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h=Message-ID:Subject:Subject:From:From:To:To:Cc:Cc:Date:Date:In-Reply-To:References:Content-Type:Content-Transfer-Encoding:MIME-Version:Message-Id:Reply-To; bh=KXBkxKNVs6wE+YT1mA1bDTsal53xZCrh2yGOcHdlcSw=; b=BgqrFMoBaDs0zKiagjcSl7Jq5fwEdYl3vhRFxtnwXWMROWIQp0E0Ae0YyyUVN4fx lzGw4Fkp0Y/eRgRLFkqLicLDOKPR7SjhXk/ZN9+qk/raS/ACF4U0+g9r/ujJIpRHQeo JMUtoC62WVcxsJobeNajXDzf14blOuWkEAyalyOY= Received: by mx.zohomail.com with SMTPS id 1729155096956592.1643472587784; Thu, 17 Oct 2024 01:51:36 -0700 (PDT) Message-ID: Subject: Re: [PATCH v6 01/14] drm/panthor: Add uAPI From: Erik Faye-Lund To: Boris Brezillon Cc: Robin Murphy , dri-devel@lists.freedesktop.org, Daniel Stone , Liviu Dudau , Steven Price , kernel@collabora.com, Chris Diamand , Ketil Johnsen Date: Thu, 17 Oct 2024 10:51:32 +0200 In-Reply-To: <20241016161833.574494ee@collabora.com> References: <20240229162230.2634044-1-boris.brezillon@collabora.com> <20240229162230.2634044-2-boris.brezillon@collabora.com> <64ff75ddede7623c16ed0272eef5e950ae34e7d5.camel@collabora.com> <1bd37b18455607b709529c8def963c4561e2ff1e.camel@collabora.com> <20241016161833.574494ee@collabora.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.54.0 (by Flathub.org) MIME-Version: 1.0 X-ZohoMailClient: External X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Wed, 2024-10-16 at 16:18 +0200, Boris Brezillon wrote: > On Wed, 16 Oct 2024 16:05:55 +0200 > Erik Faye-Lund wrote: >=20 > > On Wed, 2024-10-16 at 15:02 +0100, Robin Murphy wrote: > > > On 2024-10-16 2:50 pm, Erik Faye-Lund wrote:=C2=A0=20 > > > > On Wed, 2024-10-16 at 15:16 +0200, Erik Faye-Lund wrote:=C2=A0=20 > > > > > On Thu, 2024-02-29 at 17:22 +0100, Boris Brezillon wrote:=C2=A0= =20 > > > > > > +/** > > > > > > + * 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 =3D 0xff, > > > > > > + > > > > > > + /** @DRM_PANTHOR_SYNC_OP_HANDLE_TYPE_SYNCOBJ: > > > > > > Synchronization object type. */ > > > > > > + DRM_PANTHOR_SYNC_OP_HANDLE_TYPE_SYNCOBJ =3D 0, > > > > > > + > > > > > > + /** > > > > > > + * > > > > > > @DRM_PANTHOR_SYNC_OP_HANDLE_TYPE_TIMELINE_SYNCOBJ: > > > > > > Timeline synchronization > > > > > > + * object type. > > > > > > + */ > > > > > > + DRM_PANTHOR_SYNC_OP_HANDLE_TYPE_TIMELINE_SYNCOBJ =3D > > > > > > 1, > > > > > > + > > > > > > + /** @DRM_PANTHOR_SYNC_OP_WAIT: Wait operation. */ > > > > > > + DRM_PANTHOR_SYNC_OP_WAIT =3D 0 << 31, > > > > > > + > > > > > > + /** @DRM_PANTHOR_SYNC_OP_SIGNAL: Signal operation. > > > > > > */ > > > > > > + DRM_PANTHOR_SYNC_OP_SIGNAL =3D (int)(1u << 31),=C2=A0=20 > > > > >=20 > > > > > 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? > > > > > =C2=A0=20 > > > >=20 > > > > Seems this was proposed here: > > > > https://lore.kernel.org/dri-devel/89be8f8f-7c4e-4efd-0b7b-c30bcfbf1= d23@arm.com/ > > > >=20 > > > > ...that kinda sounds like bad advice to me. > > > >=20 > > > > 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. > > > >=20 > > > > Here's the quote from the C99 spec, section 6.3.1.3 "Signed and > > > > unsigned integers": > > > >=20 > > > > """ > > > > 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 > > > > """" > > > >=20 > > > > 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...=C2=A0=20 > > >=20 > > > 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=20 > > > represents negatives. > > >=20 > > > Robin.=C2=A0=20 > >=20 > > Ah, you're right. So that could fix the problem, indeed. >=20 > On the other hand, I hate the idea of having -1 << 31 to encode > bit31-set. That's even worse for DRM_PANTHOR_VM_BIND_OP_TYPE_xxx when > we'll reach a value above 0x7, because then the negative value is > hard > to map to its unsigned representation. If we really care about this > corner case, I'd rather go full-defines for flags and call it a day. >=20 Yeah, I suppose it can get ugly for some other cases. If we rule that out, I think there's only two options I can think of left: 1. Using #defines instead, like Boris suggested 2. Using 64 bit signed enums (e.g "1ll << 31" instead) Again, #2 here would be the smaller change. But I kinda think I lean towards #1, because... These aren't really enumerators. They are flags. ...Yeah, sure. In C the practical difference isn't huge. But if we ever wanted to support using these enums from C++ code, we'd need to add overloaded operators, because C++ doesn't allow ORing together enums out of the box. I'm not saying I have any plans on using the uAPI from C++, just saying that if we're going to tackle this, we might as well tackle it completely... Also, expanding the enum-type to 64 bits might have some additional consequences, like needlessly needing more stack-space to pass values around etc. Thoughts? Surely there must be some precedence on using the top bit for flags in the kernel, no?