Re: [PATCH] drm/fb-helper: Fix vt restore

[Alex Deucher <alexdeucher@gmail.com>]

On Wed, Jun 24, 2020 at 5:29 AM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>
> In the past we had a pile of hacks to orchestrate access between fbdev
> emulation and native kms clients. We've tried to streamline this, by
> always preferring the kms side above fbdev calls when a drm master
> exists, because drm master controls access to the display resources.
>
> Unfortunately this breaks existing userspace, specifically Xorg. When
> exiting Xorg first restores the console to text mode using the KDSET
> ioctl on the vt. This does nothing, because a drm master is still
> around. Then it drops the drm master status, which again does nothing,
> because logind is keeping additional drm fd open to be able to
> orchestrate vt switches. In the past this is the point where fbdev was
> restored, as part of the ->lastclose hook on the drm side.
>
> Now to fix this regression we don't want to go back to letting fbdev
> restore things whenever it feels like, or to the pile of hacks we've
> had before. Instead try and go with a minimal exception to make the
> KDSET case work again, and nothing else.
>
> This means that if userspace does a KDSET call when switching between
> graphical compositors, there will be some flickering with fbcon
> showing up for a bit. But a) that's not a regression and b) userspace
> can fix it by improving the vt switching dance - logind should have
> all the information it needs.
>
> While pondering all this I'm also wondering wheter we should have a
> SWITCH_MASTER ioctl to allow race-free master status handover. But
> that's for another day.
>
> v2: Somehow forgot to cc all the fbdev people.
>
> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=208179
> Cc: shlomo@fastmail.com
> Reported-and-Tested-by: shlomo@fastmail.com
> Cc: Michel Dänzer <michel@daenzer.net>
> Fixes: 64914da24ea9 ("drm/fbdev-helper: don't force restores")
> Cc: Noralf Trønnes <noralf@tronnes.org>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: dri-devel@lists.freedesktop.org
> Cc: <stable@vger.kernel.org> # v5.7+
> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Cc: Nathan Chancellor <natechancellor@gmail.com>
> Cc: Qiujun Huang <hqjagain@gmail.com>
> Cc: Peter Rosin <peda@axentia.se>
> Cc: linux-fbdev@vger.kernel.org
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> ---
>  drivers/gpu/drm/drm_fb_helper.c  | 63 +++++++++++++++++++++++++-------
>  drivers/video/fbdev/core/fbcon.c |  3 +-
>  include/uapi/linux/fb.h          |  1 +
>  3 files changed, 52 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index 170aa7689110..ae69bf8e9bcc 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -227,18 +227,9 @@ int drm_fb_helper_debug_leave(struct fb_info *info)
>  }
>  EXPORT_SYMBOL(drm_fb_helper_debug_leave);
>
> -/**
> - * drm_fb_helper_restore_fbdev_mode_unlocked - restore fbdev configuration
> - * @fb_helper: driver-allocated fbdev helper, can be NULL
> - *
> - * This should be called from driver's drm &drm_driver.lastclose callback
> - * when implementing an fbcon on top of kms using this helper. This ensures that
> - * the user isn't greeted with a black screen when e.g. X dies.
> - *
> - * RETURNS:
> - * Zero if everything went ok, negative error code otherwise.
> - */
> -int drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper)
> +static int
> +__drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper,
> +                                           bool force)
>  {
>         bool do_delayed;
>         int ret;
> @@ -250,7 +241,16 @@ int drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper)
>                 return 0;
>
>         mutex_lock(&fb_helper->lock);
> -       ret = drm_client_modeset_commit(&fb_helper->client);
> +       if (force) {
> +               /*
> +                * Yes this is the _locked version which expects the master lock
> +                * to be held. But for forced restores we're intentionally
> +                * racing here, see drm_fb_helper_set_par().
> +                */
> +               ret = drm_client_modeset_commit_locked(&fb_helper->client);
> +       } else {
> +               ret = drm_client_modeset_commit(&fb_helper->client);
> +       }
>
>         do_delayed = fb_helper->delayed_hotplug;
>         if (do_delayed)
> @@ -262,6 +262,22 @@ int drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper)
>
>         return ret;
>  }
> +
> +/**
> + * drm_fb_helper_restore_fbdev_mode_unlocked - restore fbdev configuration
> + * @fb_helper: driver-allocated fbdev helper, can be NULL
> + *
> + * This should be called from driver's drm &drm_driver.lastclose callback
> + * when implementing an fbcon on top of kms using this helper. This ensures that
> + * the user isn't greeted with a black screen when e.g. X dies.
> + *
> + * RETURNS:
> + * Zero if everything went ok, negative error code otherwise.
> + */
> +int drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper)
> +{
> +       return __drm_fb_helper_restore_fbdev_mode_unlocked(fb_helper, false);
> +}
>  EXPORT_SYMBOL(drm_fb_helper_restore_fbdev_mode_unlocked);
>
>  #ifdef CONFIG_MAGIC_SYSRQ
> @@ -1318,6 +1334,7 @@ int drm_fb_helper_set_par(struct fb_info *info)
>  {
>         struct drm_fb_helper *fb_helper = info->par;
>         struct fb_var_screeninfo *var = &info->var;
> +       bool force;
>
>         if (oops_in_progress)
>                 return -EBUSY;
> @@ -1327,7 +1344,25 @@ int drm_fb_helper_set_par(struct fb_info *info)
>                 return -EINVAL;
>         }
>
> -       drm_fb_helper_restore_fbdev_mode_unlocked(fb_helper);
> +       /*
> +        * Normally we want to make sure that a kms master takes
> +        * precedence over fbdev, to avoid fbdev flickering and
> +        * occasionally stealing the display status. But Xorg first sets
> +        * the vt back to text mode using the KDSET IOCTL with KD_TEXT,
> +        * and only after that drops the master status when exiting.
> +        *
> +        * In the past this was caught by drm_fb_helper_lastclose(), but
> +        * on modern systems where logind always keeps a drm fd open to
> +        * orchestrate the vt switching, this doesn't work.
> +        *
> +        * To no break the userspace ABI we have this special case here,

