Re: [PATCH] drm/msm: split DPU core IRQ handler under CONFIG_PREEMPT_RT

[Vishnu Saini]

On Thu, Sep 10, 2026 at 08:47:23AM +0200, Sebastian Andrzej Siewior wrote:
> On 2026-09-09 14:22:16 [+0530], vishnu.saini@xxxxxxxxxxxxxxxx wrote:
> > From: Naman S Thaker <namathak@xxxxxxxxxxxxxxxx>
> > 
> > On a PREEMPT_RT kernel, dpu_core_irq() runs as a true hardirq handler,
> 
> *why* is this the case. The code you replaces adds some ifdefs around
> request_irq() with 0 as flags. This does not make it run has hardirq.
You are right, the statement is incorrect, i will update the commit msg.
On RT kernel, the dpu_core_irq() runs in threaded context by default.
This behavior is changed in the commit by calling devm_request_threaded_irq with
IRQ_ONESHOT flag only for RT kernel using the ifdefs.

The irq is now split into two parts for RT,
1: primary handler which does not use sleeping locks, and
2: secondary threaded handler which can use sleepable locks.
For normal kernels, the request_irq() with 0 flag is still used as-is.

 
> > but it dispatches per-encoder callbacks that take sleepable locks
> > (spinlock_t becomes an rt_mutex on RT, and some DRM-core locks reached
> > through vblank/CRC/writeback handling are sleepable as well). Sleeping
> > inside a hardirq handler is not allowed and eventually crashes the
> > display, which is what happens after running GLMark2 for a while.
> 
> That is correct. That is the irq handler are threaded by default and
> only non-threaded if explicitly requested.
> 
> I suggest to stick with non-threaded by default.
> There is only one request_threaded_irq() as far as I can tell and this
> msm_dp_display_request_irq():
> 
> |         rc = devm_request_threaded_irq(&pdev->dev, dp->irq,
> |                                        msm_dp_display_irq_handler,
> |                                        msm_dp_display_irq_thread,
> |                                        IRQ_TYPE_LEVEL_HIGH,
> |                                        "dp_display_isr", dp);
> 
> and its primary handler will be threaded on PREEMPT_RT, too. So you end
> up with two threads here.
You are correct about the primary handler in msm_dp_display_request_irq() being threaded.
But the request_threaded_irq function in msm_dp_display_request_irq() is not modified in this commit.
The issue fixed by the current commit relates to msm_mdss_irq crash stack below.

[  152.108462] Call trace:
[  152.108465]  show_stack+0x18/0x30 (C)
[  152.108477]  dump_stack_lvl+0x60/0x80
[  152.108484]  dump_stack+0x18/0x24
[  152.108489]  __report_bad_irq+0x4c/0xec
[  152.108496]  note_interrupt+0x340/0x394
[  152.108502]  handle_irq_event+0x94/0xa0
[  152.108508]  handle_level_irq+0xd8/0x16c
[  152.108513]  handle_irq_desc+0x34/0x5c
[  152.108518]  generic_handle_domain_irq+0x1c/0x28
[  152.108522]  msm_mdss_irq+0x68/0x144 [msm]
[  152.108654]  handle_irq_desc+0x34/0x5c
[  152.108660]  generic_handle_domain_irq+0x1c/0x28
[  152.108664]  gic_handle_irq+0x4c/0x140
[  152.108669]  call_on_irq_stack+0x30/0x48
[  152.108673]  do_interrupt_handler+0x80/0x84
[  152.108678]  el1_interrupt+0x38/0x58
[  152.108685]  el1h_64_irq_handler+0x18/0x24
[  152.108690]  el1h_64_irq+0x70/0x74
[  152.108694]  __schedule+0x6c/0xc9c (P)
[  152.108701]  schedule_idle+0x20/0x40
[  152.108705]  do_idle+0x17c/0x2c0
[  152.108710]  cpu_startup_entry+0x38/0x40
[  152.108714]  rest_init+0xd8/0xe0
[  152.108719]  console_on_rootfs+0x0/0x6c
[  152.108726]  __primary_switched+0x88/0x90
[  152.108732] handlers:
[  152.108734] [<000000000cabe59c>] irq_default_primary_handler threaded [<00000000779fd542>] msm_irq [msm]
[  152.108859] Disabling IRQ #246

> Sebastian