[Intel-gfx] [PATCH 1/2] Give up on edid retries when i2c tells us that bus is not there

Eugeni Dodonov eugeni at dodonov.net
Mon Oct 24 07:40:14 PDT 2011

On Thu, Oct 20, 2011 at 10:33, Jean Delvare <khali at linux-fr.org> wrote:

> Just to clarify: by "connectivity is setup", do you mean code in the
> driver setting the GPIO pin direction etc., or a display being
> connected to the graphics card?
> I admit I am a little surprised. I2C buses should have their lines up
> by default, thanks to pull-up resistors, and masters and slaves should
> merely drive the lines low as needed. The absence of slaves should have
> no impact on the line behavior. If the Intel graphics chips (or the
> driver) rely on the display to hold the lines high, I'd say this is a
> seriously broken design.
> As a side note, I thought that HDMI had the capability of presence
> detection, so there may be a better way to know if a display is
> connected or not, and this information could used to dynamically
> instantiate and delete the I2C buses? That way we could skip probing
> for EDID when there is no chance of success.

Yes, I think so too.

I admit I haven't got to the root of the problem here. My test was really
simple, borrowed from the test_bus() at i2c-algo-bit.c - without HDMI cable
plugged in, I was getting the "SDA stuck high" messages; with the cable
plugged in, I wasn't getting any of those.

But in any case, I haven't investigated it deeper in the hardware direction
after figuring out that drm_get_edid would retry its attempts for retreiving
the edid for 15 times, getting the -ENXIO error for all of them.

> Well, you could always do manual line testing of the I2C bus _before_
> calling drm_do_probe_ddc_edid()? And skip the call if the test fails
> (i.e. the bus isn't ready for use.) As said before, I am willing to
> export bit_test if it helps any. I've attached a patch doing exactly
> this. Let me know if you want me to commit it.

Yes, surely, I can do this. I did a similar test in the i915-specific patch,
checking if we can talk to i2c adapter pior to call drm_do_probe_ddc_edid,
but I thought that perhaps it would be easier for all the cards relying on
drm_get_edid to have a faster return path in case of problems.

I am fine with any solution, if this problem is happening to appear on i915
cards only, we could do this in our driver. It is that 15 attempts looks a
bit overkill.

> Your proposed patch is better than I first thought. I had forgotten
> that i2c-algo-bit tries up to 3 times to get a first ack from a slave.
> So if i2c_transfer returns -ENXIO, this means that i2c-algo-bit has
> already attempted 3 times to contact the slave, with no reply, so
> there's probably no point going on. A communication problem with a
> present slave would have returned a different error code.
> Without your patch, a missing slave would cause 3 * 5 = 15 retries,
> which is definitely too much.
> That being said, even then the whole probe sequence shouldn't exceed
> 10 ms, which I wouldn't expect a user to notice. The user-reported 4
> second delay when running xrandr can't be caused by this. 4 seconds for
> 15 attempts is 250 ms per attempt, this looks like a timeout due to
> non-functional bus, not a nack. Not that 250 ms is 1 jiffy for HZ=250,
> which I guess was what the reporting user was running. So even with
> your patch, there's still 750 ms to wait, I don't think this is
> acceptable. You really have to fix that I2C bus or skip probing it.

Yep, true. I've followed the easiest route so far - find out where the
initial problem happens, and attempt at solving it. This change in
drm_get_edid solves the delay at its origin, but we certainly could have
additional delays propagated somewhere else. I couldn't reproduce the
original reporter's huge delay, so I looked at what was within my reach.

In any case - looking at a faster way to leave the drm_do_probe_ddc_edid,
while also allowing a way for having a more detailed probing - would it be
an acceptable solution to add a:

MODULE_PARM(skip_unresponsive_edid, "Ignore outputs which do not provide
edid on first attempt");

and update the patch to use this option?

Eugeni Dodonov
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