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

[Aradhya Bhatia <aradhya.bhatia@linux.dev>]

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?

> 
> 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?

> 
> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> Co-developed-by: Bin Liu <b-liu@ti.com>
> Signed-off-by: Bin Liu <b-liu@ti.com>
> Co-developed-by: Devarsh Thakkar <devarsht@ti.com>
> Signed-off-by: Devarsh Thakkar <devarsht@ti.com>
> Co-developed-by: Jonathan Cormier <jcormier@criticallink.com>
> Signed-off-by: Jonathan Cormier <jcormier@criticallink.com>
> Fixes: 32a1795f57ee ("drm/tidss: New driver for TI Keystone platform Display SubSystem")
> Cc: stable@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.

Otherwise, LGTM,

Reviewed-by: Aradhya Bhatia <aradhya.bhatia@linux.dev>

Regards
Aradhya


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