On 7/20/25 17:45, Alex Deucher wrote:
DP (and all of its variants, eDP, USB-C thunderbolt) doesn't actually
use i2c directly. It's aux; you can do i2c over aux, but in the case
of MST, it's more like a network for displays so naively messing with
i2c buses from userspace won't do what you expect. For MST, you have
a single set of i2c/aux pins for the connector which may have several
monitors on the other end.
DP is a two way communications channel. You may have the driver
training a link or communicating with other devices on the DP network
(MST hubs, monitors, etc.). You can also get requests from the
monitor to the driver via hpd interrupts. Many of these processes do
not do well if interrupted.
Alex
I get that this is a part of a very complicated protocol. I am still
irritated that the points you mention are relevant from the perspective
of userspace. From my perspective there is no expectation that the
kernel should just interrupt ongoing procedures when I access an i2c
link. I am happy to wait for the kernel to schedule the operation for
when it is convenient. After all the point of abstraction is not having
to worry about the layers underneath. And if the drm device exposes an
i2c device that I access it is the job of the drm driver to handle how
and when that transmission takes place.
Also I would like to point out that the bug I am experiencing does not
materialize in case of any external displays attached via an MST hub. It
happens with the internal display even when nothing is attached. The
point about MST Hubs is only relevant because those i2c interface can
not be matched via udev to the corresponding display. Though the can be
matched when reading edid from the device. Those interfaces then work
just fine when I use ddc to read/set monitor inputs. So those points do
not seem to really be relevant in case of the screen freezing trigger.
Anyway that is just my thoughts on the matter. I'll look into writing a
workaround to maybe avoid some i2c devices that could be problematic.
And there still might be a relation to the other screen freezing issues.
Felix
