Re: [PATCH] drm/fb-helper: Add locking to sysrq handling

From: Daniel Vetter <daniel.vetter@ffwll.ch>
To: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>,
	Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
	DRI Development <dri-devel@lists.freedesktop.org>,
	Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [PATCH] drm/fb-helper: Add locking to sysrq handling
Date: Thu, 8 Oct 2020 11:29:18 +0200	[thread overview]
Message-ID: <CAKMK7uGFqNiHU5tSC_db4dBNRybY1Wc-b-OYQXReK7efn-AN4g@mail.gmail.com> (raw)
In-Reply-To: <fedcb884-fdb0-8f32-34ce-e0a2d3238413@suse.de>

On Thu, Oct 8, 2020 at 11:22 AM Thomas Zimmermann <tzimmermann@suse.de> wrote:
>
> Hi
>
> Am 07.10.20 um 15:30 schrieb Daniel Vetter:
> > We didn't take the kernel_fb_helper_lock mutex, which protects that
> > code. While at it, simplify the code
> > - inline the function (originally shared with kgdb I think)
> > - drop the error tracking and all the complications
> > - drop the pointless early out, it served nothing
> >
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Maxime Ripard <mripard@kernel.org>
> > Cc: Thomas Zimmermann <tzimmermann@suse.de>
> > Cc: David Airlie <airlied@linux.ie>
> > Cc: Daniel Vetter <daniel@ffwll.ch>
> > ---
> >  drivers/gpu/drm/drm_fb_helper.c | 26 +++++---------------------
> >  1 file changed, 5 insertions(+), 21 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> > index 8697554ccd41..c2f72bb6afb1 100644
> > --- a/drivers/gpu/drm/drm_fb_helper.c
> > +++ b/drivers/gpu/drm/drm_fb_helper.c
> > @@ -281,18 +281,12 @@ int drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper)
> >  EXPORT_SYMBOL(drm_fb_helper_restore_fbdev_mode_unlocked);
> >
> >  #ifdef CONFIG_MAGIC_SYSRQ
> > -/*
> > - * restore fbcon display for all kms driver's using this helper, used for sysrq
> > - * and panic handling.
> > - */
> > -static bool drm_fb_helper_force_kernel_mode(void)
> > +/* emergency restore, don't bother with error reporting */
> > +static void drm_fb_helper_restore_work_fn(struct work_struct *ignored)
> >  {
> > -     bool ret, error = false;
> >       struct drm_fb_helper *helper;
> >
> > -     if (list_empty(&kernel_fb_helper_list))
> > -             return false;
> > -
> > +     mutex_lock(&kernel_fb_helper_lock);
> >       list_for_each_entry(helper, &kernel_fb_helper_list, kernel_fb_list) {
> >               struct drm_device *dev = helper->dev;
> >
> > @@ -300,22 +294,12 @@ static bool drm_fb_helper_force_kernel_mode(void)
> >                       continue;
> >
> >               mutex_lock(&helper->lock);
> > -             ret = drm_client_modeset_commit_locked(&helper->client);
> > -             if (ret)
> > -                     error = true;
> > +             drm_client_modeset_commit_locked(&helper->client);
> >               mutex_unlock(&helper->lock);
> >       }
> > -     return error;
> > +     mutex_unlock(&kernel_fb_helper_lock);
> >  }
> >
> > -static void drm_fb_helper_restore_work_fn(struct work_struct *ignored)
> > -{
> > -     bool ret;
> > -
> > -     ret = drm_fb_helper_force_kernel_mode();
> > -     if (ret == true)
> > -             DRM_ERROR("Failed to restore crtc configuration\n");
>
> Is there a specific reason for removing that warning? Even if it doesn't
> show up on screen, is it not helpful in the kernel's log?

I just found it really unhelpful, if you're trying to force-show
fbcon, what's the point if it doesn't work out? The user will notice.
Also, if we're really in a dire situation where you want this, in my
experience there's a bunch of random other reasons why this can fail,
mostly when the work we have to schedule for locking reasons never
runs. So it's also unreliable.

> In any case, the rest looks good.
>
> Acked-by: Thomas Zimmermann <tzimmermann@suse.de>

Thanks for taking a look, I'll apply it.
-Daniel

>
> Best regards
> Thomas
>
> > -}
> >  static DECLARE_WORK(drm_fb_helper_restore_work, drm_fb_helper_restore_work_fn);
> >
> >  static void drm_fb_helper_sysrq(int dummy1)
> >
>
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
> Geschäftsführer: Felix Imendörffer
>

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