Re: [PATCH 2/6] drm: Add drm_timeout_rel_to_jiffies()

On 8/9/26 21:23, Maíra Canal wrote:
> drm_timeout_abs_to_jiffies() covers the drivers whose wait UAPI takes an
> absolute deadline, but there is no equivalent for the drivers that
> express a wait as a duration. Drivers such as i915 and v3d convert the
> value themselves.
> 
> Converting a nanosecond duration to jiffies needs some care.
> nsecs_to_jiffies() returns unsigned long, so on 32-bit a large
> userspace-supplied timeout overflows its range and is silently truncated.
> 
> i915 already handles both cases in a local helper, which v3d has a copy
> of. Add the same conversion to the core, so that it is available to any
> driver and both copies can be dropped.
> 
> Signed-off-by: Maíra Canal <[email protected]>
> ---
>  drivers/gpu/drm/drm_timeout.c | 29 +++++++++++++++++++++++++++++
>  include/drm/drm_utils.h       |  1 +
>  2 files changed, 30 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_timeout.c b/drivers/gpu/drm/drm_timeout.c
> index 78e9f65e5477..30ad3e8ebc92 100644
> --- a/drivers/gpu/drm/drm_timeout.c
> +++ b/drivers/gpu/drm/drm_timeout.c
> @@ -9,6 +9,7 @@
>  #include <linux/export.h>
>  #include <linux/jiffies.h>
>  #include <linux/ktime.h>
> +#include <linux/math64.h>
>  #include <linux/sched.h>
>  
>  #include <drm/drm_utils.h>
> @@ -45,3 +46,31 @@ signed long drm_timeout_abs_to_jiffies(int64_t timeout_nsec)
>  	return timeout_jiffies64 + 1;
>  }
>  EXPORT_SYMBOL(drm_timeout_abs_to_jiffies);
> +
> +/**
> + * drm_timeout_rel_to_jiffies - calculate jiffies timeout from relative value
> + *
> + * @timeout_nsec: relative timeout in ns, 0 for poll
> + *
> + * Calculate the timeout in jiffies from a relative timeout in ns, for drivers
> + * whose UAPI expresses a wait as a duration rather than as a deadline.
> + *
> + * The result is clamped to MAX_JIFFY_OFFSET. That keeps it positive once it is
> + * converted to the signed long taken by dma_fence_wait_timeout() and friends,
> + * which matters on 32-bit, and keeps it distinct from MAX_SCHEDULE_TIMEOUT so
> + * that a finite wait is never understood as an infinite one.

Please add a comment that it is strongly discouraged to use relative timeouts in uAPIs.

The background is that relative timeouts doesn't work with restarting IOCTLs.

I think that's also part of the reason why we don't have a common helper function for that.

Regards,
Christian.

> + */
> +unsigned long drm_timeout_rel_to_jiffies(u64 timeout_nsec)
> +{
> +	/* make 0 timeout means poll, as for the absolute variant */
> +	if (timeout_nsec == 0)
> +		return 0;
> +
> +	/* nsecs_to_jiffies64() does not guard against overflow */
> +	if ((NSEC_PER_SEC % HZ) != 0 &&
> +	    div_u64(timeout_nsec, NSEC_PER_SEC) >= MAX_JIFFY_OFFSET / HZ)
> +		return MAX_JIFFY_OFFSET;
> +
> +	return min_t(u64, MAX_JIFFY_OFFSET, nsecs_to_jiffies64(timeout_nsec) + 1);
> +}
> +EXPORT_SYMBOL(drm_timeout_rel_to_jiffies);
> diff --git a/include/drm/drm_utils.h b/include/drm/drm_utils.h
> index 6a46f755daba..8c0cc9835413 100644
> --- a/include/drm/drm_utils.h
> +++ b/include/drm/drm_utils.h
> @@ -25,5 +25,6 @@ const struct drm_panel_backlight_quirk *
>  drm_get_panel_backlight_quirk(const struct drm_edid *edid);
>  
>  signed long drm_timeout_abs_to_jiffies(int64_t timeout_nsec);
> +unsigned long drm_timeout_rel_to_jiffies(u64 timeout_nsec);
>  
>  #endif
>