linux-kernel - Re: [PATCH 1/2] drm/msm: gpu: don't abuse dma

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Date:   Wed, 28 Jun 2017 22:06:57 -0700
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Rob Clark <robdclark@...il.com>,
        Jordan Crouse <jcrouse@...eaurora.org>,
        David Airlie <airlied@...ux.ie>, Eric Anholt <eric@...olt.net>,
        Sushmita Susheelendra <ssusheel@...eaurora.org>,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] drm/msm: gpu: don't abuse dma_alloc for non-DMA
 allocations

On Tue 20 Jun 13:16 PDT 2017, Arnd Bergmann wrote:

> In zap_shader_load_mdt(), we pass a pointer to a phys_addr_t
> into dmam_alloc_coherent, which the compiler warns about:
> 
> drivers/gpu/drm/msm/adreno/a5xx_gpu.c: In function 'zap_shader_load_mdt':
> drivers/gpu/drm/msm/adreno/a5xx_gpu.c:54:50: error: passing argument 3 of 'dmam_alloc_coherent' from incompatible pointer type [-Werror=incompatible-pointer-types]
> 
> The returned DMA address is later passed on to a function that
> takes a phys_addr_t, so it's clearly wrong to use the DMA
> mapping interface here: the memory may be uncached, or the
> address may be completely wrong if there is an IOMMU connected
> to the device.
> 
> My interpretation is that using dmam_alloc_coherent() had two
> purposes:
> 
>  a) get a chunk of consecutive memory that may be larger than
>     the limit for kmalloc()
> 
>  b) use the devres infrastructure to simplify the unwinding
>     in the error case.
> 
> I think ideally we'd use a devres-based version of
> alloc_pages_exact() here, but since that doesn't exist,
> let's use devm_get_free_pages() instead. This wastes a little
> memory as the size gets rounded up to a power of two, but
> is otherwise harmless. If we want to save memory here, calling
> devm_free_pages() to release the memory once it is no longer
> needed is probably better anyway.
> 
> Fixes: 7c65817e6d38 ("drm/msm: gpu: Enable zap shader for A5XX")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Acked-by: Bjorn Andersson <bjorn.andersson@...aro.org>

Regards,
Bjorn

> ---
>  drivers/gpu/drm/msm/adreno/a5xx_gpu.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/msm/adreno/a5xx_gpu.c b/drivers/gpu/drm/msm/adreno/a5xx_gpu.c
> index b4b54f1c24bc..eee9ac81aaa1 100644
> --- a/drivers/gpu/drm/msm/adreno/a5xx_gpu.c
> +++ b/drivers/gpu/drm/msm/adreno/a5xx_gpu.c
> @@ -51,11 +51,13 @@ static int zap_shader_load_mdt(struct device *dev, const char *fwname)
>  	}
>  
>  	/* Allocate memory for the firmware image */
> -	mem_region = dmam_alloc_coherent(dev, mem_size, &mem_phys, GFP_KERNEL);
> +	mem_region = (void *)devm_get_free_pages(dev, GFP_KERNEL,
> +			get_order(mem_size));
>  	if (!mem_region) {
>  		ret = -ENOMEM;
>  		goto out;
>  	}
> +	mem_phys = virt_to_phys(mem_region);
>  
>  	/* Load the rest of the MDT */
>  	ret = qcom_mdt_load(dev, fw, fwname, GPU_PAS_ID, mem_region, mem_phys,
> -- 
> 2.9.0
>