Re: [Intel-gfx] [PATCH v2] drm/i915/hwmon: Fix a build error used with clang compiler

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Fri, 28 Oct 2022 21:42:30 -0700, Gwan-gyeong Mun wrote:
>
> diff --git a/drivers/gpu/drm/i915/i915_hwmon.c b/drivers/gpu/drm/i915/i915_hwmon.c
> index 9e9781493025..c588a17f97e9 100644
> --- a/drivers/gpu/drm/i915/i915_hwmon.c
> +++ b/drivers/gpu/drm/i915/i915_hwmon.c
> @@ -101,21 +101,16 @@ hwm_field_read_and_scale(struct hwm_drvdata *ddat, i915_reg_t rgadr,
>
>  static void
>  hwm_field_scale_and_write(struct hwm_drvdata *ddat, i915_reg_t rgadr,
> -			  u32 field_msk, int nshift,
> -			  unsigned int scale_factor, long lval)
> +			  int nshift, unsigned int scale_factor, long lval)
>  {
>	u32 nval;
> -	u32 bits_to_clear;
> -	u32 bits_to_set;
>
>	/* Computation in 64-bits to avoid overflow. Round to nearest. */
>	nval = DIV_ROUND_CLOSEST_ULL((u64)lval << nshift, scale_factor);
>
> -	bits_to_clear = field_msk;
> -	bits_to_set = FIELD_PREP(field_msk, nval);
> -
>	hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,
> -					    bits_to_clear, bits_to_set);
> +					    PKG_PWR_LIM_1,
> +					    REG_FIELD_PREP(PKG_PWR_LIM_1, nval));

I registered my objection to this patch already here:

https://lore.kernel.org/intel-gfx/87ilk7pwrw.wl-ashutosh.dixit@intel.com/

the crux of which is "hwm_field_scale_and_write() pairs with
hwm_field_read_and_scale() (they are basically a set/get pair) so it is
desirable they have identical arguments. This patch breaks that symmetry".

We can merge this patch now but the moment a second caller of
hwm_field_scale_and_write arrives this patch will need to be reverted.

I have also posted my preferred way (as I previously indiecated) of fixing
this issue here (if this needs to be fixed in i915):

https://patchwork.freedesktop.org/series/110301/

IMO it would be a mistake to use REG_FIELD_PREP or FIELD_PREP here since
here the mask comes in as a function argument whereas REG_FIELD_PREP and
FIELD_PREP require that mask to be a compile time constant.

Thanks.
--
Ashutosh