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

[Robin Murphy]

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@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...

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.