Re: [PATCH v6 01/14] drm/panthor: Add uAPI

[Erik Faye-Lund <erik.faye-lund@collabora.com>]

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@arm.com/

...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...