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

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Date:	Mon, 21 May 2012 19:38:28 +0200
From:	Lucas Stach <dev@...xeye.de>
To:	Thierry Reding <thierry.reding@...onic-design.de>
Cc:	Stephen Warren <swarren@...dotorg.org>,
	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

Am Montag, den 21.05.2012, 19:31 +0200 schrieb Thierry Reding:
> * 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.

Ah, yes. There already was a plan to add properties for this to the
IOMMU API. So I'm ok with the GART driver claiming the resources. I will
try to push the needed API changes forward in the next few days.

-- Lucas


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