The goal of the current code is to allocate memory from the CMA, so one
> option would be to change it to use dma_alloc_from_contiguous. That way
> we wouldn't get the double IOMMU mapping, which would be nice.

dma_alloc_from_contiguous() is intentionally not exported to drivers,
and it would result in a page that is not mapped into your kernel
address space.

This is probably not the only driver that has this issue, so maybe a
better approach would be to fill the API gap and introduce an IOMMU
API function that takes a domain/iova/length tuple as its argument
and allocates coherent or WC memory that it maps into that address?

      Arnd