linux-kernel - Re: [PATCH 2/2] iommu/tegra: gart: Reserve iomem resource

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Date:	Mon, 21 May 2012 19:31:24 +0200
From:	Thierry Reding <thierry.reding@...onic-design.de>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	Lucas Stach <dev@...xeye.de>, Hiroshi DOYU <hdoyu@...dia.com>,
	linux-tegra@...r.kernel.org, Joerg Roedel <joerg.roedel@....com>,
	Bharat Nihalani <bnihalani@...dia.com>,
	Vandana Salve <vsalve@...dia.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] iommu/tegra: gart: Reserve iomem resource

* Stephen Warren wrote:
> On 05/21/2012 11:05 AM, Lucas Stach wrote:
> > Am Montag, den 21.05.2012, 10:41 -0600 schrieb Stephen Warren:
> >> On 05/21/2012 08:11 AM, Thierry Reding wrote:
> >>> * Hiroshi DOYU wrote:
> >>>> GART Register/Aperture range should be reserved.
> >> ...
> >>>> diff --git a/drivers/iommu/tegra-gart.c
> >>>> b/drivers/iommu/tegra-gart.c
> >> ...
> >>>> +	if (!devm_request_mem_region(dev, res_remap->start, +
> >>>> resource_size(res_remap), dev_name(dev))) { +		dev_err(dev,
> >>>> "failed to reserve GART aperture\n"); +		return -EBUSY; +	} +
> >>>
> >>> I'm not so sure that this is a good idea. If the GART driver
> >>> requests the GART aperture, then users of the GART aperture (e.g.
> >>> the DRM driver) will no longer be able to use it.
> >>
> >> Presumably the DRM driver would be obtaining access to parts of the
> >> GART aperture through the GART driver, rather than just grabbing the
> >> IO region and using it for itself?
> > 
> > With a TTM-like memory manager the DRM driver will take over the address
> > space management of the GART aperture. The GART driver is only used to
> > set up the mappings in the IOMMU. The DRM driver is the one that sets
> > up/tears down the CPU mappings on demand, so the aperture should be
> > claimed in the DRM driver.
> 
> Even if the DRM driver is managing the aperture, I still think the GART
> driver should claim it. The physical address of the aperture is a
> property of the GART, so only the GART's resources (and DT node) should
> contain a representation of where that aperture is. The DRM driver
> should call into the GART driver to find out where the aperture is, and
> hence GART is able to claim it without conflict.

That could be difficult to do. There isn't any API to allow this. I guess if
something like that is added it should probably go into the IOMMU framework.

Thierry

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