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Tue, 23 Aug 2022 05:02:22 -0700 (PDT) Received: from server.roeck-us.net ([2600:1700:e321:62f0:329c:23ff:fee3:9d7c]) by smtp.gmail.com with ESMTPSA id t15-20020a17090340cf00b0016dbce87aecsm6684594pld.182.2022.08.23.05.02.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 23 Aug 2022 05:02:21 -0700 (PDT) Sender: Guenter Roeck Date: Tue, 23 Aug 2022 05:02:19 -0700 From: Guenter Roeck To: Marco Felsch Cc: "Alice Guo (OSS)" , "wim@linux-watchdog.org" , "shawnguo@kernel.org" , "s.hauer@pengutronix.de" , "festevam@gmail.com" , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , dl-linux-imx , "kernel@pengutronix.de" , "linux-watchdog@vger.kernel.org" Subject: Re: [PATCH 2/7] watchdog: imx7ulp: Add explict memory barrier for unlock sequence Message-ID: <20220823120219.GA203169@roeck-us.net> References: <20220816043643.26569-1-alice.guo@oss.nxp.com> <20220816043643.26569-3-alice.guo@oss.nxp.com> <20220816062330.z2fvurteg337krw2@pengutronix.de> <20220822080010.ecdphpm3i26cco5f@pengutronix.de> <20220822140347.GA4087281@roeck-us.net> <20220823091027.ezyxkn64asajvjom@pengutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220823091027.ezyxkn64asajvjom@pengutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-watchdog@vger.kernel.org On Tue, Aug 23, 2022 at 11:10:27AM +0200, Marco Felsch wrote: > On 22-08-23, Alice Guo (OSS) wrote: > > > > > > > -----Original Message----- > > > From: Guenter Roeck On Behalf Of Guenter Roeck > > > Sent: Monday, August 22, 2022 10:04 PM > > > To: Marco Felsch > > > Cc: Alice Guo (OSS) ; wim@linux-watchdog.org; > > > shawnguo@kernel.org; s.hauer@pengutronix.de; festevam@gmail.com; > > > linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org; > > > dl-linux-imx ; kernel@pengutronix.de; > > > linux-watchdog@vger.kernel.org > > > Subject: Re: [PATCH 2/7] watchdog: imx7ulp: Add explict memory barrier for > > > unlock sequence > > > > > > On Mon, Aug 22, 2022 at 10:00:10AM +0200, Marco Felsch wrote: > > > > On 22-08-22, Alice Guo (OSS) wrote: > > > > > > -----Original Message----- > > > > > > From: Marco Felsch > > > > > > Sent: Tuesday, August 16, 2022 2:24 PM > > > > > > To: Alice Guo (OSS) > > > > > > Cc: wim@linux-watchdog.org; linux@roeck-us.net; > > > > > > shawnguo@kernel.org; s.hauer@pengutronix.de; festevam@gmail.com; > > > > > > linux-arm-kernel@lists.infradead.org; > > > > > > linux-kernel@vger.kernel.org; dl-linux-imx ; > > > > > > kernel@pengutronix.de; linux-watchdog@vger.kernel.org > > > > > > Subject: Re: [PATCH 2/7] watchdog: imx7ulp: Add explict memory > > > > > > barrier for unlock sequence > > > > > > > > > > > > On 22-08-16, Alice Guo (OSS) wrote: > > > > > > > From: Jacky Bai > > > > > > > > > > > > > > Add explict memory barrier for the wdog unlock sequence. > > > > > > > > > > > > Did you inspected any failures? It's not enough to say what you > > > > > > did, you need to specify the why as well. > > > > > > > > > > > > Regards, > > > > > > Marco > > > > > > > > > > Hi, > > > > > > > > > > Two 16-bit writes of unlocking the Watchdog should be completed within a > > > certain time. The first mb() is used to ensure that previous instructions are > > > completed. > > > > > The second mb() is used to ensure that the unlock sequence cannot be > > > affected by subsequent instructions. The reason will be added in the commit > > > log of v2. > > > > > > > > Hi, > > > > > > > > I know what memory barriers are. My question was, did you see any > > > > issues? Since the driver is used mainline and no one reported issues. > > > > > > > > Also just don't use the *_relaxed() versions is more common, than > > > > adding > > > > mb() calls around *_relaxed() versions. > > > > > > > > > > Agreed with both. The series is a bit short in explaining _why_ the changes are > > > made. > > > > > > Guenter > > > > > > > Regards, > > > > Marco > > > > > > > > > > > > > Hi Guenter and Marco, > > > > 1. did you see any issues? > > This WDOG Timer first appeared in i.MX7ULP, no one report issues > > probably because few people use i.MX7ULP. This issue was found when we > > did a stress test on it. When we reconfigure the WDOG Timer, there is > > a certain probability that it reset. The reason for the error is that > > when WDOG_CS[CMD32EN] is 0, the unlock sequence is two 16-bit writes > > (0xC520, 0xD928) to the CNT register within 16 bus clocks, and > > improper unlock sequence causes the WDOG to reset. Adding mb() is to > > guarantee that two 16-bit writes are finished within 16 bus clocks. > > After this explanation the whole imx7ulp_wdt_init() seems a bit buggy > because writel_relaxed() as well as writel() are 32bit access functions. > So the very first thing to do is to enable the 32-bit mode. > Agreed. This is much better than having extra code to deal with both 16-bit and 32-bit access. > Also this is a explanation worth to be added to the commit message ;) > Definitely. Also, the use of mb(), if it should indeed be needed, would have to be explained in a code comment. Thanks, Guenter > > 2. Also just don't use the *_relaxed() versions is more common, than > > adding mb() calls around *_relaxed() versions. Memory barriers cannot > > be added between two 16-bit writes. I do not know the reason. > > As written above, writel() as well as writel_relaxed() are not 16-bit > access functions. > > So to me it looks as you need first to ensure that 32-bit access mode is > enabled. After that you can write drop the to writel_relaxed() functions > and instead just write: > > writel(UNLOCK, wdt->base + WDOG_CNT); > > Also why do we need to unlock the watchdog during imx7ulp_wdt_init()? > This is handled just fine during imx7ulp_wdt_enable() and during > imx7ulp_wdt_set_timeout(). So just drop those relaxed writes and > everything should be fine. > > Regards, > Marco