Re: [Nouveau] [Intel-gfx] [PATCH v3 13/13] drm/i915: replace drm_detect_hdmi_monitor() with drm_display_info.is_hdmi

[Claudio Suarez <cssk@net-c.es>]

On Fri, Oct 22, 2021 at 03:22:57PM +0300, Ville Syrjälä wrote:
> On Fri, Oct 22, 2021 at 03:01:52PM +0300, Ville Syrjälä wrote:
> > On Fri, Oct 22, 2021 at 12:25:33PM +0200, Claudio Suarez wrote:
> > > On Thu, Oct 21, 2021 at 04:49:59PM +0300, Ville Syrjälä wrote:
> > > > On Wed, Oct 20, 2021 at 12:51:21AM +0200, Claudio Suarez wrote:
> > > > > drm_get_edid() internally calls to drm_connector_update_edid_property()
> > > > > and then drm_add_display_info(), which parses the EDID.
> > > > > This happens in the function intel_hdmi_set_edid() and
> > > > > intel_sdvo_tmds_sink_detect() (via intel_sdvo_get_edid()).
> > > > > 
> > > > > Once EDID is parsed, the monitor HDMI support information is available
> > > > > through drm_display_info.is_hdmi. Retriving the same information with
> > > > > drm_detect_hdmi_monitor() is less efficient. Change to
> > > > > drm_display_info.is_hdmi
> > > > 
> > > > I meant we need to examine all call chains that can lead to
> > > > .detect() to make sure all of them do in fact update the
> > > > display_info beforehand.
> > > 
> > > Well, I studied it carefully and, yes, all call chains that can lead to
> > > drm_display_info.is_hdmi / drm_detect_hdmi_monitor() update display_info
> > > beforehand. In the case that this doesn't happen, the code is unchanged.
> > > 
> > > Do you want I explain the changes in the code here again ? Or do you want
> > > to me change the commit message to be more clear ? In the first case, I can
> > > write here a detailed explanation. In the second case I can make a longer commit
> > > message.
> > > 
> > > Or both?
> > 
> > I want all those call chains explained in the commit message,
> > otherwise I have no easy way to confirm whether the change
> > is correct or not.
> 
> Hmm. OK, so I had a bit of a dig around and seems that what we do now
> .detect()->drm_get_edid()->drm_connector_update_edid_property()->drm_add_display_info()

Yes. I said before that I felt something was wrong when I read the
documentation and then the code. To be more explicit now, I expected that
drm_connector_update_edid_property() will be done in the
fill_modes/get_modes phase instead of when reading the edid.
The documentation suggests that but the code reads the edid in the
detect phase.
Now, since drm_connector_update_edid_property() is called in the detect
phase, it is not necessary to keep the edid data in the private connector
struct. It is in struct drm_connector from the beginning.
But this is topic for another patch.

> Now the question is when did that start happening? Looks like it was
> commit 4b4df570b41d ("drm: Update edid-derived drm_display_info fields
> at edid property set [v2]") that started to call drm_add_display_info()
> from drm_connector_update_edid_property(), and then commit 5186421cbfe2
> ("drm: Introduce epoch counter to drm_connector") started to call
> drm_connector_update_edid_property() from drm_get_edid(). Before both
> of those commits were in place display_info would still contain
> some stale garbage during .detect().
>
> That is the story I think we want in these commit messages since it
> a) explains why the old code was directly parsing the edid instead
> b) why it's now safe to change this

------------------commit-message?--------------------

drm/i915: replace drm_detect_hdmi_monitor() with drm_display_info.is_hdmi

Commit a92d083d08b0 created the new flag is_hdmi in drm_display_info
which is set when sink compliant with CEA-861 (EDID) shall be treated
as an HDMI sink.

From that day, this value can be used in some cases instead of
calling drm_detect_hdmi_monitor() and a second parse is avoided
because drm_detect_hdmi_monitor() parses. A TODO task was
registered in Documentation/gpu/todo.rst to perform that task in
the future.

The flag drm_display_info.is_hdmi is set in the function
drm_add_display_info(), which is called from
drm_connector_update_edid_property(). Since commit 5186421cbfe2,
drm_get_edid() calls drm_connector_update_edid_property() when
reading the edid data from an i2c adapter. Therefore, in these
cases drm_display_info.is_hdmi is updated to its correct
value when returning from drm_get_edid().

Replace drm_detect_hdmi_monitor() with drm_display_info.is_hdmi
in the cases when drm_detect_hdmi_monitor() is called after a
read from an i2c adapter using drm_get_edid() in the i915 driver.
-----------------------------------------------

> 
> PS. connector->force handling in drm_get_edid() looks a bit busted
> since it doesn't call drm_connector_update_edid_property() at all
> in some cases. I think there might be some path that leads there
> anywya if/when we change connector->force, but we should fix
> drm_get_edid() to do the right thing regarless.

In those cases, the edid isn't read and NULL is returned by drm_get_edid().
No problem because display_info.is_hdmi is inside an if (edid != NULL).

BTW, struct intel_connector is allocated with kzalloc, so the initial
value of is_hdmi is zero. The connector isn't HDMI by default.

BR.
Claudio Suarez.