linux-kernel - Re: [2/2] drm/msm: Add support for GPU cooling

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Message-ID: <20201013174038.GA424420@google.com>
Date:   Tue, 13 Oct 2020 10:40:38 -0700
From:   mka@...omium.org
To:     Akhil P Oommen <akhilpo@...eaurora.org>
Cc:     linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...edesktop.org, freedreno@...ts.freedesktop.org
Subject: Re: [2/2] drm/msm: Add support for GPU cooling

On Tue, Oct 13, 2020 at 07:23:34PM +0530, Akhil P Oommen wrote:
> On 10/12/2020 11:10 PM, mka@...omium.org wrote:
> > On Mon, Oct 12, 2020 at 07:03:51PM +0530, Akhil P Oommen wrote:
> > > On 10/10/2020 12:06 AM, mka@...omium.org wrote:
> > > > Hi Akhil,
> > > > 
> > > > On Thu, Oct 08, 2020 at 10:39:07PM +0530, Akhil P Oommen wrote:
> > > > > Register GPU as a devfreq cooling device so that it can be passively
> > > > > cooled by the thermal framework.
> > > > > 
> > > > > Signed-off-by: Akhil P Oommen <akhilpo@...eaurora.org>
> > > > > ---
> > > > >    drivers/gpu/drm/msm/msm_gpu.c | 13 ++++++++++++-
> > > > >    drivers/gpu/drm/msm/msm_gpu.h |  2 ++
> > > > >    2 files changed, 14 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/drivers/gpu/drm/msm/msm_gpu.c b/drivers/gpu/drm/msm/msm_gpu.c
> > > > > index 55d1648..93ffd66 100644
> > > > > --- a/drivers/gpu/drm/msm/msm_gpu.c
> > > > > +++ b/drivers/gpu/drm/msm/msm_gpu.c
> > > > > @@ -14,6 +14,7 @@
> > > > >    #include <generated/utsrelease.h>
> > > > >    #include <linux/string_helpers.h>
> > > > >    #include <linux/devfreq.h>
> > > > > +#include <linux/devfreq_cooling.h>
> > > > >    #include <linux/devcoredump.h>
> > > > >    #include <linux/sched/task.h>
> > > > > @@ -107,9 +108,18 @@ static void msm_devfreq_init(struct msm_gpu *gpu)
> > > > >    	if (IS_ERR(gpu->devfreq.devfreq)) {
> > > > >    		DRM_DEV_ERROR(&gpu->pdev->dev, "Couldn't initialize GPU devfreq\n");
> > > > >    		gpu->devfreq.devfreq = NULL;
> > > > > +		return;
> > > > >    	}
> > > > >    	devfreq_suspend_device(gpu->devfreq.devfreq);
> > > > > +
> > > > > +	gpu->cooling = of_devfreq_cooling_register(gpu->pdev->dev.of_node,
> > > > > +			gpu->devfreq.devfreq);
> > > > > +	if (IS_ERR(gpu->cooling)) {
> > > > > +		DRM_DEV_ERROR(&gpu->pdev->dev,
> > > > > +				"Couldn't register GPU cooling device\n");
> > > > > +		gpu->cooling = NULL;
> > > > > +	}
> > > > >    }
> > > > >    static int enable_pwrrail(struct msm_gpu *gpu)
> > > > > @@ -926,7 +936,6 @@ int msm_gpu_init(struct drm_device *drm, struct platform_device *pdev,
> > > > >    	msm_devfreq_init(gpu);
> > > > > -
> Will remove this unintended change.
> > > > >    	gpu->aspace = gpu->funcs->create_address_space(gpu, pdev);
> > > > >    	if (gpu->aspace == NULL)
> > > > > @@ -1005,4 +1014,6 @@ void msm_gpu_cleanup(struct msm_gpu *gpu)
> > > > >    		gpu->aspace->mmu->funcs->detach(gpu->aspace->mmu);
> > > > >    		msm_gem_address_space_put(gpu->aspace);
> > > > >    	}
> > > > > +
> > > > > +	devfreq_cooling_unregister(gpu->cooling);
> > > > 
> > > > Resources should be released in reverse order, otherwise the cooling device
> > > > could use resources that have already been freed.
> > > > Why do you think this is not the correct order? If you are thinking
> > > about devfreq struct, it is managed device resource.
> > 
> > I did not check specifically if changing the frequency really uses any of the
> > resources that are released previously, In any case it's not a good idea to
> > allow other parts of the kernel to use a half initialized/torn down device.
> > Even if it isn't a problem today someone could change the driver to use any
> > of these resources (or add a new one) in a frequency change, without even
> > thinking about the cooling device, just (rightfully) asuming that things are
> > set up and torn down in a sane order.
> 'sane order' relative to what specifically here? Should we worry about freq
> change at this point because we have already disabled gpu runtime pm and
> devfreq?

GPU runtime PM and the devfreq being disabled is not evident from the context
of the function. You are probably right that it's not a problem in practice,
but why give reason for doubts in the first place if this could be avoided
by following a common practice?