drm/tidss: Fix issue in irq handling causing irq-flood issue

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.

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.

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: 32a1795 ("drm/tidss: New driver for TI Keystone platform Display SubSystem")
Cc: stable@vger.kernel.org
Tested-by: Jonathan Cormier <jcormier@criticallink.com>
Reviewed-by: Aradhya Bhatia <aradhya.bhatia@linux.dev>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241021-tidss-irq-fix-v1-1-82ddaec94e4a@ideasonboard.com

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);
-		}
 	}
 	if (dispc->feat->subrev == DISPC_K2G)
 		return;
 
-	dispc_write(dispc, DISPC_IRQSTATUS, top_clear);
+	/* always clear the top level irqstatus */
+	dispc_write(dispc, DISPC_IRQSTATUS, dispc_read(dispc, DISPC_IRQSTATUS));
 
 	/* Flush posted writes */
 	dispc_read(dispc, DISPC_IRQSTATUS);