6.12: i915 not allowed in combination with RT

6.12: i915 not allowed in combination with RT

[Florian Bezdeka]

Hi all,

the following commit (taken from 6.6-rt branch) resides in all stable-
rt branches but is not part of mainline 6.12 and above, which makes it
impossible to use i915 in combination with PREEMPT_RT enabled.

Is that by intention? Are there any known issues that makes it
necessary to disable i915 in combination with RT in current releases?

Best regards,
Florian

commit f532acfe5c63d53756c86e3d3fb1a51b02d29762
Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date:   Mon Feb 21 17:59:14 2022 +0100

    Revert "drm/i915: Depend on !PREEMPT_RT."
    
    Once the known issues are addressed, it should be safe to enable the
    driver.
    
    Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

diff --git a/drivers/gpu/drm/i915/Kconfig b/drivers/gpu/drm/i915/Kconfig
index 3efce05d7b57..392d51703096 100644
--- a/drivers/gpu/drm/i915/Kconfig
+++ b/drivers/gpu/drm/i915/Kconfig
@@ -3,7 +3,6 @@ config DRM_I915
	tristate "Intel 8xx/9xx/G3x/G4x/HD Graphics"
	depends on DRM
	depends on X86 && PCI
-	depends on !PREEMPT_RT
	select INTEL_GTT if X86
	select INTERVAL_TREE
	# we need shmfs for the swappable backing store, and in particular

Re: 6.12: i915 not allowed in combination with RT

[Luis Claudio R. Goncalves]

On Mon, Mar 10, 2025 at 12:31:21PM +0100, Florian Bezdeka wrote:
> Hi all,
> 
> the following commit (taken from 6.6-rt branch) resides in all stable-
> rt branches but is not part of mainline 6.12 and above, which makes it
> impossible to use i915 in combination with PREEMPT_RT enabled.
> 
> Is that by intention? Are there any known issues that makes it
> necessary to disable i915 in combination with RT in current releases?

There is a set of changes that were not merged along with the PREEMPT_RT
bits that are required to have i915 behaving properly (or as well as
possible) wrt PREEMPT_RT. While those changes are not carried by the driver
(i915) itself, they are available here:

* git tree:
    https://web.git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/tree/patches?h=linux-6.12.y-rt-patches&id=b7402ed929a05397b5c02802bdfaae40df4f2fc8

* patches:
    https://cdn.kernel.org/pub/linux/kernel/projects/rt/6.12/

Best regards,
Luis

> Best regards,
> Florian
> 
> commit f532acfe5c63d53756c86e3d3fb1a51b02d29762
> Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Date:   Mon Feb 21 17:59:14 2022 +0100
> 
>     Revert "drm/i915: Depend on !PREEMPT_RT."
>     
>     Once the known issues are addressed, it should be safe to enable the
>     driver.
>     
>     Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> 
> diff --git a/drivers/gpu/drm/i915/Kconfig b/drivers/gpu/drm/i915/Kconfig
> index 3efce05d7b57..392d51703096 100644
> --- a/drivers/gpu/drm/i915/Kconfig
> +++ b/drivers/gpu/drm/i915/Kconfig
> @@ -3,7 +3,6 @@ config DRM_I915
> 	tristate "Intel 8xx/9xx/G3x/G4x/HD Graphics"
> 	depends on DRM
> 	depends on X86 && PCI
> -	depends on !PREEMPT_RT
> 	select INTEL_GTT if X86
> 	select INTERVAL_TREE
> 	# we need shmfs for the swappable backing store, and in particular
> 
---end quoted text---

Re: 6.12: i915 not allowed in combination with RT

[Sebastian Andrzej Siewior]

On 2025-03-10 12:31:21 [+0100], Florian Bezdeka wrote:
> Hi all,
Hi,

> the following commit (taken from 6.6-rt branch) resides in all stable-
> rt branches but is not part of mainline 6.12 and above, which makes it
> impossible to use i915 in combination with PREEMPT_RT enabled.
> 
> Is that by intention? Are there any known issues that makes it
> necessary to disable i915 in combination with RT in current releases?

As noted by Luis the revert is only there because the previous patches
"fix" the i915 to the point where it seems to be working at least on my
box on people's that were complaining about things.

Those patches were not merged upstream by the maintainer therefore we
have the disable patch upstream while revert is only available in the RT
queue.

I have a few other things on my list before I go exploring the i915
dungeons again.

> Best regards,
> Florian

Sebastian