Den 10.09.2019 14.48, skrev Thomas Zimmermann:
> Hi
>
> Am 10.09.19 um 13:52 schrieb Gerd Hoffmann:
>> On Mon, Sep 09, 2019 at 04:06:32PM +0200, Thomas Zimmermann wrote:
>>> Before updating the display from the console's shadow buffer, the dirty
>>> worker now waits for vblank. This allows several screen updates to pile
>>> up and acts as a rate limiter.
>>>
>>> Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
>>> ---
>>> drivers/gpu/drm/drm_fb_helper.c | 12 ++++++++++++
>>> 1 file changed, 12 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
>>> index a7ba5b4902d6..017e2f6bd1b9 100644
>>> --- a/drivers/gpu/drm/drm_fb_helper.c
>>> +++ b/drivers/gpu/drm/drm_fb_helper.c
>>> @@ -402,8 +402,20 @@ static void drm_fb_helper_dirty_work(struct work_struct *work)
>>> dirty_work);
>>> struct drm_clip_rect *clip = &helper->dirty_clip;
>>> struct drm_clip_rect clip_copy;
>>> + struct drm_crtc *crtc;
>>> unsigned long flags;
>>> void *vaddr;
>>> + int ret;
>>> +
>>> + /* rate-limit update frequency */
>>> + mutex_lock(&helper->lock);
>>> + crtc = helper->client.modesets[0].crtc;
>>> + ret = drm_crtc_vblank_get(crtc);
>>> + if (!ret) {
>>> + drm_crtc_wait_one_vblank(crtc);
>>> + drm_crtc_vblank_put(crtc);
>>> + }
>>> + mutex_unlock(&helper->lock);
>>
>> Hmm, not sure it is the best plan to sleep for a while in the worker,
>> especially while holding the lock.
>>
>> What does the lock protect against here? Accessing
>
> This lock is hold by the fbdev code during mode-setting operations (but
> not drawing operations). So *crtc might be gone if we don't hold it here.
>
>> helper->client.modesets? If so then you can unlock before going to
>> sleep in drm_crtc_wait_one_vblank() I think.
>
> I looked, but I cannot find any code that protects crtc while vblank is
> active. I'd rather not change the current code until someone can clarify.
>
The client->modesets array and the crtc struct member are invariant over
the lifetime of client (drm_client_modeset_create()). No need to take a
lock for access. See drm_client_modeset_release() for the things that
_can_ change and needs protection (protected by client->modeset_mutex).
I don't see a problem with sleeping in the worker though, but I might
miss out on something. AFAICS changes will just pile up in >dirty_clip
and the worker will be scheduled for a new run to happen when it's done
with the current update.
Noralf.
> Best regards
> Thomas
>
>>
>> cheers,
>> Gerd
>>
>
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