Re: [PATCH v3 2/7] drm: Add drm_timeout_rel_to_jiffies()

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

On Wed, 16 Sep 2026, Jani Nikula <jani.nikula@linux.intel.com> wrote:
> On Tue, 15 Sep 2026, Maíra Canal <mcanal@igalia.com> 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 <mcanal@igalia.com>
>> ---
>>  drivers/gpu/drm/drm_timeout.c | 45 +++++++++++++++++++++++++++++++++++++++++++
>>  include/drm/drm_timeout.h     |  1 +
>>  2 files changed, 46 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_timeout.c b/drivers/gpu/drm/drm_timeout.c
>> index 35e10293e0a1..ee545475429c 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_timeout.h>
>> @@ -48,3 +49,47 @@ 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.
>> + *
>> + * It's strongly discouraged to use relative timeouts in uAPIs, as they do not
>> + * survive a restarted ioctl. A signal-interrupted ioctl is re-entered with the
>> + * same arguments, so the duration starts counting from zero again. New uAPIs
>> + * should take an absolute deadline and use drm_timeout_abs_to_jiffies().
>> + *
>> + * Returns:
>> + * 0 if @timeout_nsec is 0. Otherwise the equivalent number of jiffies and
>> + * clamped to MAX_JIFFY_OFFSET.
>> + */
>> +unsigned long drm_timeout_rel_to_jiffies(u64 timeout_nsec)
>> +{
>> +	u64 secs;
>> +	u32 rem;
>> +
>> +	/* Make 0 timeout means poll, as for the absolute variant. */
>> +	if (timeout_nsec == 0)
>> +		return 0;
>> +
>> +	/*
>> +	 * As nsecs_to_jiffies64() does not guard against overflow, split
>> +	 * the timeout into whole seconds and nanoseconds. This way
>> +	 * nsecs_to_jiffies64() is always handed a value below a second.
>> +	 */
>> +	secs = div_u64_rem(timeout_nsec, NSEC_PER_SEC, &rem);
>> +	if (secs >= MAX_JIFFY_OFFSET / HZ)
>> +		return MAX_JIFFY_OFFSET;
>> +
>> +	return min_t(u64, MAX_JIFFY_OFFSET,
>> +		     secs_to_jiffies(secs) + nsecs_to_jiffies64(rem) + 1);
>> +}
>> +EXPORT_SYMBOL(drm_timeout_rel_to_jiffies);
>> diff --git a/include/drm/drm_timeout.h b/include/drm/drm_timeout.h
>> index cd9621c52062..6ee222a3e97c 100644
>> --- a/include/drm/drm_timeout.h
>> +++ b/include/drm/drm_timeout.h
>> @@ -12,5 +12,6 @@
>>  #include <linux/types.h>
>>  
>>  signed long drm_timeout_abs_to_jiffies(s64 timeout_nsec);
>> +unsigned long drm_timeout_rel_to_jiffies(u64 timeout_nsec);
>
> The signed/unsigned differences here bug me a little. Maybe explain why
> in the commit message if there's a rationale?
>
> I note that drm_timeout_rel_to_jiffies() return value is assigned to
> unsigned long variables half the time. I guess its kernel-doc could say
> it never returns negative values?

Gah, I meant drm_timeout_abs_to_jiffies() here, sorry.

>
> BR,
> Jani.
>
>
>
>
>>  
>>  #endif

-- 
Jani Nikula, Intel