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

Jean Delvare
2023-04-12T16:55:28Z

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.

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?

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.

--
Jean Delvare
SUSE L3 Support