[PATCH v2] drm/panthor: dummy read to discard the stale first GPU_TIMESTAMP read

[ryan.zhang@oss.nxp.com]

From: Ryan Zhang <ryan.zhang@oss.nxp.com>

The first read of GPU_TIMESTAMP after the GPU has been idle can return a
value that lags the real counter, while a read issued immediately after is
accurate.

Userspace correlates this timestamp with the host clock, so a stale value
shows up as skew between the device and host time domains and makes
dEQP-VK.pipeline.monolithic.timestamp.calibrated.calibration_test fail
whenever a query happens to hit a large first-read error.

Signed-off-by: Ryan Zhang <ryan.zhang@oss.nxp.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
---
v2:
 - Added the r-b in commit msg.
 - Rebased on drm-misc-next to resolve conflict.
 - Moved the dummy read into panthor_gpu_get_timestamp() to align with
   the refactored timestamp query code on drm-misc-next.
Note: The original v1 patch was based on v6.18 and applied cleanly there.
Could we also port v1 into 6.18?

 drivers/gpu/drm/panthor/panthor_gpu.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/panthor/panthor_gpu.c b/drivers/gpu/drm/panthor/panthor_gpu.c
index a383b04f101e..b8df5663f04e 100644
--- a/drivers/gpu/drm/panthor/panthor_gpu.c
+++ b/drivers/gpu/drm/panthor/panthor_gpu.c
@@ -2,6 +2,7 @@
 /* Copyright 2018 Marty E. Plummer <hanetzer@startmail.com> */
 /* Copyright 2019 Linaro, Ltd., Rob Herring <robh@kernel.org> */
 /* Copyright 2019 Collabora ltd. */
+/* Copyright 2026 NXP */
 
 #include <linux/bitfield.h>
 #include <linux/bitmap.h>
@@ -470,6 +471,9 @@ void panthor_gpu_resume(struct panthor_device *ptdev)
 
 u64 panthor_gpu_get_timestamp(struct panthor_device *ptdev)
 {
+	/* Dummy read to avoid stale values. */
+	gpu_read64_counter(ptdev->gpu->iomem, GPU_TIMESTAMP);
+
 	return gpu_read64_counter(ptdev->gpu->iomem, GPU_TIMESTAMP);
 }
 
-- 
2.43.0