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

[Jani Nikula <jani.nikula@linux.intel.com>]

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