Re: [PATCH] drm/exynos: fix vblank handling during dpms off

[Alexandre Courbot]

On Thu, Oct 9, 2014 at 7:08 PM, Thierry Reding <treding@xxxxxxxxxx> wrote:
> On Thu, Oct 09, 2014 at 02:43:02PM +0900, Alexandre Courbot wrote:
>> On 10/02/2014 08:52 PM, Andrzej Hajda wrote:
>> >Hi,
>> >
>> >+CC possible victims
>> >
>> >On 10/02/2014 12:52 PM, Inki Dae wrote:
>> >>On 2014ë 10ì 02ì 17:58, Joonyoung Shim wrote:
>> >>>Hi Andrzej,
>> >>>
>> >>>On 10/01/2014 05:14 PM, Andrzej Hajda wrote:
>> >>>>The patch disables vblanks during dpms off only if pagefilp has
>> >>>>not been finished. It also replaces drm_vblank_off with drm_crtc_vblank_put.
>> >>>>It fixes issue with page_flip ioctl not being able to acquire vblank counter.
>> >>>This problem isn't related with pageflip, it just causes from
>> >>>7ffd7a68511c710b84db3548a1997fd2625f580a commit (drm: Always reject
>> >>>drm_vblank_get() after drm_vblank_off()).
>> >>>
>> >>>We need to use drm_vblank_on() as a counterpart to drm_vblank_off()
>> >>>after the commit .
>> >
>> >This patch should break also other drivers, it seems at least following
>> >drms could be affected:
>> >armada, sti, tegra.
>>
>> Indeed we (tegra) have just been hit by this. The problem seems to come from
>> the fact that we have been using drm_vblank_pre_modeset,
>> drm_vblank_post_modeset and drm_vblank_off conjointly. All these functions
>> depend on the value of vblank->inmodeset, and 7ffd7a68511 increases the
>> vblank reference counter only in drm_vblank_off, which can result in the
>> acquired reference never being released.
>>
>> The following seems to fix this for Tegra, by stopping using
>> drm_vblank_pre/post_modeset and relying on drm_vblank_off/on instead:
>>
>> diff --git a/drivers/gpu/drm/tegra/dc.c b/drivers/gpu/drm/tegra/dc.c
>> index b08df07cad47..3955d81236d0 100644
>> --- a/drivers/gpu/drm/tegra/dc.c
>> +++ b/drivers/gpu/drm/tegra/dc.c
>> @@ -739,7 +739,6 @@ static const struct drm_crtc_funcs tegra_crtc_funcs = {
>>
>>  static void tegra_crtc_disable(struct drm_crtc *crtc)
>>  {
>> -       struct tegra_dc *dc = to_tegra_dc(crtc);
>>         struct drm_device *drm = crtc->dev;
>>         struct drm_plane *plane;
>>
>> @@ -755,7 +754,7 @@ static void tegra_crtc_disable(struct drm_crtc *crtc)
>>                 }
>>         }
>>
>> -       drm_vblank_off(drm, dc->pipe);
>> +       drm_crtc_vblank_off(crtc);
>>  }
>>
>>  static bool tegra_crtc_mode_fixup(struct drm_crtc *crtc,
>> @@ -844,7 +843,7 @@ static int tegra_crtc_mode_set(struct drm_crtc *crtc,
>>         u32 value;
>>         int err;
>>
>> -       drm_vblank_pre_modeset(crtc->dev, dc->pipe);
>> +       drm_crtc_vblank_off(crtc);
>>
>>         err = tegra_crtc_setup_clk(crtc, mode);
>>         if (err) {
>> @@ -946,7 +945,7 @@ static void tegra_crtc_commit(struct drm_crtc *crtc)
>>         value = GENERAL_ACT_REQ | WIN_A_ACT_REQ;
>>         tegra_dc_writel(dc, value, DC_CMD_STATE_CONTROL);
>>
>> -       drm_vblank_post_modeset(crtc->dev, dc->pipe);
>> +       drm_crtc_vblank_on(crtc);
>>  }
>>
>>  static void tegra_crtc_load_lut(struct drm_crtc *crtc)
>>
>> Thierry, does this look ok to you?
>
> Yes, that looks like almost the same patch that I sent out yesterday.
> The difference is that I didn't replace the drm_vblank_pre_modeset()
> call with drm_vblank_off() like you did, but rather just dropped the
> former.
>
> I /think/ your version is more correct in that regard.

Feel free to take that extra line in your patch then. ;)

>
> Thierry
>
>> But there might be another issue, which is that calls to drm_vblank_get()
>> will return -EINVAL if invoked between drm_blank_off and drm_blank_on. Is
>> this really the desired behavior? Can it at least happen? If so, how are
>> drivers supposed to react to this situation?
>
> It shouldn't happen. If drm_vblank_off() and drm_vblank_on() are called
> around a modeset they should never conflict with drm_vblank_get(), at
> least on Tegra, because the modeset and page-flip IOCTLs will be
> serialized.

Ok, that's good. I was just wondering whether this case has been thought of.

Actually, and seeing how drm_vblank_pre/post_modeset() have become
useless (if not harmful), maybe it would be a good idea to come with a
fixup patch that gets rid of them altogether and make their callers
invoke drm_vblank_off/on() instead?
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