[PATCH v2] drm/i915: Do not disable preemption for resets

[Andi Shyti]

Hi Tvrtko,

On Tue, Sep 26, 2023 at 11:08:55AM +0100, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 
> Commit ade8a0f59844 ("drm/i915: Make all GPU resets atomic") added a
> preempt disable section over the hardware reset callback to prepare the
> driver for being able to reset from atomic contexts.
> 
> In retrospect I can see that the work item at a time was about removing
> the struct mutex from the reset path. Code base also briefly entertained
> the idea of doing the reset under stop_machine in order to serialize
> userspace mmap and temporary glitch in the fence registers (see
> eb8d0f5af4ec ("drm/i915: Remove GPU reset dependence on struct_mutex"),
> but that never materialized and was soon removed in 2caffbf11762
> ("drm/i915: Revoke mmaps and prevent access to fence registers across
> reset") and replaced with a SRCU based solution.
> 
> As such, as far as I can see, today we still have a requirement that
> resets must not sleep (invoked from submission tasklets), but no need to
> support invoking them from a truly atomic context.
> 
> Given that the preemption section is problematic on RT kernels, since the
> uncore lock becomes a sleeping lock and so is invalid in such section,
> lets try and remove it. Potential downside is that our short waits on GPU
> to complete the reset may get extended if CPU scheduling interferes, but
> in practice that probably isn't a deal breaker.
> 
> In terms of mechanics, since the preemption disabled block is being
> removed we just need to replace a few of the wait_for_atomic macros into
> busy looping versions which will work (and not complain) when called from
> non-atomic sections.
> 
> v2:
>  * Fix timeouts which are now in us. (Andi)
>  * Update one comment as a drive by. (Andi)
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Chris Wilson <chris.p.wilson at intel.com>
> Cc: Paul Gortmaker <paul.gortmaker at windriver.com>
> Cc: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
> Cc: Andi Shyti <andi.shyti at linux.intel.com>

Reviewed-by: Andi Shyti <andi.shyti at linux.intel.com> 

Thanks,
Andi