[PATCH] 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>
---
 drivers/gpu/drm/panthor/panthor_drv.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/panthor/panthor_drv.c b/drivers/gpu/drm/panthor/panthor_drv.c
index 720bf889096d..956a147dc282 100644
--- a/drivers/gpu/drm/panthor/panthor_drv.c
+++ b/drivers/gpu/drm/panthor/panthor_drv.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 */
 
 #ifdef CONFIG_ARM_ARCH_TIMER
 #include <asm/arch_timer.h>
@@ -772,6 +773,10 @@ static int panthor_query_timestamp_info(struct panthor_device *ptdev,
 #else
 	arg->timestamp_frequency = 0;
 #endif
+
+	/* Dummy read to avoid stale values. */
+	gpu_read64_counter(ptdev, GPU_TIMESTAMP);
+
 	arg->current_timestamp = gpu_read64_counter(ptdev, GPU_TIMESTAMP);
 	arg->timestamp_offset = gpu_read64(ptdev, GPU_TIMESTAMP_OFFSET);
 
-- 
2.43.0