From patchwork Wed Jun 23 16:19:55 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Daniel Vetter X-Patchwork-Id: 12340115 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-16.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 597C8C4743C for ; Wed, 23 Jun 2021 16:20:07 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 07C836024A for ; Wed, 23 Jun 2021 16:20:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 07C836024A Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ffwll.ch Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=intel-gfx-bounces@lists.freedesktop.org Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id B78336E955; Wed, 23 Jun 2021 16:20:04 +0000 (UTC) Received: from mail-ed1-x52e.google.com (mail-ed1-x52e.google.com [IPv6:2a00:1450:4864:20::52e]) by gabe.freedesktop.org (Postfix) with ESMTPS id 2213E89BF8 for ; Wed, 23 Jun 2021 16:20:03 +0000 (UTC) Received: by mail-ed1-x52e.google.com with SMTP id r7so4230245edv.12 for ; Wed, 23 Jun 2021 09:20:03 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ffwll.ch; s=google; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=18nsKlNRPeSGzK/GtnkV65GHuS4sYhXYKk8mOhr9KfE=; b=TuPLi5hS5A53DBo+f8uhcVE1txIOQXShrAJF3YLB8j0FAHlj1BRP3LogkRA3DiBVjU JMzxb7oAOUi87Hr6V3vp8P3KZWvxYY7DEhTClRYkoFWpLiC5vy+s78mGKg7eivhTEidY KJ4TvOQ4DGB26WELxg8qT5ukAtQIRSPiMmEco= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=18nsKlNRPeSGzK/GtnkV65GHuS4sYhXYKk8mOhr9KfE=; b=RtF+Fgqki3KDKPxnMz1sDA3w8K+JAxpIg3NRzAnaFlBAl6n16v60ejnsxbCIkDkAXQ 6eNCwTvqlrLMR/YhyOHp8nqkTlHHqI8NjBVxyXJHeDvbaXhL7plyPcgOpeFF6BqMEhtb E54z8K1cfvju3eyqcn0f9p10fAltBk8NLve4hpaejc7zNlNZZGyCnTDGGb8uPixKByT+ lFdepgIZ7pSn2jrgQw/fVwebO7kZXl/mrWkFifTjSjL2yC4O9ZrdmP9cWog5wAmSRokB i+YU4Tvvyjh53TZMa/clGcYuE8fGQRXR+YNLsddtL2DONOFWKnvTkO3fIhe5+Xd1KYJW yD1w== X-Gm-Message-State: AOAM530r166xd6i+5B6HY8bNQalEbnfiFX6SRaIN0BzTwx2xY3H/3Vaw t6iGvnHHCfiso0U62dWljmKFJA== X-Google-Smtp-Source: ABdhPJwjxcIdheXhNHV4FNEf4j5HbPzwCWsR0AfvzJdQN1VDjWxSlGzeIsVeQ3oXGvUYSFzYSxfLGg== X-Received: by 2002:a50:85cd:: with SMTP id q13mr759508edh.216.1624465201697; Wed, 23 Jun 2021 09:20:01 -0700 (PDT) Received: from phenom.ffwll.local ([2a02:168:57f4:0:efd0:b9e5:5ae6:c2fa]) by smtp.gmail.com with ESMTPSA id h6sm296018edj.91.2021.06.23.09.20.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 23 Jun 2021 09:20:01 -0700 (PDT) From: Daniel Vetter To: DRI Development Date: Wed, 23 Jun 2021 18:19:55 +0200 Message-Id: <20210623161955.3371466-1-daniel.vetter@ffwll.ch> X-Mailer: git-send-email 2.32.0.rc2 In-Reply-To: <20210622165511.3169559-4-daniel.vetter@ffwll.ch> References: <20210622165511.3169559-4-daniel.vetter@ffwll.ch> MIME-Version: 1.0 Subject: [Intel-gfx] [PATCH] dma-buf: Document dma-buf implicit fencing/resv fencing rules X-BeenThere: intel-gfx@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Intel graphics driver community testing & development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Rob Clark , linaro-mm-sig@lists.linaro.org, Daniel Stone , Daniel Vetter , Daniel Vetter , Intel Graphics Development , Kevin Wang , Sumit Semwal , =?utf-8?q?Michel_D=C3=A4nzer?= , Luben Tuikov , "Kristian H . Kristensen" , Chen Li , Bas Nieuwenhuizen , Alex Deucher , mesa-dev@lists.freedesktop.org, Dave Airlie , =?utf-8?q?Christian_K=C3=B6nig?= , Dennis Li , Deepak R Varma Errors-To: intel-gfx-bounces@lists.freedesktop.org Sender: "Intel-gfx" Docs for struct dma_resv are fairly clear: "A reservation object can have attached one exclusive fence (normally associated with write operations) or N shared fences (read operations)." https://dri.freedesktop.org/docs/drm/driver-api/dma-buf.html#reservation-objects Furthermore a review across all of upstream. First of render drivers and how they set implicit fences: - nouveau follows this contract, see in validate_fini_no_ticket() nouveau_bo_fence(nvbo, fence, !!b->write_domains); and