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=-6.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY, 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 17A4EC728EF for ; Thu, 27 Feb 2020 14:23:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D495924688 for ; Thu, 27 Feb 2020 14:23:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582813403; bh=cXyWnGOXMDI/RpQ47AsYJXfqH3j09CyQ273gSjmt4ek=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=gl3L/N1A5xGbLr/KbJkTvDZhDtvGiITtP8JDSQHkADhg/ZmWRE9w/fLk9cMKBNopF 5PUwa5D+a5eK+62JkBvLkUdDic0s4A0PfyQAxuUvb7g9Ly6B3a0VymWcJUrYuPq9zS A01vPzEmGM08D7DUyv3TiLubgwcdrkSrXCbDTgGs= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389169AbgB0OOn (ORCPT ); Thu, 27 Feb 2020 09:14:43 -0500 Received: from mail.kernel.org ([198.145.29.99]:54050 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389160AbgB0OOk (ORCPT ); Thu, 27 Feb 2020 09:14:40 -0500 Received: from localhost (83-86-89-107.cable.dynamic.v4.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id C0BCC24691; Thu, 27 Feb 2020 14:14:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582812880; bh=cXyWnGOXMDI/RpQ47AsYJXfqH3j09CyQ273gSjmt4ek=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=KI1PEwXr55OVwEFIas5qsW+fWKpSdBUGEeqUXWa2S5sb7W3F5xaAU1MgPyFSmR9+M jCHqOkx0ScUQ/MVTdyuxDCSOXT4wzH2JLfJjZj5I1k42onDRPZW6IHTjAk1o0vmab2 g/bbYgTwGie3t4RfCqmiKyrWU/1+EKnvzlBRgWAM= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Jakub Nantl , Johan Hovold Subject: [PATCH 5.5 024/150] USB: serial: ch341: fix receiver regression Date: Thu, 27 Feb 2020 14:36:01 +0100 Message-Id: <20200227132236.387789404@linuxfoundation.org> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20200227132232.815448360@linuxfoundation.org> References: <20200227132232.815448360@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Johan Hovold commit 7c3d02285ad558691f27fde760bcd841baa27eab upstream. While assumed not to make a difference, not using the factor-2 prescaler makes the receiver more susceptible to errors. Specifically, there have been reports of problems with devices that cannot generate a 115200 rate with a smaller error than 2.1% (e.g. 117647 bps). But this can also be reproduced with a low-speed RS232 tranceiver at 115200 when the input rate matches the nominal rate. So whenever possible, enable the factor-2 prescaler and halve the divisor in order to use settings closer to that of the previous algorithm. Fixes: 35714565089e ("USB: serial: ch341: reimplement line-speed handling") Cc: stable # 5.5 Reported-by: Jakub Nantl Tested-by: Jakub Nantl Reviewed-by: Greg Kroah-Hartman Signed-off-by: Johan Hovold Signed-off-by: Greg Kroah-Hartman --- drivers/usb/serial/ch341.c | 10 ++++++++++ 1 file changed, 10 insertions(+) --- a/drivers/usb/serial/ch341.c +++ b/drivers/usb/serial/ch341.c @@ -205,6 +205,16 @@ static int ch341_get_divisor(speed_t spe 16 * speed - 16 * CH341_CLKRATE / (clk_div * (div + 1))) div++; + /* + * Prefer lower base clock (fact = 0) if even divisor. + * + * Note that this makes the receiver more tolerant to errors. + */ + if (fact == 1 && div % 2 == 0) { + div /= 2; + fact = 0; + } + return (0x100 - div) << 8 | fact << 2 | ps; }