From patchwork Tue Jul 6 10:12:09 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Daniel Vetter X-Patchwork-Id: 12360201 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,URIBL_BLOCKED,USER_AGENT_GIT autolearn=unavailable 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 A5597C07E9B for ; Tue, 6 Jul 2021 10:12:40 +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 79F9B619A2 for ; Tue, 6 Jul 2021 10:12:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 79F9B619A2 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ffwll.ch Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=dri-devel-bounces@lists.freedesktop.org Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id BC43F89E36; Tue, 6 Jul 2021 10:12:24 +0000 (UTC) Received: from mail-wr1-x42a.google.com (mail-wr1-x42a.google.com [IPv6:2a00:1450:4864:20::42a]) by gabe.freedesktop.org (Postfix) with ESMTPS id B583E89E1D for ; Tue, 6 Jul 2021 10:12:22 +0000 (UTC) Received: by mail-wr1-x42a.google.com with SMTP id v5so25410260wrt.3 for ; Tue, 06 Jul 2021 03:12:22 -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=1lSRZ4P7ggIgdNFz6D4WaF8yqxGm3+JUhNvxHpUyQxw=; b=VrKzPojpHAT+Dlp99W84NQiSzwI+IIerjeNVLBCH7GUFJaDde2u8qU/GaqfXfY+m+O 3oNwwllyfMZ+9Dgr8AJ6hL4U/JFxLmKiR6sLkuFP6LwyD4oPVmXOVdA8NOnoigwA2eMw gKIY0ZEG51D6CQ6hKcn5p+w4BbPr3mDOp1C0A= 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=1lSRZ4P7ggIgdNFz6D4WaF8yqxGm3+JUhNvxHpUyQxw=; b=mDjglqqwWwr0FK98QW2AbsIGYrS+5y3hwtsvah38xaeYcAF9DOI2Zw41VqOo+jmtZG hbwf92L21zX0QJFE1N8ye4pOQCNNPaZttiyEJMPh+hkQvpaf+KHkVITqg8L6gEuOoFQB 4B2uIeDLHGL+ESySO+MgKK8fUBM7FwvLlWYTy9smVBl7tlxodiQYJXtY6d1Z+QU2iA2X 7xnjE03B7jVFYNDZL3ZSsCsMBFNpDs5C/lQsPkEUutQbLI/PbrEd5hVYJerC31pzGhRI ZoJ4npJrxjKiU9vFOjpmiZ9edW/Je2eZaupo/aZEv5H68bs7h6F2tTmfayKSoRCTsZOy 8qmA== X-Gm-Message-State: AOAM5337Uydq8tJ4ztuY3LY3k2CHOwwaeMNyVPWHAzSZsGdwOlrzojpF QrOivO4Wn5bVbWUYElKWR2Wd5qIydwcNTA== X-Google-Smtp-Source: ABdhPJwKzoqROJvDlgnQYoMxQCRFOMdeGCAuxEjonVQRkgacAtQ0/yt6DkJAw0IdzX1LbW0bcGlI+A== X-Received: by 2002:adf:ef8b:: with SMTP id d11mr20742548wro.346.1625566341395; Tue, 06 Jul 2021 03:12:21 -0700 (PDT) Received: from phenom.ffwll.local ([2a02:168:57f4:0:efd0:b9e5:5ae6:c2fa]) by smtp.gmail.com with ESMTPSA id u2sm9862739wmc.42.2021.07.06.03.12.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 06 Jul 2021 03:12:20 -0700 (PDT) From: Daniel Vetter To: DRI Development Subject: [PATCH 7/7] dma-resv: Give the docs a do-over Date: Tue, 6 Jul 2021 12:12:09 +0200 Message-Id: <20210706101209.3034092-8-daniel.vetter@ffwll.ch> X-Mailer: git-send-email 2.32.0 In-Reply-To: <20210706101209.3034092-1-daniel.vetter@ffwll.ch> References: <20210706101209.3034092-1-daniel.vetter@ffwll.ch> MIME-Version: 1.0 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: , Cc: Daniel Vetter , Intel Graphics Development , linaro-mm-sig@lists.linaro.org, Daniel Vetter , =?utf-8?q?Christian_K=C3=B6nig?= , linux-media@vger.kernel.org Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" Specifically document the new/clarified rules around how the shared fences do not have any ordering requirements against the exclusive fence. But also document all the things a bit better, given how central struct dma_resv to dynamic buffer management the docs have been very inadequat. - Lots more links to other pieces of the puzzle. Unfortunately ttm_buffer_object has no docs, so no links :-( - Explain/complain a bit about dma_resv_locking_ctx(). I still don't like that one, but fixing the ttm call chains is going to be horrible. Plus we want to plug in real slowpath locking when we do that anyway. - Main part of the patch is some actual docs for struct dma_resv. Overall I think we still have a lot of bad naming in this area (e.g. dma_resv.fence is singular, but contains the multiple shared fences), but I think that's more indicative of how the semantics and rules are just not great. Another thing that's real awkard is how chaining exclusive fences right now means direct dma_resv.exclusive_fence pointer access with an rcu_assign_pointer. Not so great either. Signed-off-by: Daniel Vetter Cc: Sumit Semwal Cc: "Christian König" Cc: linux-media@vger.kernel.org Cc: linaro-mm-sig@lists.linaro.org Reviewed-by: Matthew Auld --- drivers/dma-buf/dma-resv.c | 22 ++++++-- include/linux/dma-resv.h | 104 +++++++++++++++++++++++++++++++++++-- 