Re: [PATCH] drm/amdgpu: Grab/put runtime PM references in atomic_commit_tail()

[Harry Wentland <harry.wentland@amd.com>]

On 2018-06-04 03:35 PM, Lyude Paul wrote:
> So, unfortunately I recently made the discovery that in the upstream
> kernel, the only reason that amdgpu is not currently suffering from
> issues with runtime PM putting the GPU into suspend while it's driving
> displays is due to the fact that on most prime systems, we have sound
> devices associated with the GPU that hold their own runtime PM ref for
> the GPU.
> 
> What this means however, is that in the event that there isn't any kind
> of sound device active (which can easily be reproduced by building a
> kernel with sound drivers disabled), the GPU will fall asleep even when
> there's displays active. This appears to be in part due to the fact that
> amdgpu has not actually ever relied on it's rpm_idle() function to be
> the only thing keeping it running, and normally grabs it's own power
> references whenever there are displays active (as can be seen with the
> original pre-DC codepath in amdgpu_display_crtc_set_config() in
> amdgpu_display.c). This means it's very likely that this bug was
> introduced during the switch over the DC.
> 
> So to fix this, we start grabbing runtime PM references every time we
> enable a previously disabled CRTC in atomic_commit_tail(). This appears
> to be the correct solution, as it matches up with what i915 does in
> i915/intel_runtime_pm.c.
> 
> The one sideaffect of this is that we ignore the variable that the
> pre-DC code used to use for tracking when it needed runtime PM refs,
> adev->have_disp_power_ref. This is mainly because there's no way for a
> driver to tell whether or not all of it's CRTCs are enabled or disabled
> when we've begun committing an atomic state, as there may be CRTC
> commits happening in parallel that aren't contained within the atomic
> state being committed. So, it's safer to just get/put a reference for
> each CRTC being enabled or disabled in the new atomic state.
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>


I'm not familiar with the runtime_pm stuff, as is painfully obvious from the fact that we missed that with the DC driver. That said, from a cursory look at runtime_pm.txt, this looks right. 

Reviewed-by: Harry Wentland <harry.wentland@amd.com>

I'll pull this in through the amd-stg tree.

> ---
> As a note, I'm not entirely happy with this fix and I wouldn't be
> surprised if I missed something while looking through amdgpu. So, please
> don't hesistate to suggest a better fix :).

I still don't really like amdgpu_dm_atomic_commit_tail and related functions. We have plans to rework these and make them more straight-forward.

Harry

> 
>  drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> index 1dd1142246c2..361b81ef6997 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> @@ -46,6 +46,7 @@
>  #include <linux/moduleparam.h>
>  #include <linux/version.h>
>  #include <linux/types.h>
> +#include <linux/pm_runtime.h>
>  
>  #include <drm/drmP.h>
>  #include <drm/drm_atomic.h>
> @@ -4211,6 +4212,8 @@ static void amdgpu_dm_atomic_commit_tail(struct drm_atomic_state *state)
>  			if (dm_old_crtc_state->stream)
>  				remove_stream(adev, acrtc, dm_old_crtc_state->stream);
>  
> +			pm_runtime_get_noresume(dev->dev);
> +
>  			acrtc->enabled = true;
>  			acrtc->hw_mode = new_crtc_state->mode;
>  			crtc->hwmode = new_crtc_state->mode;
> @@ -4396,6 +4399,16 @@ static void amdgpu_dm_atomic_commit_tail(struct drm_atomic_state *state)
>  		drm_atomic_helper_wait_for_flip_done(dev, state);
>  
>  	drm_atomic_helper_cleanup_planes(dev, state);
> +
> +	/* Finally, drop a runtime PM reference for each newly disabled CRTC,
> +	 * so we can put the GPU into runtime suspend if we're not driving any
> +	 * displays anymore
> +	 */
> +	pm_runtime_mark_last_busy(dev->dev);
> +	for_each_oldnew_crtc_in_state(state, crtc, old_crtc_state, new_crtc_state, i) {
> +		if (old_crtc_state->active && !new_crtc_state->active)
> +			pm_runtime_put_autosuspend(dev->dev);
> +	}
>  }
>  
>  
>