[PATCH] drm/i915: Order OP vs. timeout correctly in __wait_for()

[PATCH] drm/i915: Order OP vs. timeout correctly in __wait_for()

[Ville Syrjala]

From: Ville Syrjälä <ville.syrjala@linux.intel.com>

Put the barrier() before the OP so that anything we read out in
OP and check in COND will actually be read out after the timeout
has been evaluated.

Currently the only place where we use OP is __intel_wait_for_register(),
but the use there is precisely susceptible to this reordering, assuming
the ktime_*() stuff itself doesn't act as a sufficient barrier:

__intel_wait_for_register(...)
{
	...
	ret = __wait_for(reg_value = intel_uncore_read_notrace(...),
 			 (reg_value & mask) == value, ...);
	...
}

Cc: stable@vger.kernel.org
Fixes: 1c3c1dc66a96 ("drm/i915: Add compiler barrier to wait_for")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
---
 drivers/gpu/drm/i915/i915_wait_util.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_wait_util.h b/drivers/gpu/drm/i915/i915_wait_util.h
index 7376898e3bf8..e1ed7921ec70 100644
--- a/drivers/gpu/drm/i915/i915_wait_util.h
+++ b/drivers/gpu/drm/i915/i915_wait_util.h
@@ -25,9 +25,9 @@
 	might_sleep();							\
 	for (;;) {							\
 		const bool expired__ = ktime_after(ktime_get_raw(), end__); \
-		OP;							\
 		/* Guarantee COND check prior to timeout */		\
 		barrier();						\
+		OP;							\
 		if (COND) {						\
 			ret__ = 0;					\
 			break;						\
-- 
2.52.0

Re: [PATCH] drm/i915: Order OP vs. timeout correctly in __wait_for()

[Jani Nikula]

On Fri, 13 Mar 2026, Ville Syrjala <ville.syrjala@linux.intel.com> wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
>
> Put the barrier() before the OP so that anything we read out in
> OP and check in COND will actually be read out after the timeout
> has been evaluated.
>
> Currently the only place where we use OP is __intel_wait_for_register(),
> but the use there is precisely susceptible to this reordering, assuming
> the ktime_*() stuff itself doesn't act as a sufficient barrier:
>
> __intel_wait_for_register(...)
> {
> 	...
> 	ret = __wait_for(reg_value = intel_uncore_read_notrace(...),
>  			 (reg_value & mask) == value, ...);
> 	...
> }
>
> Cc: stable@vger.kernel.org
> Fixes: 1c3c1dc66a96 ("drm/i915: Add compiler barrier to wait_for")
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

Reviewed-by: Jani Nikula <jani.nikula@intel.com>

> ---
>  drivers/gpu/drm/i915/i915_wait_util.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_wait_util.h b/drivers/gpu/drm/i915/i915_wait_util.h
> index 7376898e3bf8..e1ed7921ec70 100644
> --- a/drivers/gpu/drm/i915/i915_wait_util.h
> +++ b/drivers/gpu/drm/i915/i915_wait_util.h
> @@ -25,9 +25,9 @@
>  	might_sleep();							\
>  	for (;;) {							\
>  		const bool expired__ = ktime_after(ktime_get_raw(), end__); \
> -		OP;							\
>  		/* Guarantee COND check prior to timeout */		\
>  		barrier();						\
> +		OP;							\
>  		if (COND) {						\
>  			ret__ = 0;					\
>  			break;						\

-- 
Jani Nikula, Intel