linux-kernel - Re: [Nouveau] [PATCH 01/11] drm/fb-helper: do a generic fb_setcmap helper in terms of crtc .gamma

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Message-ID: <20170621081454.ymykfka5peelkqxp@phenom.ffwll.local>
Date:   Wed, 21 Jun 2017 10:14:54 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Michel Dänzer <michel@...nzer.net>
Cc:     Peter Rosin <peda@...ntia.se>,
        Philippe Cornu <philippe.cornu@...com>,
        Christian König <christian.koenig@....com>,
        Yannick Fertre <yannick.fertre@...com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Alex Deucher <alexander.deucher@....com>,
        Dave Airlie <airlied@...hat.com>,
        Vincent Abriou <vincent.abriou@...com>,
        Ben Skeggs <bskeggs@...hat.com>, nouveau@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        virtualization@...ts.linux-foundation.org,
        amd-gfx@...ts.freedesktop.org
Subject: Re: [Nouveau] [PATCH 01/11] drm/fb-helper: do a generic fb_setcmap
 helper in terms of crtc .gamma_set

On Wed, Jun 21, 2017 at 04:59:31PM +0900, Michel Dänzer wrote:
> On 21/06/17 04:38 PM, Daniel Vetter wrote:
> > On Tue, Jun 20, 2017 at 09:25:25PM +0200, Peter Rosin wrote:
> >> This makes the redundant fb helpers .load_lut, .gamma_set and .gamma_get
> >> totally obsolete.
> >>
> >> I think the gamma_store can end up invalid on error. But the way I read
> >> it, that can happen in drm_mode_gamma_set_ioctl as well, so why should
> >> this pesky legacy fbdev stuff be any better?
> >>
> >> drm_fb_helper_save_lut_atomic justs saves the gamma lut for later. However,
> >> it saves it to the gamma_store which should already be up to date with what
> >> .gamma_get would return and is thus a nop. So, zap it.
> > 
> > Removing drm_fb_helper_save_lut_atomic should be a separate patch I
> > think.
> >  
> >> Signed-off-by: Peter Rosin <peda@...ntia.se>
> 
> [...]
> 
> >> @@ -1167,50 +1150,6 @@ void drm_fb_helper_set_suspend_unlocked(struct drm_fb_helper *fb_helper,
> >>  }
> >>  EXPORT_SYMBOL(drm_fb_helper_set_suspend_unlocked);
> >>  
> >> -static int setcolreg(struct drm_crtc *crtc, u16 red, u16 green,
> >> -		     u16 blue, u16 regno, struct fb_info *info)
> >> -{
> >> -	struct drm_fb_helper *fb_helper = info->par;
> >> -	struct drm_framebuffer *fb = fb_helper->fb;
> >> -
> >> -	if (info->fix.visual == FB_VISUAL_TRUECOLOR) {
> > 
> > This case here seems gone, and it was also the pièce de résistance when I
> > tried tackling fbdev lut support. As far as I understand this stuff we do
> > not support FB_VISUAL_TRUECOLOR palette, and all that bitshifting here is
> > pointless. But I'm honestly not really clear.
> > 
> > I think removing this case should also be a separate patch, and I'd very
> > much prefer that someone with some fbdev-clue would ack it.
> 
> No need for anyone with fbdev-clue, just run fbset -i. :) fbcon uses a
> TRUECOLOR visual at depths > 8.

Then I understand even less what exactly this code does ... Should we keep
it? Is it just pure cargo-cult? Only needed when we'd change the color
depth of the fbdev buffer, which we never do at runtime?

> > It's a pre-existing bug, but should we also try to restore the fbdev lut
> > in drm_fb_helper_restore_fbdev_mode_unlocked()? Would be yet another bug,
> > but might be relevant for your use-case. Just try to run both an fbdev
> > application and some kms-native thing, and then SIGKILL the native kms
> > app.
> > 
> > But since pre-existing not really required, and probably too much effort.
> 
> I suspect something like that indeed needs to be done though. E.g.
> killing Xorg results in fbcon continuing to use the LUT set by Xorg.

Ok, the only trouble with that is atomic kms locking, which requires that
we can only do 1 commit per lock-holding time, and also
drm_modeset_lock_all is an evil hack and needs to be phased out. Two
solutions:

- We just update the lut after we've dropped the locks again in
  restore_fbdev_mode.
- We need both a legacy path, in restore_fbdev_mode_legacy, and an atomic
  path in restore_fbdev_mode_atomic. The latter cannot use the gamme_set
  callback, since that would call drm_atomic_helper_legacy_gamma_set for
  atomic drivers, which would result in two calls to drm_atomic_commit in
  one critical sections. Instead that needs to open-code the logic in
  drm_atomic_helper_legacy_gamma_set and integrate it into the overall
  fbdev restore commit.

The 2nd option is a bit more typing, but much cleaner. It also fits better
into some of the refactoring plans I have (I want to get rid of
drm_modeset_lock_all in the fbdev emulation). And has the benefit of
giving drivers a bit more complex fbdev emulation atomic commits, which
would be good for testing I think.

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch