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 <groeck7@xxxxxxxxx> On Behalf Of Guenter Roeck > > > Sent: Monday, August 22, 2022 10:04 PM > > > To: Marco Felsch <m.felsch@xxxxxxxxxxxxxx> > > > Cc: Alice Guo (OSS) <alice.guo@xxxxxxxxxxx>; wim@xxxxxxxxxxxxxxxxxx; > > > shawnguo@xxxxxxxxxx; s.hauer@xxxxxxxxxxxxxx; festevam@xxxxxxxxx; > > > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > > > dl-linux-imx <linux-imx@xxxxxxx>; kernel@xxxxxxxxxxxxxx; > > > linux-watchdog@xxxxxxxxxxxxxxx > > > 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 <m.felsch@xxxxxxxxxxxxxx> > > > > > > Sent: Tuesday, August 16, 2022 2:24 PM > > > > > > To: Alice Guo (OSS) <alice.guo@xxxxxxxxxxx> > > > > > > Cc: wim@xxxxxxxxxxxxxxxxxx; linux@xxxxxxxxxxxx; > > > > > > shawnguo@xxxxxxxxxx; s.hauer@xxxxxxxxxxxxxx; festevam@xxxxxxxxx; > > > > > > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; > > > > > > linux-kernel@xxxxxxxxxxxxxxx; dl-linux-imx <linux-imx@xxxxxxx>; > > > > > > kernel@xxxxxxxxxxxxxx; linux-watchdog@xxxxxxxxxxxxxxx > > > > > > 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 <ping.bai@xxxxxxx> > > > > > > > > > > > > > > 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
