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Subject: Re: [2/2] drm/msm: Add support for GPU cooling
To:     mka@chromium.org
Cc:     linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org,
        dri-devel@freedesktop.org, freedreno@lists.freedesktop.org
References: <1602176947-17385-2-git-send-email-akhilpo@codeaurora.org>
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 <20201012174035.GA44627@google.com>
From:   Akhil P Oommen <akhilpo@codeaurora.org>
Message-ID: <80ded484-a058-70fc-be9d-045be2933563@codeaurora.org>
Date:   Tue, 13 Oct 2020 19:23:34 +0530
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On 10/12/2020 11:10 PM, mka@chromium.org wrote:
> 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);
>>>> -
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?

-Akhil.
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> 


-Akhil.