Re: [PATCH v13 04/16] drm/xe/hwmon: Add helpers to validate thermal sensor readings

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] drm/xe/hwmon: mbx_temp_valid() fails to reject 0x80 (-0°C) as invalid
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commit cc835292455ece8a38adb02fe6e4c345fc3aa89a
Author: Karthik Poosa <karthik.poosa@intel.com>

drm/xe/hwmon: Add helpers to validate thermal sensor readings

This commit adds mmio_temp_valid() and mbx_temp_valid() to validate
temperature sensor readings. It uses the has_temp_sentinel flag to gate
these checks and rejects 0xFF for mailbox and 0xFFFFFFFF for MMIO.

> diff --git a/drivers/gpu/drm/xe/xe_hwmon.c b/drivers/gpu/drm/xe/xe_hwmon.c
> index 1b4efbf4ac999..883a0ae1d90e3 100644
> --- a/drivers/gpu/drm/xe/xe_hwmon.c
> +++ b/drivers/gpu/drm/xe/xe_hwmon.c
[ ... ]
> @@ -834,6 +837,30 @@ static int xe_hwmon_pcode_read_thermal_info(struct xe_hwmon *hwmon)
>  	return ret;
>  }
[ ... ]
> +/* Mailbox temperature is sign-magnitude (bit 7 sign, bits 6:0 magnitude). */
> +static inline bool mbx_temp_valid(const struct xe_hwmon *hwmon, u8 value)
> +{
> +	if (value == TEMP_MBX_INVALID)
> +		return false;
> +
> +	return hwmon->xe->info.has_temp_sentinel || value != 0;

[Severity: Low]
Does this raw byte check fail to reject 0x80?

Since 0x80 represents -0 degrees Celsius in the sign-magnitude format (sign
bit 7 is set, magnitude bits 6:0 are zero), it evaluates to true here
because 0x80 is not equal to 0.

> +}
> +
> +/* Decode an 8-bit sign-magnitude mailbox temperature to degrees Celsius. */
> +static int mbx_temp_decode(u8 value)
> +{
> +	int temp = value & TEMP_MBX_MAG;
> +
> +	return (value & TEMP_MBX_SIGN) ? -temp : temp;

If 0x80 is passed here, it decodes to 0 degrees Celsius.

This means the fallback safety logic in mbx_temp_valid() intended to reject
uninitialized 0 degrees Celsius readings can be bypassed if the hardware
returns 0x80 instead of 0x00. This could fold invalid readings into the
temperature average.

> +}

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Sashiko AI review · https://sashiko.dev/#/patchset/20261014115806.831459-1-karthik.poosa@intel.com?part=4