Re: [PATCH] i2c: piix4: Print FCH::PM::S5_RESET_STATUS

[Yazen Ghannam <yazen.ghannam@amd.com>]

On 4/12/23 12:53, Jean Delvare wrote:
> Hi Yazen,
> 
> On Fri, 07 Apr 2023 15:37:20 -0500, Yazen Ghannam wrote:
>> The following register contains bits that indicate the cause for the
>> previous reset.
>>
>>         PMx000000C0 (FCH::PM::S5_RESET_STATUS)
>>
>> This is helpful for debug, etc., and it only needs to be read once from
>> a single FCH within the system. The register definition is AMD-specific.
>>
>> Print it when the FCH MMIO space is first mapped. This register is not
>> related to I2C functionality, but read it here to leverage the existing
>> mapping.
>>
>> Use an "info" log level so that it is printed every boot without requiring
>> the user to enable debug messages. This is beneficial when debugging
>> issues that cause spontaneous reboots and are hard to reproduce.
>>
>> Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
>> ---
>>  drivers/i2c/busses/i2c-piix4.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/i2c/busses/i2c-piix4.c b/drivers/i2c/busses/i2c-piix4.c
>> index 809fbd014cd6..043b29f1e33c 100644
>> --- a/drivers/i2c/busses/i2c-piix4.c
>> +++ b/drivers/i2c/busses/i2c-piix4.c
>> @@ -100,6 +100,7 @@
>>  
>>  #define SB800_PIIX4_FCH_PM_ADDR			0xFED80300
>>  #define SB800_PIIX4_FCH_PM_SIZE			8
>> +#define SB800_PIIX4_FCH_PM_S5_RESET_STATUS	0xC0
>>  
>>  /* insmod parameters */
>>  
>> @@ -200,6 +201,9 @@ static int piix4_sb800_region_request(struct device *dev,
>>  
>>  		mmio_cfg->addr = addr;
>>  
>> +		addr += SB800_PIIX4_FCH_PM_S5_RESET_STATUS;
>> +		pr_info_once("S5_RESET_STATUS = 0x%08x", ioread32(addr));
>> +
>>  		return 0;
>>  	}
>>  
> 
> I'm skeptical. For one thing, the register you read is outside of the
> mapped MMIO area. SB800_PIIX4_FCH_PM_SIZE is 8 which is less than 0xC0.
> 

Hi Jean,

Ah sorry, I overlooked that.

> For another, printing an hexadecimal value which is AMD-specific is not
> going to be really helpful in practice. Is there public documentation
> available to decode the value?
> 

Yes, this register is listed in public documentation. But I expect this value
is only helpful to hardware debug folks. The intent is to have this
information available without requiring any system changes by the user.

> Lastly, I can't see why this should happen in
> piix4_sb800_region_request() which is going to called repeatedly at
> runtime, rather than in piix4_setup_sb800_smba() which is only called
> once when the driver is loaded. If this goes in the i2c-piix4 driver at
> all... sp5100_tco might be more suitable as that driver is at least
> somewhat related to system reset.
> 
> Looks like a hack really, and while I understand it is cheap, it would
> seem cleaner to put that code in its own platform/x86 driver. Or
> arch/x86/kernel/quirks.c maybe.
> 

Yes, that's fair. I'll check out these other places and see if there's a
better fit.

Thank you!

-Yazen