that last boolean controls whether the exclusive or shared fence slot is used. - radeon follows this contract by setting p->relocs[i].tv.num_shared = !r->write_domain; in radeon_cs_parser_relocs(), which ensures that the call to ttm_eu_fence_buffer_objects() in radeon_cs_parser_fini() will do the right thing. - vmwgfx seems to follow this contract with the shotgun approach of always setting ttm_val_buf->num_shared = 0, which means ttm_eu_fence_buffer_objects() will only use the exclusive slot. - etnaviv follows this contract, as can be trivially seen by looking at submit_attach_object_fences() - i915 is a bit a convoluted maze with multiple paths leading to i915_vma_move_to_active(). Which sets the exclusive flag if EXEC_OBJECT_WRITE is set. This can either come as a buffer flag for softpin mode, or through the write_domain when using relocations. It follows this contract. - lima follows this contract, see lima_gem_submit() which sets the exclusive fence when the LIMA_SUBMIT_BO_WRITE flag is set for that bo - msm follows this contract, see msm_gpu_submit() which sets the exclusive flag when the MSM_SUBMIT_BO_WRITE is set for that buffer - panfrost follows this contract with the shotgun approach of just always setting the exclusive fence, see panfrost_attach_object_fences(). Benefits of a single engine I guess - v3d follows this contract with the same shotgun approach in v3d_attach_fences_and_unlock_reservation(), but it has at least an XXX comment that maybe this should be improved - v4c uses the same shotgun approach of always setting an exclusive fence, see vc4_update_bo_seqnos() - vgem also follows this contract, see vgem_fence_attach_ioctl() and the VGEM_FENCE_WRITE. This is used in some igts to validate prime sharing with i915.ko without the need of a 2nd gpu - vritio follows this contract again with the shotgun approach of always setting an exclusive fence, see virtio_gpu_array_add_fence() This covers the setting of the exclusive fences when writing. Synchronizing against the exclusive fence is a lot more tricky, and I only spot checked a few: - i915 does it, with the optional EXEC_OBJECT_ASYNC to skip all implicit dependencies (which is used by vulkan) - etnaviv does this. Implicit dependencies are collected in submit_fence_sync(), again with an opt-out flag ETNA_SUBMIT_NO_IMPLICIT. These are then picked up in etnaviv_sched_dependency which is the drm_sched_backend_ops->dependency callback. - v4c seems to not do much here, maybe gets away with it by not having a scheduler and only a single engine. Since all newer broadcom chips than the OG vc4 use v3d for rendering, which follows this contract, the impact of this issue is fairly small. - v3d does this using the drm_gem_fence_array_add_implicit() helper, which then it's drm_sched_backend_ops->dependency callback v3d_job_dependency() picks up. - panfrost is nice here and tracks the implicit fences in panfrost_job->implicit_fences, which again the drm_sched_backend_ops->dependency callback panfrost_job_dependency() picks up. It is mildly questionable though since it only picks up exclusive fences in panfrost_acquire_object_fences(), but not buggy in practice because it also always sets the exclusive fence. It should pick up both sets of fences, just in case there's ever going to be a 2nd gpu in a SoC with a mali gpu. Or maybe a mali SoC with a pcie port and a real gpu, which might actually happen eventually. A bug, but easy to fix. Should probably use the drm_gem_fence_array_add_implicit() helper. - lima is nice an easy, uses drm_gem_fence_array_add_implicit() and the same schema as v3d. - msm is mildly entertaining. It also supports MSM_SUBMIT_NO_IMPLICIT, but because it doesn't use the drm/scheduler it handles fences from the wrong context with a synchronous dma_fence_wait. See submit_fence_sync() leading to msm_gem_sync_object(). Investing