"To not break"
With that fixed:
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

> +        * which is only used for the above case. Everything else uses
> +        * the normal commit function, which ensures that we never steal
> +        * the display from an active drm master.
> +        */
> +       force = var->activate & FB_ACTIVATE_KD_TEXT;
> +
> +       __drm_fb_helper_restore_fbdev_mode_unlocked(fb_helper, force);
>
>         return 0;
>  }
> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> index 9d28a8e3328f..e2a490c5ae08 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -2402,7 +2402,8 @@ static int fbcon_blank(struct vc_data *vc, int blank, int mode_switch)
>                 ops->graphics = 1;
>
>                 if (!blank) {
> -                       var.activate = FB_ACTIVATE_NOW | FB_ACTIVATE_FORCE;
> +                       var.activate = FB_ACTIVATE_NOW | FB_ACTIVATE_FORCE |
> +                               FB_ACTIVATE_KD_TEXT;
>                         fb_set_var(info, &var);
>                         ops->graphics = 0;
>                         ops->var = info->var;
> diff --git a/include/uapi/linux/fb.h b/include/uapi/linux/fb.h
> index b6aac7ee1f67..4c14e8be7267 100644
> --- a/include/uapi/linux/fb.h
> +++ b/include/uapi/linux/fb.h
> @@ -205,6 +205,7 @@ struct fb_bitfield {
>  #define FB_ACTIVATE_ALL               64       /* change all VCs on this fb    */
>  #define FB_ACTIVATE_FORCE     128      /* force apply even when no change*/
>  #define FB_ACTIVATE_INV_MODE  256       /* invalidate videomode */
> +#define FB_ACTIVATE_KD_TEXT   512       /* for KDSET vt ioctl */
>
>  #define FB_ACCELF_TEXT         1       /* (OBSOLETE) see fb_info.flags and vc_mode */
>
> --
> 2.27.0
>
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