From patchwork Wed Jun 15 12:46:34 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: David Jander X-Patchwork-Id: 12882293 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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id C8E8FCCA481 for ; Wed, 15 Jun 2022 12:47:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348511AbiFOMrz (ORCPT ); Wed, 15 Jun 2022 08:47:55 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52964 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1348537AbiFOMrs (ORCPT ); Wed, 15 Jun 2022 08:47:48 -0400 Received: from smtp16.bhosted.nl (smtp16.bhosted.nl [IPv6:2a02:9e0:8000::27]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 829A1E01C for ; Wed, 15 Jun 2022 05:47:36 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=protonic.nl; s=202111; h=content-transfer-encoding:mime-version:references:in-reply-to:message-id:date: subject:cc:to:from:from; bh=jg6S3PjeI5qTEyeBZrsTnAtoxgxaUwKcgRhBM8dhK4U=; b=rHTeOE4tzt7ZLcoRW1pRINDmnDeVvcgsgC0c7jYVjEfw55H9fGrmr1ZZDQag4GWIa3dd32PhnTEoy 5l5D1uvwp/9W3Zp/lReZOWruG+7UBgk0yBBLN3HcgrJEhrkpwR3F/C2t3Wf2C0bia0U515jyHOvBy+ IKwHqzJMx69zCPXpYRTbo6RSDmZBnAxOnbKWftErPpQSGihTkDaRgq8eB8nr0SJqiWCmInp5StUrXC YmgKrm+ahbWO9qBDCLmawvFls69qU3VP+HnyZusms/B0lLB4Uks8ZPxl2kHVAWRElDMBjW/02qAKUa bI5as1XXqVWUJ9Ee4J9ft7d3nK5fPpA== X-MSG-ID: 53d59342-eca9-11ec-9051-0050569d2c73 From: David Jander To: Mark Brown Cc: linux-spi@vger.kernel.org, Marc Kleine-Budde , Andrew Lunn , David Jander Subject: [RFC] [PATCH v2 11/11] spi: opportunistically skip ctlr->cur_msg_completion Date: Wed, 15 Jun 2022 14:46:34 +0200 Message-Id: <20220615124634.3302867-12-david@protonic.nl> X-Mailer: git-send-email 2.32.0 In-Reply-To: <20220615124634.3302867-1-david@protonic.nl> References: <20220615124634.3302867-1-david@protonic.nl> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-spi@vger.kernel.org There are only a few drivers that do not call spi_finalize_current_message() in the context of transfer_one_message(), and even for those cases the completion ctlr->cur_msg_completion is not needed always. The calls to complete() and wait_for_completion() each take a spin-lock, which is costly. This patch makes it possible to avoid those calls in the big majority of cases, by introducing two flags that with the help of ordering via barriers can avoid using the completion safely. In case of a race with the context calling spi_finalize_current_message(), the scheme errs on the safe side and takes the completion. The impact of this patch is worth the effort: On a i.MX8MM SoC, the time the SPI bus is idle between two consecutive calls to spi_sync(), is reduced from 19.6us to 16.8us... roughly 15%. Signed-off-by: David Jander --- drivers/spi/spi.c | 27 +++++++++++++++++++++++++-- include/linux/spi/spi.h | 8 ++++++++ 2 files changed, 33 insertions(+), 2 deletions(-) diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c index db08cb868652..ef37f043fd17 100644 --- a/drivers/spi/spi.c +++ b/drivers/spi/spi.c @@ -1613,14 +1613,34 @@ static int __spi_pump_transfer_message(struct spi_controller *ctlr, } } + /* + * Drivers implementation of transfer_one_message() must arrange for + * spi_finalize_current_message() to get called. Most drivers will do + * this in the calling context, but some don't. For those cases, a + * completion is used to guarantee that this function does not return + * until spi_finalize_current_message() is done accessing + * ctlr->cur_msg. + * Use of the following two flags enable to opportunistically skip the + * use of the completion since its use involves expensive spin locks. + * In case of a race with the context that calls + * spi_finalize_current_message() the completion will always be used, + * due to strict ordering of these flags using barriers. + */ + WRITE_ONCE(ctlr->cur_msg_incomplete, true); + WRITE_ONCE(ctlr->cur_msg_need_completion, false); reinit_completion(&ctlr->cur_msg_completion); + smp_wmb(); /* make these available to spi_finalize_current_message */ + ret = ctlr->transfer_one_message(ctlr, msg); if (ret) { dev_err(&ctlr->dev, "failed to transfer one message from queue\n"); return ret; } else { - wait_for_completion(&ctlr->cur_msg_completion); + WRITE_ONCE(ctlr->cur_msg_need_completion, true); + smp_mb(); /* see spi_finalize_current_message()... */ + if (READ_ONCE(ctlr->cur_msg_incomplete)) + wait_for_completion(&ctlr->cur_msg_completion); } return 0; @@ -1942,7 +1962,10 @@ void spi_finalize_current_message(struct spi_controller *ctlr) mesg->prepared = false; - complete(&ctlr->cur_msg_completion); + WRITE_ONCE(ctlr->cur_msg_incomplete, false); + smp_mb(); /* See __spi_pump_transfer_message()... */ + if (READ_ONCE(ctlr->cur_msg_need_completion)) + complete(&ctlr->cur_msg_completion); trace_spi_message_done(mesg); diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h index c56e0d240a58..eb0d316e3c36 100644 --- a/include/linux/spi/spi.h +++ b/include/linux/spi/spi.h @@ -385,6 +385,12 @@ extern struct spi_device *spi_new_ancillary_device(struct spi_device *spi, u8 ch * @queue: message queue * @cur_msg: the currently in-flight message * @cur_msg_completion: a completion for the current in-flight message + * @cur_msg_incomplete: Flag used internally to opportunistically skip + * the @cur_msg_completion. This flag is used to check if the driver has + * already called spi_finalize_current_message(). + * @cur_msg_need_completion: Flag used internally to opportunistically skip + * the @cur_msg_completion. This flag is used to signal the context that + * is running spi_finalize_current_message() that it needs to complete() * @cur_msg_mapped: message has been mapped for DMA * @last_cs: the last chip_select that is recorded by set_cs, -1 on non chip * selected @@ -617,6 +623,8 @@ struct spi_controller { struct list_head queue; struct spi_message *cur_msg; struct completion cur_msg_completion; + bool cur_msg_incomplete; + bool cur_msg_need_completion; bool busy; bool running; bool rt;