[PATCH 13/19] drm: do not steal the display if we have a master

[David Herrmann]

Hi

On Wed, Nov 27, 2013 at 9:24 PM, Paulo Zanoni <przanoni at gmail.com> wrote:
> From: Paulo Zanoni <paulo.r.zanoni at intel.com>
>
> Sometimes we want to disable all the screens on a system, because that
> will allow the graphics card to be put into low-power states. The
> problem is that, for example, while all screens are disabled, if we
> get a hotplug interrupt, fbcon will decide to set a mode instead of
> keeping everything disabled, which will remove us from our low power
> states.
>
> Let's assume that if there's a DRM master, it will be able to do
> whatever is appropriate when we get the hotplug.
>
> This problem can be reproduced by the runtime PM test program from
> intel-gpu-tools: we disable all the screens so the graphics device can
> be put into D3, then something triggers a hotplug interrupt, fbcon
> sets a mode and breaks our test suite. The problem can be reproduced
> more easily by the "i2c" subtest.
>
> Other approaches considered for the problem:
>     - Return "false" if "bound == 0" and the caller of
>       drm_fb_helper_is_bound is a hotplug handler. This would break
>       the case where the machine boots with no outputs connected, then
>       the user plugs a monitor.
>     - Add a new IOCTL to force fbcon to not set modes. This would keep
>       all the current applications behaving the same, but adding a new
>       IOCTL is not always the greatest idea.
>     - Return false only if "dev->primary->master && bound == 0". This
>       was my first implementation, but Chris suggested we should do
>       the check irrespective of the "bound" variable.
>
> Thanks to Daniel Vetter for the investigation, ideas and the
> implementation of the hotplug alternative.
>
> v2: - Do the check first, irrespective of "bound".
>     - Cc dri-devel

Thank god all that will get dropped with CONFIG_FB=n. Anyhow:

Reviewed-by: David Herrmann <dh.herrmann at gmail.com>

Thanks
David

> Cc: dri-devel at lists.freedesktop.org
> Credits-to: Daniel Vetter <daniel.vetter at ffwll.ch>
> Signed-off-by: Paulo Zanoni <paulo.r.zanoni at intel.com>
> ---
>  drivers/gpu/drm/drm_fb_helper.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index 0a19401..98a0363 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -359,6 +359,11 @@ static bool drm_fb_helper_is_bound(struct drm_fb_helper *fb_helper)
>         struct drm_crtc *crtc;
>         int bound = 0, crtcs_bound = 0;
>
> +       /* Sometimes user space wants everything disabled, so don't steal the
> +        * display if there's a master. */
> +       if (dev->primary->master)
> +               return false;
> +
>         list_for_each_entry(crtc, &dev->mode_config.crtc_list, head) {
>                 if (crtc->fb)
>                         crtcs_bound++;
> @@ -368,6 +373,7 @@ static bool drm_fb_helper_is_bound(struct drm_fb_helper *fb_helper)
>
>         if (bound < crtcs_bound)
>                 return false;
> +
>         return true;
>  }
>
> --
> 1.8.3.1
>
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