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@xxxxxxxxxxxxxxx>
> > 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
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