linux-kernel - Re: watchdog: sp5100_tco support for AMD V/R/E series

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Date:   Mon, 7 Sep 2020 13:45:18 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Jan Kiszka <jan.kiszka@...mens.com>, linux-watchdog@...r.kernel.org
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        linux-kernel@...r.kernel.org,
        "Awan, Arsalan" <Arsalan_Awan@...tor.com>,
        "Hombourger, Cedric" <Cedric_Hombourger@...tor.com>,
        "Farnsworth, Wade" <wade_farnsworth@...tor.com>
Subject: Re: watchdog: sp5100_tco support for AMD V/R/E series

On 9/7/20 12:18 PM, Guenter Roeck wrote:
> On 9/7/20 8:46 AM, Jan Kiszka wrote:
>> On 07.09.20 17:31, Guenter Roeck wrote:
>>> On 9/7/20 4:20 AM, Jan Kiszka wrote:
>>>> Hi all,
>>>>
>>>> Arsalan reported that the upstream driver for sp5100_tco does not work
>>>> for embedded Ryzen. Meanwhile, I was able to confirm that on an R1505G:
>>>>
>>>> [   11.607251] sp5100_tco: SP5100/SB800 TCO WatchDog Timer Driver
>>>> [   11.607337] sp5100-tco sp5100-tco: Using 0xfed80b00 for watchdog MMIO address
>>>> [   11.607344] sp5100-tco sp5100-tco: Watchdog hardware is disabled
>>>>
>>>> ..and fix it:
>>>>
>>>> diff --git a/drivers/watchdog/sp5100_tco.c b/drivers/watchdog/sp5100_tco.c
>>>> index 85e9664318c9..5482154fde42 100644
>>>> --- a/drivers/watchdog/sp5100_tco.c
>>>> +++ b/drivers/watchdog/sp5100_tco.c
>>>> @@ -193,7 +193,8 @@ static void tco_timer_enable(struct sp5100_tco *tco)
>>>>  		/* Set the Watchdog timer resolution to 1 sec and enable */
>>>>  		sp5100_tco_update_pm_reg8(EFCH_PM_DECODEEN3,
>>>>  					  ~EFCH_PM_WATCHDOG_DISABLE,
>>>> -					  EFCH_PM_DECODEEN_SECOND_RES);
>>>> +					  EFCH_PM_DECODEEN_SECOND_RES |
>>>> +					  EFCH_PM_DECODEEN_WDT_TMREN);
>>>
>>> Confusing. The register in question is a 32-bit register, but only a byte
>>> is written into it. Bit 24-25 are supposed to be the resolution, bit 25-26
>>> set to 0 enable the watchdog. Bit 7 is supposed to enable MMIO decoding.
>>> This is from AMD Publication 52740. So something in the existing code
>>> is (or seems to be) wrong, but either case I don't see how setting bit 7
>>> (or 31 ?) would enable the watchdog hardware.
>>>
>>> Hmm, I wrote that code. Guess I'll need to to spend some time figuring out
>>> what is going on.
>>
>> The logic came from [1] which inspired [2] - that's where I pointed out
>> the large overlap with the existing upstream driver. I would love to see
>> all that consolidated.
>>
>> BTW, the R1505G is family 0x17. Maybe something changed there, and that
>> bit 7 was just reserved/ignored so far. ENOSPECS
>>
> 
> Thanks for the pointers.
> 
> I think you are talking about bit 31. Bit 7 is and was WatchdogTmrEn, but that
> supposedly only enables watchdog timer memory access at 0xfeb00000. From what
> I glance from the other drivers, the existing code is wrong. It should set
> the disable and resolution bits in register offset 3 (bit 24..27), not 0.
> In other words, EFCH_PM_DECODEEN3 should be defined as 0x03, not as 0x00.
> Which actually makes sense from the name.
> 
> Playing with my hardware, turns out that setting bit 7 in EFCH_PM_DECODEEN
> (register offset 0) does indeed enable the watchdog. I'll need to check
> if it actually works. Either case, -ENOSPECS is really a problem here.
> 

... and it does work. After playing with it, it seems that on Family 17h
CPUs EFCH_PM_DECODEEN_WDT_TMREN not only enables watchdog timer memory
access at 0xfeb0000, but also enables the watchdog itself.

Also, turns out the documentation is now public, at least for some of the
Family 17h CPUs (though oddly enough not for all of them). See processor
reference manuals at https://www.amd.com/en/support/tech-docs. The documents
for model 18h and model 20h include a note stating that bit 7 of
EFCH_PM_DECODEEN enables both memory access and the watchdog hardware.

So we'll need two patches - one to fix the value of EFCH_PM_DECODEEN3,
and one to enable the watchdog bit setting bit 7 of EFCH_PM_DECODEEN
for Family 17h CPUs.

Thanks,
Guenter

> Guenter
> 
>> Jan
>>
>> [1]
>> https://git.yoctoproject.org/cgit/cgit.cgi/meta-amd/commit/meta-amd-bsp/recipes-kernel/amd-wdt/files/amd_wdt.c?id=cd760c9f04d276382a0f5156dabdb766594ace56
>> [2]
>> https://github.com/siemens/efibootguard/commit/3a702aa96d193f26571ea4e70db29ef01a0d4d5f
>>
>