Hi
Am 28.09.26 um 20:09 schrieb Maíra Canal:
Hi Thomas,
Thank you for your review!
On 28/09/26 13:37, Thomas Zimmermann wrote:
[...]
+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);
The timeout is controled by user space, right? Can these additions
overflow? I see that secs is tested against MAX_JIFFY_OFFSET, but is
that sufficient?
Although the timeout is controlled by user-space, the additions cannot
overflow here. The secs test bounds the multiplication, which is why it
is written as secs >= MAX_JIFFY_OFFSET / HZ rather than secs * HZ >=
MAX_JIFFY_OFFSET (which could overflow).
Past that check, we know that the remaining is smaller than one second
and therefore, nsecs_to_jiffies64(rem) <= HZ - 1. So, the sum + 1 is at
most MAX_JIFFY_OFFSET.
BTW there was this NSEC % HZ test in the original code? What was it
good for? It is no longer useful?
I believe that NSEC_PER_SEC % HZ was only useful to check if it was a
plain division, which wouldn't overflow. With the current approach, I
believe it's no longer needed.
Thanks for answering my questions.
Reviewed-by: Thomas Zimmermann <[email protected]>
Best regards
Thomas
Best regards,
- Maíra
Best regards
Thomas
+}
+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);
#endif
--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Stefan Gaiser, Jochen Jaser, Abhinav Puri, (HRB 36809, AG Nürnberg)
