Re: [PATCH v2 4/5] drm/panthor: Let IRQ handlers clear the interrupts themselves

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

On Wed, 2 Apr 2025 11:20:17 +0100
Steven Price <steven.price@arm.com> wrote:

> On 01/04/2025 19:23, Boris Brezillon wrote:
> > MMU handler needs to be in control of the job interrupt clears because
> > clearing the interrupt also unblocks the writer/reader that triggered
> > the fault, and we don't want it to be unblocked until we've had a chance
> > to process the IRQ.
> > 
> > Since clearing the clearing is just one line, let's make it explicit
> > instead of doing it in the generic code path.
> > 
> > Changes in v2:
> > - Move the MMU_INT_CLEAR around
> > 
> > Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>  
> 
> Look at this made me look at panthor_mmu_irq_handler() and I can't
> understand this bit of code:
> 
> > static void panthor_mmu_irq_handler(struct panthor_device *ptdev, u32 status)
> > {
> > 	bool has_unhandled_faults = false;
> > 
> > 	status = panthor_mmu_fault_mask(ptdev, status);
> > 	while (status) {
> > 		u32 as = ffs(status | (status >> 16)) - 1;  
> 
> panthor_mmu_fault_mask() masks status with GENMASK(15, 0), so AFAICT
> (status >> 16) is always 0.
> 
> Which isn't a big issue, other than you are now only clearing IRQs which
> are recognised by the loop.
> 
> So this changes the behaviour to not clear interrupt bits 16-31 (the
> COMMAND_COMPLETED bits). I believe we don't actually care about them and
> AFAICT we always mask those interrupt bits in the IRQ mask - so it
> should be safe...
> 
> TLDR; I think this change is fine, but I'm not sure if it's what you
> intended and it would be good to include something in the commit message
> about the functional change.

Didn't realize it was changing the behavior, but I was aware we were
only unmasking fault interrupts, and I left COMPLETED ones uncleared on
purpose. I'll add a comment about that in the commit message.

> 
> Thanks,
> Steve
> 
> > ---
> >  drivers/gpu/drm/panthor/panthor_device.h | 2 --
> >  drivers/gpu/drm/panthor/panthor_fw.c     | 2 ++
> >  drivers/gpu/drm/panthor/panthor_gpu.c    | 2 ++
> >  drivers/gpu/drm/panthor/panthor_mmu.c    | 5 +++++
> >  4 files changed, 9 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/panthor/panthor_device.h b/drivers/gpu/drm/panthor/panthor_device.h
> > index da6574021664..4c27b6d85f46 100644
> > --- a/drivers/gpu/drm/panthor/panthor_device.h
> > +++ b/drivers/gpu/drm/panthor/panthor_device.h
> > @@ -383,8 +383,6 @@ static irqreturn_t panthor_ ## __name ## _irq_threaded_handler(int irq, void *da
> >  		if (!status)									\
> >  			break;									\
> >  												\
> > -		gpu_write(ptdev, __reg_prefix ## _INT_CLEAR, status);				\
> > -												\
> >  		__handler(ptdev, status);							\
> >  		ret = IRQ_HANDLED;								\
> >  	}											\
> > diff --git a/drivers/gpu/drm/panthor/panthor_fw.c b/drivers/gpu/drm/panthor/panthor_fw.c
> > index 0f52766a3120..446bb377b953 100644
> > --- a/drivers/gpu/drm/panthor/panthor_fw.c
> > +++ b/drivers/gpu/drm/panthor/panthor_fw.c
> > @@ -1008,6 +1008,8 @@ static void panthor_fw_init_global_iface(struct panthor_device *ptdev)
> >  
> >  static void panthor_job_irq_handler(struct panthor_device *ptdev, u32 status)
> >  {
> > +	gpu_write(ptdev, JOB_INT_CLEAR, status);
> > +
> >  	if (!ptdev->fw->booted && (status & JOB_INT_GLOBAL_IF))
> >  		ptdev->fw->booted = true;
> >  
> > diff --git a/drivers/gpu/drm/panthor/panthor_gpu.c b/drivers/gpu/drm/panthor/panthor_gpu.c
> > index 671049020afa..32d678a0114e 100644
> > --- a/drivers/gpu/drm/panthor/panthor_gpu.c
> > +++ b/drivers/gpu/drm/panthor/panthor_gpu.c
> > @@ -150,6 +150,8 @@ static void panthor_gpu_init_info(struct panthor_device *ptdev)
> >  
> >  static void panthor_gpu_irq_handler(struct panthor_device *ptdev, u32 status)
> >  {
> > +	gpu_write(ptdev, GPU_INT_CLEAR, status);
> > +
> >  	if (status & GPU_IRQ_FAULT) {
> >  		u32 fault_status = gpu_read(ptdev, GPU_FAULT_STATUS);
> >  		u64 address = ((u64)gpu_read(ptdev, GPU_FAULT_ADDR_HI) << 32) |
> > diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
> > index 7cca97d298ea..4ac95a31907d 100644
> > --- a/drivers/gpu/drm/panthor/panthor_mmu.c
> > +++ b/drivers/gpu/drm/panthor/panthor_mmu.c
> > @@ -1710,6 +1710,11 @@ static void panthor_mmu_irq_handler(struct panthor_device *ptdev, u32 status)
> >  			access_type, access_type_name(ptdev, fault_status),
> >  			source_id);
> >  
> > +		/* We don't handle VM faults at the moment, so let's just clear the
> > +		 * interrupt and let the writer/reader crash.
> > +		 */
> > +		gpu_write(ptdev, MMU_INT_CLEAR, mask);
> > +
> >  		/* Ignore MMU interrupts on this AS until it's been
> >  		 * re-enabled.
> >  		 */  
>