2 files changed, 116 insertions(+), 10 deletions(-) diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c index f26c71747d43..898f8d894bbd 100644 --- a/drivers/dma-buf/dma-resv.c +++ b/drivers/dma-buf/dma-resv.c @@ -48,6 +48,8 @@ * write operations) or N shared fences (read operations). The RCU * mechanism is used to protect read access to fences from locked * write-side updates. + * + * See struct dma_resv for more details. */ DEFINE_WD_CLASS(reservation_ww_class); @@ -137,7 +139,11 @@ EXPORT_SYMBOL(dma_resv_fini); * @num_fences: number of fences we want to add * * Should be called before dma_resv_add_shared_fence(). Must - * be called with obj->lock held. + * be called with @obj locked through dma_resv_lock(). + * + * Note that the preallocated slots need to be re-reserved if @obj is unlocked + * at any time before callind dma_resv_add_shared_fence(). This is validate when + * CONFIG_DEBUG_MUTEXES is enabled. * * RETURNS * Zero for success, or -errno @@ -234,8 +240,10 @@ EXPORT_SYMBOL(dma_resv_reset_shared_max); * @obj: the reservation object * @fence: the shared fence to add * - * Add a fence to a shared slot, obj->lock must be held, and + * Add a fence to a shared slot, @obj must be locked with dma_resv_lock(), and * dma_resv_reserve_shared() has been called. + * + * See also &dma_resv.fence for a discussion of the semantics. */ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence) { @@ -280,7 +288,9 @@ EXPORT_SYMBOL(dma_resv_add_shared_fence); * @obj: the reservation object * @fence: the shared fence to add * - * Add a fence to the exclusive slot. The obj->lock must be held. + * Add a fence to the exclusive slot. @obj must be locked with dma_resv_lock(). + * Note that this function replaces all fences attached to @obj, see also + * &dma_resv.fence_excl for a discussion of the semantics. */ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence) { @@ -609,9 +619,11 @@ static inline int dma_resv_test_signaled_single(struct dma_fence *passed_fence) * fence * * Callers are not required to hold specific locks, but maybe hold - * dma_resv_lock() already + * dma_resv_lock() already. + * * RETURNS - * true if all fences signaled, else false + * + * True if all fences signaled, else false. */ bool dma_resv_test_signaled(struct dma_resv *obj, bool test_all) { diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h index e1ca2080a1ff..c77fd54d033f 100644 --- a/include/linux/dma-resv.h +++ b/include/linux/dma-resv.h @@ -62,16 +62,90 @@ struct dma_resv_list { /** * struct dma_resv - a reservation object manages fences for a buffer - * @lock: update side lock - * @seq: sequence count for managing RCU read-side synchronization - * @fence_excl: the exclusive fence, if there is one currently - * @fence: list of current shared fences + * + * There are multiple uses for this, with sometimes slightly different rules in + * how the fence slots are used. + * + * One use is to synchronize cross-driver access to a struct dma_buf, either for + * dynamic buffer management or just to handle implicit synchronization between + * different users of the buffer in userspace. See &dma_buf.resv for a more + * in-depth discussion. + * + * The other major use is to manage access and locking within a driver in a + * buffer based memory manager. struct ttm_buffer_object is the canonical + * example here, since this is were reservation objects originated from. But use + * in drivers is spreading and some drivers also manage struct + * drm_gem_object with the same scheme. */ struct dma_resv { + /** + * @lock: + * + * Update side lock. Don't use directly, instead use the wrapper + * functions like dma_resv_lock() and dma_resv_unlock(). + * + * Drivers which use the reservation object to manage memory dynamically + * also use this lock to protect buffer object state like placement, + * allocation policies or throughout command submission. + */ struct ww_mutex lock; + + /** + * @seq: + * + * Sequence count for managing RCU read-side synchronization, allows + * read-only access to @fence_excl and @fence while ensuring we take a + * consistent snapshot. + */ seqcount_ww_mutex_t seq; + /** + * @fence_excl: + * + * The exclusive fence, if there is one currently. + * + * There are two was to update this fence: + * + * - First by calling dma_resv_add_excl_fence(), which