linux-kernel - Re: [PATCH v1 1/3] gpu: host1x: Remove implicit IOMMU backing on client's registration

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Date:   Thu, 24 Oct 2019 16:35:13 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/3] gpu: host1x: Remove implicit IOMMU backing on
 client's registration

24.10.2019 14:50, Thierry Reding пишет:
> On Sun, Jun 23, 2019 at 08:37:41PM +0300, Dmitry Osipenko wrote:
>> On ARM32 we don't want any of the clients device to be backed by the
>> implicit domain, simply because we can't afford such a waste on older
>> Tegra SoCs that have very few domains available in total. The recent IOMMU
>> support addition for the Video Decoder hardware uncovered the problem
>> that an unfortunate drivers probe order results in the DRM driver probe
>> failure if CONFIG_ARM_DMA_USE_IOMMU=y due to a shortage of IOMMU domains
>> caused by the implicit backing. The host1x_client_register() is a common
>> function that is invoked by all of the relevant DRM drivers during theirs
>> probe and hence it is convenient to remove the implicit backing there,
>> resolving the problem.
>>
>> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
>> ---
>>  drivers/gpu/host1x/bus.c | 19 +++++++++++++++++++
>>  1 file changed, 19 insertions(+)
> 
> I don't really want to do this in a central place like this. If we
> really do need this, why can't we do it in the individual drivers?

Why do you want to duplicate the same action for each driver instead of
doing it in a single common place?

> we already call host1x_client_iommu_attach() from all the drivers and
> that detaches from the IOMMU as well. So I'm not sure I understand why
> this is needed.

Wish you could ask that couple months ago. I'll try to refresh the details.