Re: [PATCH v2 06/11] drm/panthor: Prepare the scheduler logic for FW events in IRQ context

[Boris Brezillon <boris.brezillon@collabora.com>]

On Tue, 12 May 2026 14:04:43 -0700
Chia-I Wu <olvaffe@gmail.com> wrote:

> On Tue, May 12, 2026 at 5:14 AM Boris Brezillon
> <boris.brezillon@collabora.com> wrote:
> >
> > Add a specific spinlock for events processing, and force processing
> > of events in the panthor_sched_report_fw_events() path rather than
> > deferring it to a work item. We also fast-track fence signalling by
> > making the job completion logic IRQ-safe.
> >
> > Note that it requires changing a couple spin_lock() into
> > spin_lock_irqsave() when those are taken inside a events_lock section.
> >
> > Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> > ---
> >  drivers/gpu/drm/panthor/panthor_sched.c | 332 +++++++++++++++-----------------
> >  1 file changed, 155 insertions(+), 177 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/panthor/panthor_sched.c b/drivers/gpu/drm/panthor/panthor_sched.c
> > index 5b34032deff8..fbf76b59b7ef 100644
> > --- a/drivers/gpu/drm/panthor/panthor_sched.c
> > +++ b/drivers/gpu/drm/panthor/panthor_sched.c
> > @@ -177,18 +177,6 @@ struct panthor_scheduler {
> >          */
> >         struct work_struct sync_upd_work;
> >
> > -       /**
> > -        * @fw_events_work: Work used to process FW events outside the interrupt path.
> > -        *
> > -        * Even if the interrupt is threaded, we need any event processing
> > -        * that require taking the panthor_scheduler::lock to be processed
> > -        * outside the interrupt path so we don't block the tick logic when
> > -        * it calls panthor_fw_{csg,wait}_wait_acks(). Since most of the
> > -        * event processing requires taking this lock, we just delegate all
> > -        * FW event processing to the scheduler workqueue.
> > -        */
> > -       struct work_struct fw_events_work;
> > -
> >         /**
> >          * @fw_events: Bitmask encoding pending FW events.
> >          */  
> If we process all fw events in the irq context, we can remove
> fw_events as well. More on this below.

Oops, forgot to remove this field, indeed.

> > @@ -254,6 +242,15 @@ struct panthor_scheduler {
> >                 struct list_head waiting;
> >         } groups;
> >
> > +       /**
> > +        * @events_lock: Lock taken when processing events.
> > +        *
> > +        * This also needs to be taken when csg_slots are updated, to make sure
> > +        * the event processing logic doesn't touch groups that have left the CSG
> > +        * slot.
> > +        */
> > +       spinlock_t events_lock;
> > +
> >         /**
> >          * @csg_slots: FW command stream group slots.  
> It looks like read access can use either lock (process context) or
> events_lock (irq context), while write access must use events_lock
> (process context). Can we put that into the comment, or if makes
> sense, enforce that with accessor functions?

You're right. I'll mention that updates to csg_slots[] must be done
with both the ::lock and ::events_lock held, while reads can be done
with any of them held.

> 
> 
> >          */
> > @@ -676,9 +673,6 @@ struct panthor_group {
> >          */
> >         struct panthor_kernel_bo *protm_suspend_buf;
> >
> > -       /** @sync_upd_work: Work used to check/signal job fences. */
> > -       struct work_struct sync_upd_work;
> > -  
> Can we make this a preparatory commit, where group_sync_upd_work is
> replaced by group_check_job_completion?

I'll try to split that up.

> 
> Multiple things happen in this commit. I try to identify things that
> can be separate commits. If this does not make sense, feel free to
> ignore.
> 
> >         /** @tiler_oom_work: Work used to process tiler OOM events happening on this group. */
> >         struct work_struct tiler_oom_work;
> >

[...]

> >  /**
> >   * panthor_sched_report_fw_events() - Report FW events to the scheduler.
> >   * @ptdev: Device.
> > @@ -1902,8 +1953,19 @@ void panthor_sched_report_fw_events(struct panthor_device *ptdev, u32 events)  
> This can be renamed to panthor_sched_handle_fw_events.

It's not quite handling events though. For most of them, it's really
just deferring the processing to work items, SYNC_UPDATE is the
exception.

> 
> >         if (!ptdev->scheduler)
> >                 return;
> >
> > -       atomic_or(events, &ptdev->scheduler->fw_events);
> > -       sched_queue_work(ptdev->scheduler, fw_events);
> > +       guard(spinlock_irqsave)(&ptdev->scheduler->events_lock);
> > +
> > +       if (events & JOB_INT_GLOBAL_IF) {
> > +               sched_process_global_irq_locked(ptdev);
> > +               events &= ~JOB_INT_GLOBAL_IF;
> > +       }
> > +
> > +       while (events) {
> > +               u32 csg_id = ffs(events) - 1;
> > +
> > +               sched_process_csg_irq_locked(ptdev, csg_id);
> > +               events &= ~BIT(csg_id);
> > +       }  
> This handles all fw events in the irq context. Are there concerns that
> it may take too long? I might be wrong, but it seems possible to
> handle only CSG_SYNC_UPDATE and defer the rest as before.

I started with just the SYNC_UPDATE processing done in the hard-irq
context, but after auditing the other stuff done in the handler, I
realized it's basically just deferring all actual processing to work
items. Yes, there's the overhead of demuxing the events from the
ack/req regs, but part of this is already done to get to SYNC_UPDATE
anyway, so at this point we're probably better off demuxing everything
and scheduling works for all kind of events.

I also compared the perfs between the two approaches (though I didn't
do as much testing as I did with the new version, so I might have
missed something), and it didn't seem to matter at all, because the
interrupts we receive the most are SYNC_UPDATE and IDLE events, and
those are at the same level.