On Wed, 2024-10-16 at 15:02 +0100, Robin Murphy wrote:
> 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.
Ah, you're right. So that could fix the problem, indeed.
