[PATCH 3/3] drm/panfrost: Stay in the threaded MMU IRQ handler until we've handled all IRQs

[Boris Brezillon]

On Mon, 1 Feb 2021 13:24:00 +0000
Steven Price <steven.price at arm.com> wrote:

> On 01/02/2021 12:59, Boris Brezillon wrote:
> > On Mon, 1 Feb 2021 12:13:49 +0000
> > Steven Price <steven.price at arm.com> wrote:
> >   
> >> On 01/02/2021 08:21, Boris Brezillon wrote:  
> >>> Doing a hw-irq -> threaded-irq round-trip is counter-productive, stay
> >>> in the threaded irq handler as long as we can.
> >>>
> >>> Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>  
> >>
> >> Looks fine to me, but I'm interested to know if you actually saw a
> >> performance improvement. Back-to-back MMU faults should (hopefully) be
> >> fairly uncommon.  
> > 
> > I actually didn't check the perf improvement or the actual number of
> > back-to-back MMU faults, but
> > dEQP-GLES31.functional.draw_indirect.compute_interop.large.drawelements_combined_grid_1000x1000_drawcount_5000
> > seemed to generate a few of those, so I thought it'd be good to
> > optimize that case given how trivial it is.  
> 
> Fair enough! I was just a little concerned that Panfrost was somehow 
> provoking enough interrupts that this was a measurable performance 
> improvement.
> 
> I assume you'll push these to drm-misc-next (/fixes) as appropriate.

I think I'll just queue everything to misc-next and let the stable
maintainers backport the fixes (no one complained about this issue so
far). 

> 
> Thanks,
> 
> Steve
> 
> >>
> >> Regardless:
> >>
> >> Reviewed-by: Steven Price <steven.price at arm.com>
> >>  
> >>> ---
> >>>    drivers/gpu/drm/panfrost/panfrost_mmu.c | 7 +++++++
> >>>    1 file changed, 7 insertions(+)
> >>>
> >>> diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> >>> index 21e552d1ac71..65bc20628c4e 100644
> >>> --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> >>> +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> >>> @@ -580,6 +580,8 @@ static irqreturn_t panfrost_mmu_irq_handler_thread(int irq, void *data)
> >>>    	u32 status = mmu_read(pfdev, MMU_INT_RAWSTAT);
> >>>    	int i, ret;
> >>>    
> >>> +again:
> >>> +
> >>>    	for (i = 0; status; i++) {
> >>>    		u32 mask = BIT(i) | BIT(i + 16);
> >>>    		u64 addr;
> >>> @@ -628,6 +630,11 @@ static irqreturn_t panfrost_mmu_irq_handler_thread(int irq, void *data)
> >>>    		status &= ~mask;
> >>>    	}
> >>>    
> >>> +	/* If we received new MMU interrupts, process them before returning. */
> >>> +	status = mmu_read(pfdev, MMU_INT_RAWSTAT);
> >>> +	if (status)
> >>> +		goto again;
> >>> +
> >>>    	mmu_write(pfdev, MMU_INT_MASK, ~0);
> >>>    	return IRQ_HANDLED;
> >>>    };
> >>>      
> >>  
> >   
>