Re: Regression: DDC I2C Display Freezing for internal displays

[Mario Limonciello <superm1@kernel.org>]

On 7/17/2025 2:42 PM, Felix Richter wrote:
> Hi,
> 
> just tested that this Bug still exists in kernel version 6.16-rc6. The 
> example trigger from my previous mail still works triggering screen 
> freezing shortly after invocation.
> 
> I also learned that setting kernel param `amdgpu.dcdebugmask=0x10` works 
> as a workaround.
> 
> Kind regards,
> Felix Richter
> 
> On 4/22/25 21:44, Felix Richter wrote:
>> Hi,
>>
>> it has been quite at while since I first started experiencing this 
>> particular bug I am about to describe. Suffice it to say during my 
>> Easter holiday I finally had the time to dig into it. It all started 
>> with an update of linux LTS from 6.6 to 6.12.
>>
>> I am a user of the sway tiling window manager and have written a small 
>> utility to manage my display configuration across different setups. 
>> With the added twist that I wrote some code to determine which monitor 
>> inputs is currently in use using the monitor command interface. Anyway 
>> the interesting detail here is that, starting with kernel 6.12 I 
>> started running into the following problem. With my display management 
>> daemon running and attaching my Laptop to an external display my 
>> internal display would just freeze with no way to bring it back apart 
>> from power cycling the entire device. When my management daemon was 
>> not running this would not happen, I would then need to manually 
>> configure my display setup. Further investigation into the what is 
>> triggering the display freeze lead me into the part of the code where 
>> I am enumerating attached displays and am trying to match `i2c` 
>> devices to their corresponding display.
>>
>> To get more specific the procedure is as follows, using udev enumerate 
>> all `i2c` busses and filter them base on some heuristics like device 
>> name and devices with parent devices drm / graphics device. Sadly this 
>> is not quite enough to already match an `i2c` command interface to the 
>> corresponding monitor, in many cases it is required to manually read 
>> the EDID information via the i2c interface and compare it to the known 
>> attached displays to get the match. And this is where the trigger for 
>> the display freeze is to be found.
>>
>> Here is the output when scanning sysfs for my internal laptop display:
>> ```
>> # ls -al /sys/devices/pci0000:00/0000:00:08.1/0000:04:00.0/drm/card1/ 
>> card1-eDP-1
>> total 0
>> drwxr-xr-x  6 root root    0 22. Apr 18:07 .
>> drwxr-xr-x 11 root root    0 22. Apr 18:07 ..
>> drwxr-xr-x  3 root root    0 22. Apr 18:07 amdgpu_bl1
>> -r--r--r--  1 root root 4096 22. Apr 18:07 connector_id
>> lrwxrwxrwx  1 root root    0 22. Apr 18:07 ddc -> ../../../i2c-3
>> lrwxrwxrwx  1 root root    0 22. Apr 18:07 device -> ../../card1
>> -r--r--r--  1 root root 4096 22. Apr 18:07 dpms
>> drwxr-xr-x  3 root root    0 22. Apr 18:07 drm_dp_aux0
>> -r--r--r--  1 root root    0 22. Apr 18:07 edid
>> -r--r--r--  1 root root 4096 22. Apr 18:07 enabled
>> drwxr-xr-x  4 root root    0 22. Apr 18:07 i2c-11
>> -r--r--r--  1 root root 4096 22. Apr 18:07 modes
>> drwxr-xr-x  2 root root    0 22. Apr 18:07 power
>> -rw-r--r--  1 root root 4096 22. Apr 18:07 status
>> lrwxrwxrwx  1 root root    0 22. Apr 18:07 subsystem - 
>> > ../../../../../../../class/drm
>> -rw-r--r--  1 root root 4096 22. Apr 18:07 uevent
>> ```
>>
>> As can be seen there are two i2c devices present, i2c-3 (as ddc 
>> symlink) and i2c-11. Now from the perspective of udev i2c-11 has the 
>> parent set to card1-eDP-1 while i2c-3 has the parent set to the drm 
>> device itself. More importantly I can not rule out i2c-3 as a valid 
>> command interface because in some cases valid command channels are 
>> never assigned to the corresponding display output directly but only 
>> live directly on the drm device, this is especially true when monitors 
>> are not attached directly but via a docking station. So I do have to 
>> look at each i2c device on its own. The freeze is trigged by trying to 
>> read edid from i2c-3: This is the code snipped I used to trigger the 
>> bug: https://github.com/ju6ge/libmonitor/ 
>> blob/918b2543eafb96aca29f66debc70fd18fa21ee11/examples/via-i2c-dev.rs 
>> (adjusted target i2c interface accordingly). To be absolutely clear 
>> this is not the i2c device that is expected to work in every case of 
>> trying this with kernel 6.6 to 6.12 I get the following error message: 
>> DdcError(CommunicationError(ReceiveError(EIO: I/O error))). That is 
>> expected internal laptop displays do not support the command interface 
>> in most cases anyway. But what I do not expect to happen is that my 
>> Laptop screen freezes! And since this did not happen with kernel 6.6 
>> but started happening with 6.12 this seems to be a software issue and 
>> with that a regression!
>>
>> Next I bisected the kernel from 6.6 to 6.12 to determine when this 
>> regression was introduced. I attached the full bisect log to the email ;)
>>
>> The offending commit seems to be:
>>
>> [58a261bfc96763a851cb48b203ed57da37e157b8] drm/amd/display: use a more 
>> lax vblank enable policy for older ASICs
>>
>> Since this is quite a small commit I validated this by reverting the 
>> changes on a newer kernel version (patch attached as well). Testing 
>> actually shows that reverting the change resolves the screen freezing 
>> behavior for me.
>>
>> Now I am not deep enough into graphics drivers to claim that just 
>> reverting the commit should be considered a valid fix. Just that the 
>> change is definitely responsible for the screen freezing now as 
>> apposed to before.
>>
>> So what should be done here? I can validate any other suggested fixes 
>> against my setup or provide more information if need be.
>>
>> Kind regards,
>> Felix Richter
>>
>> #regzbot introduced: v6.6..v6.12
> 
> 

At least to me, this issue sounds like a case that multiple entities are 
trying to communicate with the panel at the same time.

By setting dcdebugmask=0x10 what you're essentially doing is stopping 
the display hardware from trying to put the panel into PSR.  So there is 
"less" I2C traffic to fight with.

*Why* are you using I2C to read the EDID like this?  Could you instead 
use /sys/class/drm/cardX-inputY/edid?  Or even better - can you use the 
information from drm_info to make decisions?

I think the less I2C traffic done directly from userspace the better 
when it comes to synchronization issues..