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

[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?

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?

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