[PATCH] drm/tegra: sor: Grab runtime PM reference across reset

[Thierry Reding]

From: Thierry Reding <treding at nvidia.com>

The SOR resets are exclusively shared with the SOR power domain. This
means that exclusive access can only be granted temporarily and in order
for that to work, a rigorous sequence must be observed. To ensure that a
single consumer gets exclusive access to a reset, each consumer must
implement a rigorous protocol using the reset_control_acquire() and
reset_control_release() functions.

However, these functions alone don't provide any guarantees at the
system level. Drivers need to ensure that the only a single consumer has
access to the reset at the same time. In order for the SOR to be able to
exclusively access its reset, it must therefore ensure that the SOR
power domain is not powered off by holding on to a runtime PM reference
to that power domain across the reset assert/deassert operation.

This used to work fine by accident, but was revealed when recently more
devices started to rely on the SOR power domain.

Fixes: 11c632e1cfd3 ("drm/tegra: sor: Implement acquire/release for reset")
Reported-by: Jonathan Hunter <jonathanh at nvidia.com>
Signed-off-by: Thierry Reding <treding at nvidia.com>
---
 drivers/gpu/drm/tegra/sor.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/tegra/sor.c b/drivers/gpu/drm/tegra/sor.c
index f02a035dda45..7b88261f57bb 100644
--- a/drivers/gpu/drm/tegra/sor.c
+++ b/drivers/gpu/drm/tegra/sor.c
@@ -3115,6 +3115,12 @@ static int tegra_sor_init(struct host1x_client *client)
 	 * kernel is possible.
 	 */
 	if (sor->rst) {
+		err = pm_runtime_resume_and_get(sor->dev);
+		if (err < 0) {
+			dev_err(sor->dev, "failed to get runtime PM: %d\n", err);
+			return err;
+		}
+
 		err = reset_control_acquire(sor->rst);
 		if (err < 0) {
 			dev_err(sor->dev, "failed to acquire SOR reset: %d\n",
@@ -3148,6 +3154,7 @@ static int tegra_sor_init(struct host1x_client *client)
 		}
 
 		reset_control_release(sor->rst);
+		pm_runtime_put(sor->dev);
 	}
 
 	err = clk_prepare_enable(sor->clk_safe);
-- 
2.30.2