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

[Nick Desaulniers <ndesaulniers@google.com>]

On Thu, Oct 27, 2022 at 9:53 AM Dixit, Ashutosh
<ashutosh.dixit@intel.com> wrote:
>
> On Thu, 27 Oct 2022 09:35:24 -0700, Nick Desaulniers wrote:
> >
>
> Hi Nick,
>
> > On Tue, Oct 25, 2022 at 5:18 PM Andi Shyti <andi.shyti@linux.intel.com> wrote:
> > >
> > > Hi Ashutosh,
> > >
> > > > But I'd wait to hear from clang/llvm folks first.
> > >
> > > Yeah! Looking forward to getting some ideas :)
> >
> > Gwan-gyeong, which tree and set of configs are necessary to reproduce
> > the observed warning?
> >
> > Warnings are treated as errors, so I don't want this breaking our CI.
>
> The following or equivalent should do it:
>
> git clone https://anongit.freedesktop.org/git/drm/drm-tip
> git checkout drm-tip
>
> Kernel config:
> CONFIG_DRM_I915=m
> CONFIG_HWMON=y
>
> Files:
> drivers/gpu/drm/i915/i915_hwmon.c/.h
>
> Thanks for taking a look.

Thanks, I can repro now.

I haven't detangled the macro soup, but I noticed:

1. FIELD_PREP is defined in include/linux/bitfield.h which has the
following comment:
 18  * Mask must be a compilation time constant.

2. hwm_field_scale_and_write only has one callsite.

The following patch works:

```
diff --git a/drivers/gpu/drm/i915/i915_hwmon.c
b/drivers/gpu/drm/i915/i915_hwmon.c
index 9e9781493025..6ac29d90b92a 100644
--- a/drivers/gpu/drm/i915/i915_hwmon.c
+++ b/drivers/gpu/drm/i915/i915_hwmon.c
@@ -101,7 +101,7 @@ 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,
+                         int nshift,
                          unsigned int scale_factor, long lval)
 {
        u32 nval;
@@ -111,8 +111,8 @@ hwm_field_scale_and_write(struct hwm_drvdata
*ddat, i915_reg_t rgadr,
        /* 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);
+       bits_to_clear = PKG_PWR_LIM_1;
+       bits_to_set = FIELD_PREP(PKG_PWR_LIM_1, nval);

        hwm_locked_with_pm_intel_uncore_rmw(ddat, rgadr,
                                            bits_to_clear, bits_to_set);
@@ -406,7 +406,6 @@ hwm_power_write(struct hwm_drvdata *ddat, u32
attr, int chan, long val)
        case hwmon_power_max:
                hwm_field_scale_and_write(ddat,
                                          hwmon->rg.pkg_rapl_limit,
-                                         PKG_PWR_LIM_1,
                                          hwmon->scl_shift_power,
                                          SF_POWER, val);
                return 0;
```
Though I'm not sure if you're planning to add further callsites of
hwm_field_scale_and_write with different field_masks?

Alternatively, (without the above diff),

```
diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index c9be1657f03d..6f40f12bcf89 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -8,6 +8,7 @@
 #define _LINUX_BITFIELD_H

 #include <linux/build_bug.h>
+#include <linux/const.h>
 #include <asm/byteorder.h>

 /*
@@ -62,7 +63,7 @@

 #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)                      \
        ({                                                              \
-               BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),          \
+               BUILD_BUG_ON_MSG(!__is_constexpr(_mask),                \
                                 _pfx "mask is not constant");          \
                BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero");    \
                BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?           \
```
will produce:
error: call to __compiletime_assert_407 declared with 'error'
attribute: FIELD_PREP: mask is not constant

I haven't tested if that change is also feasible (on top of fixing
this specific instance), but I think it might help avoid more of these
subtleties wrt. __builtin_constant_p that depende heavily on compiler,
compiler version, optimization level.
-- 
Thanks,
~Nick Desaulniers