replaces all + * fences attached to the reservation object. To guarantee that no + * fences are lost this new fence must signal only after all previous + * fences, both shared and exclusive, have signalled. In some cases it + * is convenient to achieve that by attaching a struct dma_fence_array + * with all the new and old fences. + * + * - Alternatively the fence can be set directly, which leaves the + * shared fences unchanged. To guarantee that no fences are lost this + * new fence must signale only after the previous exclusive fence has + * singalled. Since the shared fences are staying intact, it is not + * necessary to maintain any ordering against those. If semantically + * only a new access is added without actually treating the previous + * one as a dependency the exclusive fences can be strung together + * using struct dma_fence_chain. + * + * Note that actual semantics of what an exclusive or shared fence mean + * is defined by the user, for reservation objects shared across drivers + * see &dma_buf.resv. + */ struct dma_fence __rcu *fence_excl; + + /** + * @fence: + * + * List of current shared fences. + * + * There are no ordering constraints of shared fences against the + * exclusive fence slot. If a waiter needs to wait for all access, it + * has to wait for both set of fences to signal. + * + * A new fence is added by calling dma_resv_add_shared_fence(). Since + * this often needs to be done past the point of no return in command + * submission it cannot fail, and therefor sufficient slots need to be + * reserved by calling dma_resv_reserve_shared(). + * + * Note that actual semantics of what an exclusive or shared fence mean + * is defined by the user, for reservation objects shared across drivers + * see &dma_buf.resv. + */ struct dma_resv_list __rcu *fence; }; @@ -98,6 +172,13 @@ static inline void dma_resv_reset_shared_max(struct dma_resv *obj) {} * undefined order, a #ww_acquire_ctx is passed to unwind if a cycle * is detected. See ww_mutex_lock() and ww_acquire_init(). A reservation * object may be locked by itself by passing NULL as @ctx. + * + * When a die situation is indicated by returning -EDEADLK all locks held by + * @ctx must be unlocked and then dma_resv_lock_slow() called on @obj. + * + * Unlocked by calling dma_resv_lock(). + * + * See also dma_resv_lock_interruptible() for the interruptible variant. */ static inline int dma_resv_lock(struct dma_resv *obj, struct ww_acquire_ctx *ctx) @@ -119,6 +200,12 @@ static inline int dma_resv_lock(struct dma_resv *obj, * undefined order, a #ww_acquire_ctx is passed to unwind if a cycle * is detected. See ww_mutex_lock() and ww_acquire_init(). A reservation * object may be locked by itself by passing NULL as @ctx. + * + * When a die situation is indicated by returning -EDEADLK all locks held by + * @ctx must be unlocked and then dma_resv_lock_slow_interruptible() called on + * @obj. + * + * Unlocked by calling dma_resv_lock(). */ static inline int dma_resv_lock_interruptible(struct dma_resv *obj, struct ww_acquire_ctx *ctx) @@ -134,6 +221,8 @@ static inline int dma_resv_lock_interruptible(struct dma_resv *obj, * Acquires the reservation object after a die case. This function * will sleep until the lock becomes available. See dma_resv_lock() as * well. + * + * See also dma_resv_lock_slow_interruptible() for the interruptible variant. */ static inline void dma_resv_lock_slow(struct dma_resv *obj, struct ww_acquire_ctx *ctx) @@ -167,7 +256,7 @@ static inline int dma_resv_lock_slow_interruptible(struct dma_resv *obj, * if they overlap with a writer. * * Also note that since no context is provided, no deadlock protection is - * possible. + * possible, which is also not needed for a trylock. * * Returns true if the lock was acquired, false otherwise. */ @@ -193,6 +282,11 @@ static inline bool dma_resv_is_locked(struct dma_resv *obj) * * Returns the context used to lock a reservation object or NULL if no context * was used or the object is not locked at all. + * + * WARNING: This interface is pretty horrible, but TTM needs it because it + * doesn't pass the struct ww_acquire_ctx around in some very long callchains. + * Everyone else just uses it to check whether they're holding a reservation or + * not. */ static inline struct ww_acquire_ctx *dma_resv_locking_ctx(struct dma_resv *obj) {