[Intel-gfx] [PATCH] drm/i915/hdmi: Fix weak connector detection

[Ezequiel Garcia]

On 01 Apr 06:46 PM, Ville Syrjälä wrote:
> On Fri, Apr 01, 2016 at 12:38:11PM -0300, Ezequiel Garcia wrote:
> > El abr. 1, 2016 11:47 AM, "Ville Syrjälä" <ville.syrjala at linux.intel.com>
> > escribió:
> > >
> > > On Thu, Mar 31, 2016 at 05:55:03PM -0300, Ezequiel Garcia wrote:
> > > > Currently, our implementation of drm_connector_funcs.detect is
> > > > based on getting a valid EDID.
> > > >
> > > > This requirement makes the driver fail to detect connected
> > > > connectors in case of EDID corruption, which prevents from falling
> > > > back to modes provided by builtin or user-provided EDIDs.
> > >
> > > So why are you getting corrupted EDIDs?
> > >
> > 
> > Does it matter?
> 
> Yes. We should fix the real cause (if possible) instead of adding
> more duct tape.
> 

So, there are two things involved in this patch:

1.
There are several reasons why EDID can get screwed, this is
documented at length [1], and it's the motivation for
CONFIG_DRM_LOAD_EDID_FIRMWARE to exist.

You can find lots of reports on the internet of people getting
corrupt EDID from their monitors. For instance, here's one [2].

And even if no firmware is provided using CONFIG_DRM_LOAD_EDID_FIRMWARE,
the DRM core will provide a 1024x768 fallback mode:

int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
                                            uint32_t maxX, uint32_t maxY)
{
[..]
        if (count == 0 && connector->status == connector_status_connected)
                count = drm_add_modes_noedid(connector, 1024, 768);

But, this only works if the connector is detected.

Since I'm interested in backporting this patch to apply it on the kernels
I maintain (which are currently deployed on hundreds of machines), I tried
to find a simple solution. Hence, this patch.

There's no issue to fix here, because broken hardware is a fact of life,
and not something we can fix or ignore [3].

2.
On the other side, the i915 implementation looks suspicious. IMHO,
drm_connector_funcs.detect should not try to read a valid EDID,
and just try to detect if the connector is connected or disconnected.

The EDID can be read in drm_connector_helper_funcs.get_modes, as other
drm/connector drivers are doing (tda998x, tfp410, tegra).

However, I think it's safer to get a simple fix now, and do this
as follow-up patches.

How does it sound?

[1] Documentation/EDID/HOWTO.txt
[2] http://www.blaicher.com/2012/06/howto-fixing-a-broken-edid-eeprom-with-a-bus-pirate-v4/
[3] https://marc.info/?l=linux-kernel&m=112838038415265&w=4
-- 
Ezequiel Garcia, VanguardiaSur
www.vanguardiasur.com.ar