From patchwork Tue May 8 13:00:25 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Timo Aaltonen X-Patchwork-Id: 910176 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Authentication-Results: ozlabs.org; spf=none (mailfrom) smtp.mailfrom=lists.ubuntu.com (client-ip=91.189.94.19; helo=huckleberry.canonical.com; envelope-from=kernel-team-bounces@lists.ubuntu.com; receiver=) Authentication-Results: ozlabs.org; dmarc=none (p=none dis=none) header.from=ubuntu.com Received: from huckleberry.canonical.com (huckleberry.canonical.com [91.189.94.19]) by ozlabs.org (Postfix) with ESMTP id 40gKNR1FWkz9s2k; Tue, 8 May 2018 23:00:39 +1000 (AEST) Received: from localhost ([127.0.0.1] helo=huckleberry.canonical.com) by huckleberry.canonical.com with esmtp (Exim 4.86_2) (envelope-from ) id 1fG2EH-0001i0-Qg; Tue, 08 May 2018 13:00:29 +0000 Received: from youngberry.canonical.com ([91.189.89.112]) by huckleberry.canonical.com with esmtps (TLS1.0:DHE_RSA_AES_128_CBC_SHA1:128) (Exim 4.86_2) (envelope-from ) id 1fG2EF-0001hk-3y for kernel-team@lists.ubuntu.com; Tue, 08 May 2018 13:00:27 +0000 Received: from kryptik.nebulazone.fi ([83.145.237.38] helo=deckard.tyrell) by youngberry.canonical.com with esmtpsa (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.76) (envelope-from ) id 1fG2EE-0006WJ-QG for kernel-team@lists.ubuntu.com; Tue, 08 May 2018 13:00:26 +0000 From: Timo Aaltonen To: kernel-team@lists.ubuntu.com Subject: [SRU][OEM-B][PATCH] UBUNTU: SAUCE: drm/i915/execlists: Use rmb() to order CSB reads Date: Tue, 8 May 2018 16:00:25 +0300 Message-Id: <20180508130025.30095-1-tjaalton@ubuntu.com> X-Mailer: git-send-email 2.17.0 MIME-Version: 1.0 X-BeenThere: kernel-team@lists.ubuntu.com X-Mailman-Version: 2.1.20 Precedence: list List-Id: Kernel team discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: kernel-team-bounces@lists.ubuntu.com Sender: "kernel-team" From: Chris Wilson BugLink: http://bugs.launchpad.net/bugs/1769843 We assume that the CSB is written using the normal ringbuffer coherency protocols, as outlined in kernel/events/ring_buffer.c: * (HW) (DRIVER) * * if (LOAD ->data_tail) { LOAD ->data_head * (A) smp_rmb() (C) * STORE $data LOAD $data * smp_wmb() (B) smp_mb() (D) * STORE ->data_head STORE ->data_tail * } So we assume that the HW fulfils it's ordering requirements, and so we should use a complimentary rmb() to ensure that our read of its WRITE pointer is completed before we start accessing the data. The final mb() is implied by the uncached mmio we perform to inform the HW of our READ pointer. References: https://bugs.freedesktop.org/show_bug.cgi?id=105064 References: https://bugs.freedesktop.org/show_bug.cgi?id=105888 References: https://bugs.freedesktop.org/show_bug.cgi?id=106185 References: 61bf9719fa17 ("drm/i915/cnl: Use mmio access to context status buffer") Suggested-by: Mika Kuoppala Signed-off-by: Chris Wilson Cc: Mika Kuoppala Cc: Joonas Lahtinen Cc: Tvrtko Ursulin Cc: Michał Winiarski Cc: Rafael Antognolli Cc: Michel Thierry Cc: Timo Aaltonen Signed-off-by: Timo Aaltonen --- drivers/gpu/drm/i915/intel_lrc.c | 1 + 1 file changed, 1 insertion(+) diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c index e8af93e9235a..c4bcaa0cf677 100644 --- a/drivers/gpu/drm/i915/intel_lrc.c +++ b/drivers/gpu/drm/i915/intel_lrc.c @@ -838,6 +838,7 @@ static void intel_lrc_irq_handler(unsigned long data) head = execlists->csb_head; tail = READ_ONCE(buf[write_idx]); + rmb(); /* Hopefully paired with a wmb() in HW */ } while (head != tail) {