Re: [PATCH/RFT v1 0/6] drm/via: drop use of deprecated headers drmP.h and drm_os_linux.h

[Sam Ravnborg <sam@ravnborg.org>]

Hi Michael.

On Fri, Jul 19, 2019 at 11:05:44AM +0200, Michel Dänzer wrote:
> On 2019-07-19 8:07 a.m., Sam Ravnborg wrote:
> > On Thu, Jul 18, 2019 at 05:37:31PM +0200, Sam Ravnborg wrote:
> >> This is some janitorial updates to the via driver
> >> that is required to get rid of deprecated headers
> >> in the drm subsystem.
> >>
> >> The first three patches are trivial, where
> >> the dependencies on drmP.h and drm_os_linux are dropped.
> >>
> >> The remaining three patches drop use of DRM_WAIT_ON().
> >> They are replaced by wait_event_interruptible_timeout().
> >> These patches could use a more critical review.
> > 
> > The differences between DRM_WAIT_ON() and
> > wait_event_interruptible_timeout() are bigger than anticipated.
> > 
> > The conversion I did for drm_vblank.c is bogus thus I expect
> > the conversion done for via is also bogus.
> 
> What exactly is the problem though? Can you share information about the
> failures you're seeing?
> 
> There was some discussion about DRM_WAIT_ON() "polling" on IRC. I assume
> that refers to it only sleeping for up to 0.01s before checking the
> condition again. In contrast, wait_event_interruptible_timeout() checks
> the condition once, then sleeps up to the full timeout before checking
> it again.
Correct - it was based on the feedback on irc from airlied and ickle
that made me conclude that the via part may not be good.
I cannot say if the polling versus timeout is properly dealt with in the
via driver and I am inclided to just move DRM_WAIT_ON() to via_drv.h and
name it VIA_WAIT_ON().
Then the changes to this legacy driver is minimal and it will not
prevent us from gettting rid of drm_os_linux.h

> 
> If that makes a difference for drm_wait_vblank_ioctl, it indicates that
> some other code which updates the vblank count or clears vblank->enabled
> doesn't wake up the vblank->queue.
Let me analyse a little...

In drm_handle_vblank() there is a call to wake_up(&vblank->queue);
And this is called from an interrupt - OK.

drm_vblank_enable() is called outside an interrupt - no need for
wake_up()

drm_crtc_accurate_vblank_count() is called outside interrupt - no need
for wake_up()

drm_vblank_disable_and_save() is called outside interrupt - no need for
wake_up()'

That is all functions I could dig up that updates the vblank counter.

So based on this short analysis I actually start to think that
I can use the variant that uses wait_event_interruptible_timeout()
anyway.
I will post a v3 and await feedback on that version.

As for via - introducing VIA_WAIT_ON() is the simple solution.
So I will post a v2 of this series using VIA_WAIT_ON().

	Sam
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