into a scheduler might be a good idea. - all the remaining drivers are ttm based, where I hope they do appropriately obey implicit fences already. I didn't do the full audit there because a) not follow the contract would confuse ttm quite well and b) reading non-standard scheduler and submit code which isn't based on drm/scheduler is a pain. Onwards to the display side. - Any driver using the drm_gem_plane_helper_prepare_fb() helper will correctly. Overwhelmingly most drivers get this right, except a few totally dont. I'll follow up with a patch to make this the default and avoid a bunch of bugs. - I didn't audit the ttm drivers, but given that dma_resv started there I hope they get this right. In conclusion this IS the contract, both as documented and overwhelmingly implemented, specically as implemented by all render drivers except amdgpu. Amdgpu tried to fix this already in commit 049aca4363d8af87cab8d53de5401602db3b9999 Author: Christian König Date: Wed Sep 19 16:54:35 2018 +0200 drm/amdgpu: fix using shared fence for exported BOs v2 but this fix falls short on a number of areas: - It's racy, by the time the buffer is shared it might be too late. To make sure there's definitely never a problem we need to set the fences correctly for any buffer that's potentially exportable. - It's breaking uapi, dma-buf fds support poll() and differentitiate between, which was introduced in commit 9b495a5887994a6d74d5c261d012083a92b94738 Author: Maarten Lankhorst Date: Tue Jul 1 12:57:43 2014 +0200 dma-buf: add poll support, v3 - Christian König wants to nack new uapi building further on this dma_resv contract because it breaks amdgpu, quoting "Yeah, and that is exactly the reason why I will NAK this uAPI change. "This doesn't works for amdgpu at all for the reasons outlined above." https://lore.kernel.org/dri-devel/f2eb6751-2f82-9b23-f57e-548de5b729de@gmail.com/ Rejecting new development because your own driver is broken and violates established cross driver contracts and uapi is really not how upstream works. Now this patch will have a severe performance impact on anything that runs on multiple engines. So we can't just merge it outright, but need a bit a plan: - amdgpu needs a proper uapi for handling implicit fencing. The funny thing is that to do it correctly, implicit fencing must be treated as a very strange IPC mechanism for transporting fences, where both setting the fence and dependency intercepts must be handled explicitly. Current best practices is a per-bo flag to indicate writes, and a per-bo flag to to skip implicit fencing in the CS ioctl as a new chunk. - Since amdgpu has been shipping with broken behaviour we need an opt-out flag from the butchered implicit fencing model to enable the proper explicit implicit fencing model. - for kernel memory fences due to bo moves at least the i915 idea is to use ttm_bo->moving. amdgpu probably needs the same. - since the current p2p dma-buf interface assumes the kernel memory fence is in the exclusive dma_resv fence slot we need to add a new fence slot for kernel fences, which must never be ignored. Since currently only amdgpu supports this there's no real problem here yet, until amdgpu gains a NO_IMPLICIT CS flag. - New userspace needs to ship in enough desktop distros so that users wont notice the perf impact. I think we can ignore LTS distros who upgrade their kernels but not their mesa3d snapshot. - Then when this is all in place we can merge this patch here. What is not a solution to this problem here is trying to make the dma_resv rules in the kernel more clever. The fundamental issue here is that the amdgpu CS uapi is the least expressive one across all drivers (only equalled by panfrost, which has an actual excuse) by not allowing any userspace control over how implicit sync is conducted. Until this is fixed it's completely pointless to make the kernel more