Re: [PATCH 1/2] drm/i915: Prevent HW access during init from SDVO TV get_modes hook

[Imre Deak <imre.deak@intel.com>]

On Wed, Feb 07, 2024 at 04:10:28PM +0200, Imre Deak wrote:
> On Wed, Feb 07, 2024 at 03:45:15PM +0200, Hogander, Jouni wrote:
> > On Wed, 2024-02-07 at 15:26 +0200, Hogander, Jouni wrote:
> > > On Tue, 2024-02-06 at 17:39 +0200, Imre Deak wrote:
> > > > Prevent accessing the HW from the SDVO/TV get_modes connector hook.
> > > > Returning 0 from the hook will make the caller -
> > > > drm_helper_probe_single_connector_modes() - keep the previously
> > > > detected
> > > > mode list of the connector.
> > >
> >
> > > I don't see where this is done? Not sure if looking at wrong place,
> > > but I see it tries using some override edid and in case that fails
> > > as well uses drm_add_modes_noedid?
> 
> Some default and EDID override modes are also added to the connector
> mode list, similarly to when the HW access in intel_sdvo_get_tv_modes()
> fails (along with any modes specified via the kernel command line, which
> happens unconditionally, see drm_helper_probe_add_cmdline_mode()).
> 
> All the modes detected by the encoder detect and the connector get_modes
> hooks gets added to the drm_connector::probed_modes list. From this
> __drm_helper_update_and_validate() will copy any new modes to the
> drm_connector::modes list (which will be returned to user space/kernel
> client). Thus, returning 0 from the above TV connector get_modes hook
> will preserve the list of modes on the drm_connector::modes list from an
> earlier encoder detect/connector get_modes call.

Drat, I realized the above is not quite correct. All the modes on
drm_connector:modes will be flagged as MODE_STALE and removed by
drm_mode_prune_invalid(). This means if get_modes() returns 0 only the
default/override modes will be returned, not the ones detected in an
earlier detect/get_modes call. I think this is ok, matching the case
where HW access fails, but needs at least a clarficiation in the
comment. Thanks for questioning it..

> 
> > >
> > > >
> > > > Signed-off-by: Imre Deak <imre.deak@intel.com>
> > > > ---
> > > >  drivers/gpu/drm/i915/display/intel_sdvo.c | 4 ++++
> > > >  1 file changed, 4 insertions(+)
> > > >
> > > > diff --git a/drivers/gpu/drm/i915/display/intel_sdvo.c
> > > > b/drivers/gpu/drm/i915/display/intel_sdvo.c
> > > > index 2571ef5a1b211..ccea0efbd136f 100644
> > > > --- a/drivers/gpu/drm/i915/display/intel_sdvo.c
> > > > +++ b/drivers/gpu/drm/i915/display/intel_sdvo.c
> > > > @@ -2287,6 +2287,7 @@ static const struct drm_display_mode
> > > > sdvo_tv_modes[] = {
> > > >  static int intel_sdvo_get_tv_modes(struct drm_connector
> > > > *connector)
> > >
> > > I see intel_sdvo_get_tv_modes is called from intel_sdvo_get_modes.
> > > Why
> > > there is no need to do the same in intel_sdvo_get_lvds_modes and
> > > intel_sdvo_get_ddc_modes as well?
> > 
> > It seems you are taking care of intel_svdo_get_ddc_modes in next patch.
> > Intel_svdo_get_lvds_modes doesn't touch the hw -> nothing to do there.
> > 
> > >
> > > BR,
> > >
> > > Jouni Högander
> > >
> > > >  {
> > > >         struct intel_sdvo *intel_sdvo =
> > > > intel_attached_sdvo(to_intel_connector(connector));
> > > > +       struct drm_i915_private *i915 = to_i915(intel_sdvo-
> > > > > base.base.dev);
> > > >         struct intel_sdvo_connector *intel_sdvo_connector =
> > > >                 to_intel_sdvo_connector(connector);
> > > >         const struct drm_connector_state *conn_state = connector-
> > > > > state;
> > > > @@ -2298,6 +2299,9 @@ static int intel_sdvo_get_tv_modes(struct
> > > > drm_connector *connector)
> > > >         DRM_DEBUG_KMS("[CONNECTOR:%d:%s]\n",
> > > >                       connector->base.id, connector->name);
> > > >
> > > > +       if (!intel_display_driver_check_access(i915))
> > > > +               return 0;
> > > > +
> > > >         /*
> > > >          * Read the list of supported input resolutions for the
> > > > selected TV
> > > >          * format.
> > >
> >