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 X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 37E08C07E96 for ; Tue, 13 Jul 2021 17:47:40 +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 CEC8160720 for ; Tue, 13 Jul 2021 17:47:38 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CEC8160720 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com 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 034A66E106; Tue, 13 Jul 2021 17:47:38 +0000 (UTC) Received: from mga01.intel.com (mga01.intel.com [192.55.52.88]) by gabe.freedesktop.org (Postfix) with ESMTPS id D3CAB6E105; Tue, 13 Jul 2021 17:47:36 +0000 (UTC) X-IronPort-AV: E=McAfee;i="6200,9189,10044"; a="232019174" X-IronPort-AV: E=Sophos;i="5.84,237,1620716400"; d="scan'208";a="232019174" Received: from fmsmga003.fm.intel.com ([10.253.24.29]) by fmsmga101.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 13 Jul 2021 10:47:35 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.84,237,1620716400"; d="scan'208";a="493199519" Received: from stinkbox.fi.intel.com (HELO stinkbox) ([10.237.72.171]) by FMSMGA003.fm.intel.com with SMTP; 13 Jul 2021 10:47:32 -0700 Received: by stinkbox (sSMTP sendmail emulation); Tue, 13 Jul 2021 20:47:31 +0300 Date: Tue, 13 Jul 2021 20:47:31 +0300 From: Ville =?iso-8859-1?Q?Syrj=E4l=E4?= To: Matthew Auld Subject: Re: [Intel-gfx] [PATCH 1/5] drm/i915: document caching related bits Message-ID: References: <20210713104554.2381406-1-matthew.auld@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Patchwork-Hint: comment 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 , Matthew Auld , ML dri-devel Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Tue, Jul 13, 2021 at 05:13:37PM +0100, Matthew Auld wrote: > On Tue, 13 Jul 2021 at 16:55, Ville Syrjälä > wrote: > > > > On Tue, Jul 13, 2021 at 11:45:50AM +0100, Matthew Auld wrote: > > > + /** > > > + * @cache_coherent: > > > + * > > > + * Track whether the pages are coherent with the GPU if reading or > > > + * writing through the CPU cache. > > > + * > > > + * This largely depends on the @cache_level, for example if the object > > > + * is marked as I915_CACHE_LLC, then GPU access is coherent for both > > > + * reads and writes through the CPU cache. > > > + * > > > + * Note that on platforms with shared-LLC support(HAS_LLC) reads through > > > + * the CPU cache are always coherent, regardless of the @cache_level. On > > > + * snooping based platforms this is not the case, unless the full > > > + * I915_CACHE_LLC or similar setting is used. > > > + * > > > + * As a result of this we need to track coherency separately for reads > > > + * and writes, in order to avoid superfluous flushing on shared-LLC > > > + * platforms, for reads. > > > + * > > > + * I915_BO_CACHE_COHERENT_FOR_READ: > > > + * > > > + * When reading through the CPU cache, the GPU is still coherent. Note > > > + * that no data has actually been modified here, so it might seem > > > + * strange that we care about this. > > > + * > > > + * As an example, if some object is mapped on the CPU with write-back > > > + * caching, and we read some page, then the cache likely now contains > > > + * the data from that read. At this point the cache and main memory > > > + * match up, so all good. But next the GPU needs to write some data to > > > + * that same page. Now if the @cache_level is I915_CACHE_NONE and the > > > + * the platform doesn't have the shared-LLC, then the GPU will > > > + * effectively skip invalidating the cache(or however that works > > > + * internally) when writing the new value. This is really bad since the > > > + * GPU has just written some new data to main memory, but the CPU cache > > > + * is still valid and now contains stale data. As a result the next time > > > + * we do a cached read with the CPU, we are rewarded with stale data. > > > + * Likewise if the cache is later flushed, we might be rewarded with > > > + * overwriting main memory with stale data. > > > + * > > > + * I915_BO_CACHE_COHERENT_FOR_WRITE: > > > + * > > > + * When writing through the CPU cache, the GPU is still coherent. Note > > > + * that this also implies I915_BO_CACHE_COHERENT_FOR_READ. > > > + * > > > + * This is never set when I915_CACHE_NONE is used for @cache_level, > > > + * where instead we have to manually flush the caches after writing > > > + * through the CPU cache. For other cache levels this should be set and > > > + * the object is therefore considered coherent for both reads and writes > > > + * through the CPU cache. > > > > I don't remember why we have this read vs. write split and this new > > documentation doesn't seem to really explain it either. > > Hmm, I attempted to explain that earlier: > > * Note that on platforms with shared-LLC support(HAS_LLC) reads through > * the CPU cache are always coherent, regardless of the @cache_level. On > * snooping based platforms this is not the case, unless the full > * I915_CACHE_LLC or similar setting is used. > * > * As a result of this we need to track coherency separately for reads > * and writes, in order to avoid superfluous flushing on shared-LLC > * platforms, for reads. > > So AFAIK it's just because shared-LLC can be coherent for reads, while > also not being coherent for writes(CACHE_NONE), CPU vs. GPU is fully coherent when it comes to LLC. Or at least I've never heard of any mechanism that would make it only partially coherent. -- Ville Syrjälä Intel