Re: [RFC PATCH] drm/xe/hwmon: report a single fan for DG2 instead of two

[Raag Jadav]

On Wed, May 27, 2026 at 07:53:11PM +0800, Zhan Wei wrote:
> xe_hwmon_pcode_read_fan_control() currently hardcodes *uval = 2 when
> queried with FSC_READ_NUM_FANS on DG2. This causes fan2_input to be
> exposed via sysfs, but on the tested Arc A750 LE (DG2 G10, PCI ID
> 0x56a1) fan2_input reads 0 RPM permanently while fan1_input correctly
> reports ~800 RPM with both physical fan physically spinning.
> 
> The RPM is calculated delta-based from a tach pulse counter:
> 
>     rotations = (reg_val - fi->reg_val_prev) / 2;
> 
> so a constant-zero RPM means the register at offset 0x138170
> (BMG_FAN_2_SPEED) simply does not accumulate pulses on DG2 silicon.
> The i915 driver does not expose fan2 on DG2 at all -- it only maps
> PCU_PWM_FAN_SPEED (0x138140, identical to BMG_FAN_1_SPEED), consistent
> with the observation that only one fan tach register is wired on DG2.

i915 is for legacy cards (like DG1) which only has a single channel
in hardware. I just happen to extend the support to DG2 for the folks
that might be using it.

> Report a single fan for DG2 to keep the phantom fan2_input out of
> sysfs.  Battlemage paths are unchanged.
> 
> Tested on Arc A750 LE (DG2 G10): with this patch applied, fan2_input
> no longer appears in /sys/class/hwmon/hwmonX/ and `sensors xe-pci-0300`
> shows fan1 only.
> 
> Fixes: 28f79ac609de ("drm/xe/hwmon: expose fan speed")
> Signed-off-by: Zhan Wei <zhanwei919@xxxxxxxxx>
> ---
> Open questions for reviewers: this is verified only on DG2 G10. Owners
> of G11 (e.g. ASRock Challenger A750) and G12 (e.g. Sparkle Titan A750
> with three physical fans) -- does fan2_input or fan3_input ever read
> non-zero in your setup? If so, the right fix is a per-subplatform
> table rather than a flat 1.

There's no straight answer here :)

root@DUT2147DG2FRD:/home/gta# cat /sys/class/drm/card0/device/device
0x56a1

root@DUT2147DG2FRD:/home/gta# sensors xe-pci-0300
xe-pci-0300
Adapter: PCI adapter
pkg:         758.00 mV
fan1:         636 RPM
fan2:         652 RPM
pkg:          +47.0°C
vram:         +50.0°C
pkg:              N/A  (max = 190.00 W)
pkg:          14.37 kJ


The way this works is upto the OEMs how they design their cards. Some reuse
a single channel for multiple physical fans while some use 1:1 mapped multiple
channels for each fan.

This is unfortunately not possible to figure out from the driver without
FSC_READ_NUM_FANS command (which has been found to be not working on some
cards and hence the hardcoded value).

Raag

>  drivers/gpu/drm/xe/xe_hwmon.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_hwmon.c b/drivers/gpu/drm/xe/xe_hwmon.c
> index de3f2aeffc3f..2a60a76b1971 100644
> --- a/drivers/gpu/drm/xe/xe_hwmon.c
> +++ b/drivers/gpu/drm/xe/xe_hwmon.c
> @@ -860,9 +860,15 @@ static int xe_hwmon_pcode_read_fan_control(const struct xe_hwmon *hwmon, u32 sub
>  {
>  	struct xe_tile *root_tile = xe_device_get_root_tile(hwmon->xe);
>  
> -	/* Platforms that don't return correct value */
> +	/*
> +	 * The PCODE FAN_SPEED_CONTROL subcommands return an error on DG2, so we
> +	 * answer the FSC_READ_NUM_FANS query here. DG2 only wires a single fan
> +	 * tachometer register (BMG_FAN_1_SPEED == 0x138140, shared with i915's
> +	 * PCU_PWM_FAN_SPEED); BMG_FAN_2/3_SPEED read 0 on DG2 silicon. Reporting
> +	 * one fan keeps a phantom fan2_input that always reads 0 out of sysfs.
> +	 */
>  	if (hwmon->xe->info.platform == XE_DG2 && subcmd == FSC_READ_NUM_FANS) {
> -		*uval = 2;
> +		*uval = 1;
>  		return 0;
>  	}
>  
> -- 
> 2.43.0
>