Re: [PATCH v7.2-rc6] drm/amd/display: fix usage of DC_FPU_{BEGIN,END} with PREEMPT_RT

[Bert Karwatzki]

Am Donnerstag, dem 27.08.2026 um 14:49 +0200 schrieb Sebastian Andrzej Siewior:
> On 2026-08-07 14:49:42 [+0200], Bert Karwatzki wrote:
> > On PREEMPT_RT kernels kvzalloc_obj() can sleep because spin_lock is
> > converted to rt_mutex. dc_create_plane_state() can be called while
> > inside an FPU-guarded region, resuling in "scheduling while atomic"
> > errors on PREEMPT_RT kernels.
> >  Fix this by calling kvzalloc_obj() with DC_RUN_WITH_PREEMPTION_ENABLED().
> > Also fix the error path in dc_create_stream_for_sink().
> > 
> > Fixes: 3539437f354b ("drm/amd/display: Move FPU Guards From DML To DC - Part 1")
> > Link: https://lore.kernel.org/lkml/20260723123449.6494-1-spasswolf@xxxxxx/
> > Signed-off-by: Bert Karwatzki <spasswolf@xxxxxx>
> 
> Thank you Bert. Got this somewhere in the meantime?

No, this is seems to be stuck somewhere ...

> 
> Could the FPU regions with disabled preemption be limited to where we
> have actually have FPU usage in way that you don't have to worry when it
> is needed to use DC_RUN_WITH_PREEMPTION_ENABLED() and when not? Also the
> dc_fpu_begin()/ end() can nest and if they do the usage of
> DC_RUN_WITH_PREEMPTION_ENABLED() is futile, isn't it?
> 

I'm not very familiar with the amd display engine, and it's a lot of code,
but there I think there's some room for improvement, e.g. dml2_destroy():

dml2_destroy is called from dc_state_free() from within DC_FP_{START,END}()
and then basically just calls vfree with DC_RUN_WITH_PREEMPTION_ENABLED().
(directly or through dml21_destroy()). Here both the DC_FP_*() tags and
DC_RUN_WITH_PREEMPTION_ENABLED() could be dropped I think (not tested, yet)

Regarding nesting DC_FP_*()s: When I was searching for the cause of the
kernel panics I monitored every DC_FP_{START,END}() with printk(), and I
don't think I encountered nesting (the logs are lost unfortunately)


> The first usage kernel_fpu_begin() saves the FPU state to the user task.
> kernel_fpu_end() does not restore it. Therefore the subsequent
> invocation of kernel_fpu_begin() is cheaper.
> 
> Sebastian

Bert Karwatzki