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

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Message-ID: <CAK8P3a3JJrA_4Zur47ngWmR7VeKBopJLNZCaqOPzzmnFoV3=Ew@mail.gmail.com>
Date:   Thu, 20 Apr 2017 10:25:01 +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 9:02 AM, Mikko Perttunen <cyndis@...si.fi> wrote:
> On 19.04.2017 21:24, Arnd Bergmann wrote:
>>
>> When dma_addr_t and phys_addr_t are not the same size, we get a warning
>> from the dma_alloc_wc function:
>>
>> drivers/gpu/host1x/cdma.c: In function 'host1x_pushbuffer_init':
>> drivers/gpu/host1x/cdma.c:94:48: error: passing argument 3 of
>> 'dma_alloc_wc' from incompatible pointer type
>> [-Werror=incompatible-pointer-types]
>>    pb->mapped = dma_alloc_wc(host1x->dev, size, &pb->phys,
>>                                                 ^
>> In file included from drivers/gpu/host1x/cdma.c:22:0:
>> include/linux/dma-mapping.h:761:37: note: expected 'dma_addr_t * {aka long
>> long unsigned int *}' but argument is of type 'phys_addr_t * {aka unsigned
>> int *}'
>>  static noinline_if_stackbloat void *dma_alloc_wc(struct device *dev,
>> size_t size,
>>                                      ^~~~~~~~~~~~
>> drivers/gpu/host1x/cdma.c:113:48: error: passing argument 3 of
>> 'dma_alloc_wc' from incompatible pointer type
>> [-Werror=incompatible-pointer-types]
>>    pb->mapped = dma_alloc_wc(host1x->dev, size, &pb->phys,
>>                                                 ^
>> In file included from drivers/gpu/host1x/cdma.c:22:0:
>> include/linux/dma-mapping.h:761:37: note: expected 'dma_addr_t * {aka long
>> long unsigned int *}' but argument is of type 'phys_addr_t * {aka unsigned
>> int *}'
>>  static noinline_if_stackbloat void *dma_alloc_wc(struct device *dev,
>> size_t size,
>>                                      ^~~~~~~~~~~~
>>
>> The problem here is that dma_alloc_wc() returns a pointer to a dma_addr_t
>> that may already be translated by an IOMMU, but the driver passes this
>> into iommu_map() as a physical address. This works by accident only when
>> the IOMMU does not get registered with the DMA API and there is a 1:1
>> mapping between physical addresses as seen by the CPU and the device.
>>
>> The fundamental problem here is the lack of a generic API to do what the
>> driver wants: allocating CPU-visible memory for use by a device through
>> user-defined IOMMU settings. Neither the dma-mapping API nor the IOMMU
>> API can do this by itself, and combining the two is not well-defined.
>>
>> This patch addresses the type mismatch by adding a third pointer into the
>> push_buffer structure: in addition to the address as seen from the CPU
>> and the address inside of the local IOMMU domain, the pb->alloc pointer
>> is the token returned by dma_alloc_wc(), and this is what we later need
>> to pass into dma_free_wc().
>>
>> The address we pass into iommu_map() however is the physical address
>> computed from virt_to_phys(), assuming that the pointer we have here
>> is part of the linear mapping (which is also questionable, e.g. when we
>> have a non-coherent device on ARM32 this may be false). Also, when
>> the DMA API uses the IOMMU to allocate the pointer for the default
>> domain, we end up with two IOMMU mappings for the same physical address.
>>
>
> 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.

     Arnd