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[176.86.136.19]) by smtp.gmail.com with ESMTPSA id c30sm132033252wrb.15.2019.07.31.13.34.32 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 31 Jul 2019 13:34:32 -0700 (PDT) Subject: Re: [PATCH 4.19 024/113] tty: serial: msm_serial: avoid system lockup condition To: Pavel Machek , Greg Kroah-Hartman Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org, Sasha Levin References: <20190729190655.455345569@linuxfoundation.org> <20190729190701.631193260@linuxfoundation.org> <20190731190533.GA4630@amd> From: Jorge Ramirez Message-ID: Date: Wed, 31 Jul 2019 22:34:30 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <20190731190533.GA4630@amd> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/31/19 21:05, Pavel Machek wrote: > Hi! hi Pavel, > >> [ Upstream commit ba3684f99f1b25d2a30b6956d02d339d7acb9799 ] >> >> The function msm_wait_for_xmitr can be taken with interrupts >> disabled. In order to avoid a potential system lockup - demonstrated >> under stress testing conditions on SoC QCS404/5 - make sure we wait >> for a bounded amount of time. >> >> Tested on SoC QCS404. > > How long did it take to timeout? > > Because... this is supposed to loop for 0.5 second with interrupts > disabled, but 500000*udelay(1) is probably going to wait for more than > that. > > Is 500msec reasonable with interrupts disabled? considering the original unbounded definition, it is hard to determine what would be a good amount of time to wait (msm_serial can be used for BT comms and I am not sure how critical that link might be for different clients..and I didnt want to create a regression hence the half a second delay). yeah, I don't think disabling interrupts for half a second is a good idea on most systems hence why I chose it that big. > > Should it use something like 5000*udelay(100), instead, as that has > chance to result in closer-to-500msec wait? the half a second timeout didnt mean to be accurate but a worst case scenario...I am not sure accuracy matters. > >> +++ b/drivers/tty/serial/msm_serial.c >> @@ -383,10 +383,14 @@ static void msm_request_rx_dma(struct msm_port *msm_port, resource_size_t base) >> >> static inline void msm_wait_for_xmitr(struct uart_port *port) >> { >> + unsigned int timeout = 500000; >> + >> while (!(msm_read(port, UART_SR) & UART_SR_TX_EMPTY)) { >> if (msm_read(port, UART_ISR) & UART_ISR_TX_READY) >> break; >> udelay(1); >> + if (!timeout--) >> + break; >> } >> msm_write(port, UART_CR_CMD_RESET_TX_READY, UART_CR); >> } > > Plus, should it do some kind of dev_err() to let users know that > something went very wrong with their serial? I did consider this but then I thought that 1/2 second without interrupts on the core should not go unnoticed. But I might be wrong. > > Thanks, > Pavel >