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

[Ezequiel Garcia]

(Adding Jani again, who got dropped for some reason)

On 1 April 2016 at 16:50, Ezequiel Garcia <ezequiel at vanguardiasur.com.ar> wrote:
> 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



-- 
Ezequiel García, VanguardiaSur
www.vanguardiasur.com.ar