[PATCH 1/7] drm/tidss: Fix issue in irq handling causing irq-flood issue

[Tomi Valkeinen]

Hi,

On 24/11/2024 19:18, Aradhya Bhatia wrote:
> Hi Tomi, Devarsh,
> 
> On 10/21/24 19:37, Tomi Valkeinen wrote:
>> It has been observed that sometimes DSS will trigger an interrupt and
>> the top level interrupt (DISPC_IRQSTATUS) is not zero, but the VP and
>> VID level interrupt-statuses are zero.
> 
> Does this mean that there was a legitimate interrupt that potentially
> went unrecognized? Or that there was a, for the lack of a better word,
> fake interrupt trigger that doesn't need handling but just clearing?

I don't have an answer to that. I haven't been able to trigger this 
issue, and I guess it's difficult to say for certain in any case.

My guess is that it's some kind of race issue either in the HW or the 
combination of HW+SW.

>> As the top level irqstatus is supposed to tell whether we have VP/VID
>> interrupts, the thinking of the driver authors was that this particular
>> case could never happen. Thus the driver only clears the DISPC_IRQSTATUS
>> bits which has corresponding interrupts in VP/VID status. So when this
>> issue happens, the driver will not clear DISPC_IRQSTATUS, and we get an
>> interrupt flood.
>>
>> It is unclear why the issue happens. It could be a race issue in the
>> driver, but no such race has been found. It could also be an issue with
>> the HW. However a similar case can be easily triggered by manually
>> writing to DISPC_IRQSTATUS_RAW. This will forcibly set a bit in the
>> DISPC_IRQSTATUS and trigger an interrupt, and as the driver never clears
>> the bit, we get an interrupt flood.
>>
>> To fix the issue, always clear DISPC_IRQSTATUS. The concern with this
>> solution is that if the top level irqstatus is the one that triggers the
>> interrupt, always clearing DISPC_IRQSTATUS might leave some interrupts
>> unhandled if VP/VID interrupt statuses have bits set. However, testing
>> shows that if any of the irqstatuses is set (i.e. even if
>> DISPC_IRQSTATUS == 0, but a VID irqstatus has a bit set), we will get an
>> interrupt.
> 
> Does this mean if VID/VP irqstatus has been set right around the time
> the equivalent DISPC_IRQSTATUS bit is being cleared, the equivalent
> DISPC_IRQSTATUS bit is going to get set again, and make the driver
> handle the event as we expect it to?

(If I recall right) no, DISPC_IRQSTATUS won't be set. But it doesn't 
matter, the interrupt will be triggered anyway, and the driver will 
handle the interrupt.

>>
>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen at ideasonboard.com>
>> Co-developed-by: Bin Liu <b-liu at ti.com>
>> Signed-off-by: Bin Liu <b-liu at ti.com>
>> Co-developed-by: Devarsh Thakkar <devarsht at ti.com>
>> Signed-off-by: Devarsh Thakkar <devarsht at ti.com>
>> Co-developed-by: Jonathan Cormier <jcormier at criticallink.com>
>> Signed-off-by: Jonathan Cormier <jcormier at criticallink.com>
>> Fixes: 32a1795f57ee ("drm/tidss: New driver for TI Keystone platform Display SubSystem")
>> Cc: stable at vger.kernel.org
>> ---
>>   drivers/gpu/drm/tidss/tidss_dispc.c | 12 ++++--------
>>   1 file changed, 4 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/tidss/tidss_dispc.c b/drivers/gpu/drm/tidss/tidss_dispc.c
>> index 1ad711f8d2a8..f81111067578 100644
>> --- a/drivers/gpu/drm/tidss/tidss_dispc.c
>> +++ b/drivers/gpu/drm/tidss/tidss_dispc.c
>> @@ -780,24 +780,20 @@ static
>>   void dispc_k3_clear_irqstatus(struct dispc_device *dispc, dispc_irq_t clearmask)
>>   {
>>   	unsigned int i;
>> -	u32 top_clear = 0;
>>   
>>   	for (i = 0; i < dispc->feat->num_vps; ++i) {
>> -		if (clearmask & DSS_IRQ_VP_MASK(i)) {
>> +		if (clearmask & DSS_IRQ_VP_MASK(i))
>>   			dispc_k3_vp_write_irqstatus(dispc, i, clearmask);
>> -			top_clear |= BIT(i);
>> -		}
>>   	}
>>   	for (i = 0; i < dispc->feat->num_planes; ++i) {
>> -		if (clearmask & DSS_IRQ_PLANE_MASK(i)) {
>> +		if (clearmask & DSS_IRQ_PLANE_MASK(i))
>>   			dispc_k3_vid_write_irqstatus(dispc, i, clearmask);
>> -			top_clear |= BIT(4 + i);
>> -		}
>>   	}
> 
> nit: Maybe these for-loop braces could be dropped as well.

I like to have braces if there are multiple lines under it.

> Otherwise, LGTM,
> 
> Reviewed-by: Aradhya Bhatia <aradhya.bhatia at linux.dev>

Thanks!

  Tomi