linux-kernel - Re: [Freedreno] [PATCH v7 6/6] drm/msm: iommu: Replace runtime calls with runtime suppliers

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Date:   Wed, 14 Feb 2018 11:12:23 -0500
From:   Rob Clark <robdclark@...il.com>
To:     Tomasz Figa <tfiga@...omium.org>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org, linux-pm@...r.kernel.org,
        David Airlie <airlied@...ux.ie>,
        Will Deacon <will.deacon@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <iommu@...ts.linux-foundation.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Robin Murphy <robin.murphy@....com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>
Subject: Re: [Freedreno] [PATCH v7 6/6] drm/msm: iommu: Replace runtime calls
 with runtime suppliers

On Wed, Feb 14, 2018 at 10:48 AM, Jordan Crouse <jcrouse@...eaurora.org> wrote:
> On Wed, Feb 14, 2018 at 12:31:29PM +0900, Tomasz Figa wrote:
>>
>> - When submitting commands to the GPU, the GPU driver will
>> pm_runtime_get_sync() on the GPU device, which will automatically do
>> the same on all the linked suppliers, which would also include the
>> SMMU itself. The role of device links here is exactly that the GPU
>> driver doesn't have to care which other devices need to be brought up.
>
> This is true.  Assuming that the device link works correctly we would not need
> to explicitly power the SMMU which makes my point entirely moot.

Just to point out what motivated this patchset, the biggest problem is
iommu_unmap() because that can happen when GPU is not powered on (or
in the v4l2 case, because some other device dropped it's reference to
the dma-buf allowing it to be free'd).  Currently we pm get/put the
GPU device around unmap, but it is kinda silly to boot up the GPU just
to unmap a buffer.

(Semi-related, I would also like to batch map/unmap's, I just haven't
gotten around to implementing it yet.. but that would be another case
where a single get_supplier()/put_supplier() outside of the iommu
would make sense instead of pm_get/put() inside the iommu driver's
->unmap().)

If you really dislike the get/put_supplier() approach, then perhaps we
need iommu_pm_get()/iommu_pm_put() operations that the iommu user
could use to accomplish the same thing?

BR,
-R