On 26/09/2023 11:26, Andi Shyti wrote:
Hi Tvrtko, On Tue, Sep 26, 2023 at 11:08:55AM +0100, Tvrtko Ursulin wrote:From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx> 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@xxxxxxxxx> Cc: Chris Wilson <chris.p.wilson@xxxxxxxxx> Cc: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx> Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Cc: Andi Shyti <andi.shyti@xxxxxxxxxxxxxxx>Reviewed-by: Andi Shyti <andi.shyti@xxxxxxxxxxxxxxx>
Thank you, pushed to drm-intel-gt-next! Regards, Tvrtko
