From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 20CECD206B4 for ; Wed, 16 Oct 2024 14:18:41 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id A662F10E0AE; Wed, 16 Oct 2024 14:18:40 +0000 (UTC) Authentication-Results: gabe.freedesktop.org; dkim=pass (2048-bit key; unprotected) header.d=collabora.com header.i=@collabora.com header.b="Osw11/Qa"; dkim-atps=neutral Received: from bali.collaboradmins.com (bali.collaboradmins.com [148.251.105.195]) by gabe.freedesktop.org (Postfix) with ESMTPS id 87E2210E0AE for ; Wed, 16 Oct 2024 14:18:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1729088318; bh=NABsgyIOWHP+iLDQJhvWhlSE0nvmcrftRvz/MLXs7H4=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=Osw11/QaYvfmwX+1gndJi0VC2y4cjPMyhcJw3qXygzCtSk4I569tEtGU4pcMlcVBR dbWdo4HFwAKnngj5lHDQr0TIh546TXdjBIzkRMFGLsbWb72BLYtSDToGSGEtvVcoon 6/8owYW1i75/kqwOfkc8Qif7XbokRfkzd1Q5caUdmSWyQJpjuaDVoQA/PYLawdwKjs c7rGqLOZA3jIenZy2DCbuVaPTPYfBv/ZsIZV7YKeSCPW2nCePDW88Kh8yBnzvRxbYN G5oIl7Ayimo//Qr7wK8Pg79tETbrn0GEhrCl08hwNg/dnzXkNmNjVtIIzIQKXefDIB gGD85TIZDUFYg== Received: from localhost (unknown [IPv6:2a01:e0a:2c:6930:5cf4:84a1:2763:fe0d]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: bbrezillon) by bali.collaboradmins.com (Postfix) with ESMTPSA id CA4C117E3613; Wed, 16 Oct 2024 16:18:37 +0200 (CEST) Date: Wed, 16 Oct 2024 16:18:33 +0200 From: Boris Brezillon To: Erik Faye-Lund Cc: Robin Murphy , dri-devel@lists.freedesktop.org, Daniel Stone , Liviu Dudau , Steven Price , kernel@collabora.com, Chris Diamand , Ketil Johnsen Subject: Re: [PATCH v6 01/14] drm/panthor: Add uAPI Message-ID: <20241016161833.574494ee@collabora.com> In-Reply-To: References: <20240229162230.2634044-1-boris.brezillon@collabora.com> <20240229162230.2634044-2-boris.brezillon@collabora.com> <64ff75ddede7623c16ed0272eef5e950ae34e7d5.camel@collabora.com> <1bd37b18455607b709529c8def963c4561e2ff1e.camel@collabora.com> Organization: Collabora X-Mailer: Claws Mail 4.3.0 (GTK 3.24.43; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit 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, 16 Oct 2024 16:05:55 +0200 Erik Faye-Lund wrote: > 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@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. > > Ah, you're right. So that could fix the problem, indeed. 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.