clever to improve amdgpu, because all we're doing is papering over this uapi design issue. amdgpu needs to attain the status quo established by other drivers first, once that's achieved we can tackle the remaining issues in a consistent way across drivers. v2: Bas pointed me at AMDGPU_GEM_CREATE_EXPLICIT_SYNC, which I entirely missed. This is great because it means the amdgpu specific piece for proper implicit fence handling exists already, and that since a while. The only thing that's now missing is - fishing the implicit fences out of a shared object at the right time - setting the exclusive implicit fence slot at the right time. Jason has a patch series to fill that gap with a bunch of generic ioctl on the dma-buf fd: https://lore.kernel.org/dri-devel/20210520190007.534046-1-jason@jlekstrand.net/ v3: Since Christian has fixed amdgpu now in commit 8c505bdc9c8b955223b054e34a0be9c3d841cd20 (drm-misc/drm-misc-next) Author: Christian König Date: Wed Jun 9 13:51:36 2021 +0200 drm/amdgpu: rework dma_resv handling v3 Use the audit covered in this commit message as the excuse to update the dma-buf docs around dma_buf.resv usage across drivers. Since dynamic importers have different rules also hammer these in again while we're at it. v4: - Add the missing "through the device" in the dynamic section that I overlooked. - Fix a kerneldoc markup mistake, the link didn't connect Reviewed-by: Christian König (v3) Cc: mesa-dev@lists.freedesktop.org Cc: Bas Nieuwenhuizen Cc: Dave Airlie Cc: Rob Clark Cc: Kristian H. Kristensen Cc: Michel Dänzer Cc: Daniel Stone Cc: Sumit Semwal Cc: "Christian König" Cc: Alex Deucher Cc: Daniel Vetter Cc: Deepak R Varma Cc: Chen Li Cc: Kevin Wang Cc: Dennis Li Cc: Luben Tuikov Cc: linaro-mm-sig@lists.linaro.org Signed-off-by: Daniel Vetter Reviewed-by: Dave Airlie --- include/linux/dma-buf.h | 39 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 39 insertions(+) diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h index 81cebf414505..494f639ee486 100644 --- a/include/linux/dma-buf.h +++ b/include/linux/dma-buf.h @@ -386,6 +386,45 @@ struct dma_buf { * @resv: * * Reservation object linked to this dma-buf. + * + * IMPLICIT SYNCHRONIZATION RULES: + * + * Drivers which support implicit synchronization of buffer access as + * e.g. exposed in `Implicit Fence Poll Support`_ should follow the + * below rules. + * + * - Drivers should add a shared fence through + * dma_resv_add_shared_fence() for anything the userspace API + * considers a read access. This highly depends upon the API and + * window system: E.g. OpenGL is generally implicitly synchronized on + * Linux, but explicitly synchronized on Android. Whereas Vulkan is + * generally explicitly synchronized for everything, and window system + * buffers have explicit API calls (which then need to make sure the + * implicit fences store here in @resv are updated correctly). + * + * - Similarly drivers should set the exclusive fence through + * dma_resv_add_excl_fence() for anything the userspace API considers + * write access. + * + * - Drivers may just always set the exclusive fence, since that only + * causes unecessarily synchronization, but no correctness issues. + * + * - Some drivers only expose a synchronous userspace API with no + * pipelining across drivers. These do not set any fences for their + * access. An example here is v4l. + * + * DYNAMIC IMPORTER RULES: + * + * Dynamic importers, see dma_buf_attachment_is_dynamic(), have + * additional constraints on how they set up fences: + * + * - Dynamic importers must obey the exclusive fence and wait for it to + * signal before allowing access to the buffer's underlying storage + * through the device. + * + * - Dynamic importers should set fences for any access that they can't + * disable immediately from their &dma_buf_attach_ops.move_notify + * callback. */ struct dma_resv *resv;