linux-kernel - [PATCH v2 3/4] drm: xlnx: zynqmp_dpsub: Filter interrupts against mask

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Date: Thu, 18 Jan 2024 21:54:36 -0800
From: Anatoliy Klymenko <anatoliy.klymenko@....com>
To: <laurent.pinchart@...asonboard.com>, <maarten.lankhorst@...ux.intel.com>,
	<mripard@...nel.org>, <tzimmermann@...e.de>, <airlied@...il.com>,
	<daniel@...ll.ch>, <michal.simek@....com>, <dri-devel@...ts.freedesktop.org>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: [PATCH v2 3/4] drm: xlnx: zynqmp_dpsub: Filter interrupts against mask

Filter out status register against the interrupts' mask.

Some events are being reported via DP status register, even if
corresponding interrupts have been disabled. One instance of such event
leads to generation of VBLANK when the driver is in DRM bridge mode,
which in turn results in NULL pointer dereferencing. We should avoid
processing such events in an interrupt handler context.

This problem is less noticeable when the driver operates in DMA mode, as
in this case we have DRM CRTC object instantiated and DRM framework
simply discards unwanted VBLANKs in drm_handle_vblank().

Signed-off-by: Anatoliy Klymenko <anatoliy.klymenko@....com>
---
 drivers/gpu/drm/xlnx/zynqmp_dp.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/xlnx/zynqmp_dp.c b/drivers/gpu/drm/xlnx/zynqmp_dp.c
index d60b7431603f..9ed12244a429 100644
--- a/drivers/gpu/drm/xlnx/zynqmp_dp.c
+++ b/drivers/gpu/drm/xlnx/zynqmp_dp.c
@@ -1624,8 +1624,17 @@ static irqreturn_t zynqmp_dp_irq_handler(int irq, void *data)
 	u32 status, mask;
 
 	status = zynqmp_dp_read(dp, ZYNQMP_DP_INT_STATUS);
+	/* clear status register as soon as we read its content */
+	zynqmp_dp_write(dp, ZYNQMP_DP_INT_STATUS, status);
 	mask = zynqmp_dp_read(dp, ZYNQMP_DP_INT_MASK);
-	if (!(status & ~mask))
+
+	/*
+	 * Status register may report some events, which corresponding interrupts
+	 * have been disabled. Filter out those events against interrupts' mask.
+	 */
+	status &= ~mask;
+
+	if (!status)
 		return IRQ_NONE;
 
 	/* dbg for diagnostic, but not much that the driver can do */
@@ -1634,7 +1643,6 @@ static irqreturn_t zynqmp_dp_irq_handler(int irq, void *data)
 	if (status & ZYNQMP_DP_INT_CHBUF_OVERFLW_MASK)
 		dev_dbg_ratelimited(dp->dev, "overflow interrupt\n");
 
-	zynqmp_dp_write(dp, ZYNQMP_DP_INT_STATUS, status);
 
 	if (status & ZYNQMP_DP_INT_VBLANK_START)
 		zynqmp_dpsub_drm_handle_vblank(dp->dpsub);
-- 
2.25.1