linux-kernel - Re: [PATCH 2/2] drm/tegra: Set the DMA mask

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Message-ID: <20160223160440.GE27656@ulmo>
Date:	Tue, 23 Feb 2016 17:04:40 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Alexandre Courbot <acourbot@...dia.com>
Cc:	Terje Bergström <tbergstrom@...dia.com>,
	Stephen Warren <swarren@...dotorg.org>,
	dri-devel@...ts.freedesktop.org, linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org, gnurou@...il.com
Subject: Re: [PATCH 2/2] drm/tegra: Set the DMA mask

On Tue, Feb 23, 2016 at 03:25:54PM +0900, Alexandre Courbot wrote:
> The default DMA mask covers a 32 bits address range, but tegradrm can
> address more than that. Set the DMA mask to the actual addressable range
> to avoid the use of unneeded bounce buffers.
> 
> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
> ---
> Thierry, I am not absolutely sure whether the size is correct and applies
> to all Tegra generations - please let me know if this needs to be
> reworked.
> 
>  drivers/gpu/drm/tegra/drm.c | 1 +
>  1 file changed, 1 insertion(+)

This kind of depends on whether or not the device is behind an IOMMU. If
it is, then the IOMMU DMA MASK would apply, which can be derived from
the number of address bits that the IOMMU can handle. The SMMU supports
32 address bits on Tegra30 and Tegra114, 34 address bits on more recent
generations.

I think for now it's safer to leave the DMA mask at the default (32 bit)
to avoid the need to distinguish between IOMMU and non-IOMMU devices.

Thierry

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