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Date:   Mon, 12 Oct 2020 10:40:35 -0700
From:   mka@chromium.org
To:     Akhil P Oommen <akhilpo@codeaurora.org>
Cc:     freedreno@lists.freedesktop.org, robh@kernel.org,
        robdclark@gmail.com, linux-arm-msm@vger.kernel.org,
        dri-devel@freedesktop.org, linux-kernel@vger.kernel.org
Subject: Re: [2/2] drm/msm: Add support for GPU cooling
Message-ID: <20201012174035.GA44627@google.com>
References: <1602176947-17385-2-git-send-email-akhilpo@codeaurora.org>
 <20201009183640.GB1292413@google.com>
 <cab2105e-7a8c-988f-dcc1-056692a94e8b@codeaurora.org>
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In-Reply-To: <cab2105e-7a8c-988f-dcc1-056692a94e8b@codeaurora.org>
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On Mon, Oct 12, 2020 at 07:03:51PM +0530, Akhil P Oommen wrote:
> On 10/10/2020 12:06 AM, mka@chromium.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@codeaurora.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);
> > > -
> > >   	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.