linux-kernel - Re: [PATCH] Revert "drm/fb-helper: improve DRM fbdev emulation device names"

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Date:   Wed, 13 Oct 2021 14:27:24 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc:     Javier Martinez Canillas <javierm@...hat.com>,
        linux-kernel@...r.kernel.org, Johannes Stezenbach <js@...21.net>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel@...ts.freedesktop.org, Ben Skeggs <bskeggs@...hat.com>,
        nouveau@...ts.freedesktop.org
Subject: Re: [PATCH] Revert "drm/fb-helper: improve DRM fbdev emulation
 device names"

On Fri, Oct 08, 2021 at 02:02:40PM +0300, Ville Syrjälä wrote:
> On Fri, Oct 08, 2021 at 09:17:08AM +0200, Javier Martinez Canillas wrote:
> > This reverts commit b3484d2b03e4c940a9598aa841a52d69729c582a.
> > 
> > That change attempted to improve the DRM drivers fbdev emulation device
> > names to avoid having confusing names like "simpledrmdrmfb" in /proc/fb.
> > 
> > But unfortunately there are user-space programs, such as pm-utils that
> > query that information and so broke after the mentioned commit. Since
> > the names in /proc/fb are used programs that consider it an ABI, let's
> > restore the old names even when this lead to silly naming like the one
> > mentioned above as an example.
> 
> The usage Johannes listed was this specificially:
>  using_kms() { grep -q -E '(nouveau|drm)fb' /proc/fb; }                                                        
> 
> So it actually looks like  Daniel's
> commit f243dd06180a ("drm/nouveau: Use drm_fb_helper_fill_info")
> also broke the abi. But for the pm-utils use case at least
> just having the "drmfb" in there should cover even nouveau.
> 
> Cc: stable@...r.kernel.org
> Reviewed-by: Ville Syrjälä <ville.syrjala@...ux.intel.com>
> 
> > 
> > Reported-by: Johannes Stezenbach <js@...21.net>
> > Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
> > ---
> > 
> >  drivers/gpu/drm/drm_fb_helper.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> > index 3ab07832104..8993b02e783 100644
> > --- a/drivers/gpu/drm/drm_fb_helper.c
> > +++ b/drivers/gpu/drm/drm_fb_helper.c
> > @@ -1737,7 +1737,7 @@ void drm_fb_helper_fill_info(struct fb_info *info,
> >  			       sizes->fb_width, sizes->fb_height);
> >  
> >  	info->par = fb_helper;
> > -	snprintf(info->fix.id, sizeof(info->fix.id), "%s",

Please add a comment here that drmfb is uapi because pm-utils matches
against it ...

Otherwise this will be lost in time again :-(
-Daniel
> > +	snprintf(info->fix.id, sizeof(info->fix.id), "%sdrmfb",
> >  		 fb_helper->dev->driver->name);
> >  
> >  }
> > -- 
> > 2.31.1
> 
> -- 
> Ville Syrjälä
> Intel

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