On 2024-10-16 2:50 pm, Erik Faye-Lund wrote:
On Wed, 2024-10-16 at 15:16 +0200, Erik Faye-Lund wrote:
On Thu, 2024-02-29 at 17:22 +0100, Boris Brezillon wrote:
+/**
+ * enum drm_panthor_sync_op_flags - Synchronization operation
flags.
+ */
+enum drm_panthor_sync_op_flags {
+ /** @DRM_PANTHOR_SYNC_OP_HANDLE_TYPE_MASK: Synchronization
handle type mask. */
+ DRM_PANTHOR_SYNC_OP_HANDLE_TYPE_MASK = 0xff,
+
+ /** @DRM_PANTHOR_SYNC_OP_HANDLE_TYPE_SYNCOBJ:
Synchronization object type. */
+ DRM_PANTHOR_SYNC_OP_HANDLE_TYPE_SYNCOBJ = 0,
+
+ /**
+ * @DRM_PANTHOR_SYNC_OP_HANDLE_TYPE_TIMELINE_SYNCOBJ:
Timeline synchronization
+ * object type.
+ */
+ DRM_PANTHOR_SYNC_OP_HANDLE_TYPE_TIMELINE_SYNCOBJ = 1,
+
+ /** @DRM_PANTHOR_SYNC_OP_WAIT: Wait operation. */
+ DRM_PANTHOR_SYNC_OP_WAIT = 0 << 31,
+
+ /** @DRM_PANTHOR_SYNC_OP_SIGNAL: Signal operation. */
+ DRM_PANTHOR_SYNC_OP_SIGNAL = (int)(1u << 31),
Why do we cast to int here? 1u << 31 doesn't fit in a 32-bit signed
integer, so isn't this undefined behavior in C?
Seems this was proposed here:
https://lore.kernel.org/dri-devel/89be8f8f-7c4e-4efd-0b7b-c30bcfbf1d23@xxxxxxx/
...that kinda sounds like bad advice to me.
Also, it's been pointed out to me elsewhere that this isn't
*technically speaking* undefined, it's "implementation defined". But as
far as kernel interfaces goes, that's pretty much the same; we can't
guarantee that the kernel and the user-space is using the same
implementation.
Here's the quote from the C99 spec, section 6.3.1.3 "Signed and
unsigned integers":
"""
Otherwise, the new type is signed and the value cannot be represented
in it; either the result is implementation-defined or an
implementation-defined signal is raised
""""
I think a better approach be to use -1 << 31, which is well-defined.
But the problem then becomes assigning it into
drm_panthor_sync_op::flags in a well-defined way... Could we make the
field signed? That seems a bit bad as well...
Is that a problem? Signed->unsigned conversion is always well-defined
(6.3.1.3 again), since it doesn't depend on how the signed type
represents negatives.
Robin.
