[PATCH v3 04/17] drm/panthor: Make sure reset requests in the post reset path are not lost

[Boris Brezillon <boris.brezillon@collabora.com>]

In theory, there might be MMU/FW faults happening after the FW has
successfully started, and since we clear the reset.pending bit after
panthor_fw_post_reset() has returned, there's a short window during
which a reset request can be ignored.

The other case is a reset condition in other subcomponents that would
not prevent the FW to boot, but given what's currently done in the
post_reset() helpers, I don't see how this can happen. Anyway, it's
probably safer to reset the pending bit just before the SOFT_RESET is
issued, so there's absolutely no timeframe during which a reset event
can be lost. The risk is an infinite reset loop if the reset condition
doesn't prevent the FW to boot, and keeps happening in subsequent resets.

Fixes: 5fe909cae118 ("drm/panthor: Add the device logical block")
Reported-by: sashiko-bot@kernel.org
Closes: https://sashiko.dev/#/patchset/20260804-panthor-unplug-fixes-v1-0-abbbd2d41b13@collabora.com?part=2
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
---
 drivers/gpu/drm/panthor/panthor_device.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/panthor/panthor_device.c b/drivers/gpu/drm/panthor/panthor_device.c
index 1a8f5ac24399..ffaff8c7088e 100644
--- a/drivers/gpu/drm/panthor/panthor_device.c
+++ b/drivers/gpu/drm/panthor/panthor_device.c
@@ -156,11 +156,18 @@ static void panthor_device_reset_work(struct work_struct *work)
 	panthor_sched_pre_reset(ptdev);
 	panthor_fw_pre_reset(ptdev, true);
 	panthor_mmu_pre_reset(ptdev);
+
+	/* Reset the pending bit just before the SOFT_RESET to catch any reset
+	 * condition happening in the post reset path. If we're in such a bad
+	 * state we can't even resume the FW, we will bail out and unplug
+	 * anyway, at which point the reset work is disabled, which should
+	 * prevent an infinite reset loop.
+	 */
+	atomic_set(&ptdev->reset.pending, 0);
 	panthor_hw_soft_reset(ptdev);
 	panthor_hw_l2_power_on(ptdev);
 	panthor_mmu_post_reset(ptdev);
 	ret = panthor_fw_post_reset(ptdev);
-	atomic_set(&ptdev->reset.pending, 0);
 	panthor_sched_post_reset(ptdev, ret != 0);
 	drm_dev_exit(cookie);
 

-- 
2.55.0