Re: [Intel-xe] [PATCH 2/3] linux/bits.h: Add fixed-width GENMASK and BIT macros
From: Lucas De Marchi
Date: Tue Jun 20 2023 - 14:03:45 EST
On Tue, Jun 20, 2023 at 08:41:10PM +0300, Andy Shevchenko wrote:
On Tue, Jun 20, 2023 at 10:25:21AM -0700, Lucas De Marchi wrote:
On Tue, Jun 20, 2023 at 05:55:19PM +0300, Andy Shevchenko wrote:
> On Tue, Jun 20, 2023 at 05:47:34PM +0300, Jani Nikula wrote:
> > On Thu, 15 Jun 2023, Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > > On Fri, May 12, 2023 at 02:45:19PM +0300, Jani Nikula wrote:
> > >> On Fri, 12 May 2023, Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > >> > On Fri, May 12, 2023 at 02:25:18PM +0300, Jani Nikula wrote:
> > >> >> On Fri, 12 May 2023, Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > >> >> > On Mon, May 08, 2023 at 10:14:02PM -0700, Lucas De Marchi wrote:
> > >> >> >> Add GENMASK_U32(), GENMASK_U16() and GENMASK_U8() macros to create
> > >> >> >> masks for fixed-width types and also the corresponding BIT_U32(),
> > >> >> >> BIT_U16() and BIT_U8().
> > >> >> > Why?
> > >> >>
> > >> >> The main reason is that GENMASK() and BIT() size varies for 32/64 bit
> > >> >> builds.
> > >> >
> > >> > When needed GENMASK_ULL() can be used (with respective castings perhaps)
> > >> > and BIT_ULL(), no?
> > >>
> > >> How does that help with making them the same 32-bit size on both 32 and
> > >> 64 bit builds?
> > >
> > > u32 x = GENMASK();
> > > u64 y = GENMASK_ULL();
> > >
> > > No? Then use in your code either x or y. Note that I assume that the parameters
> > > to GENMASK*() are built-time constants. Is it the case for you?
> >
> > What's wrong with wanting to define macros with specific size, depending
> > on e.g. hardware registers instead of build size?
>
> Nothing, but I think the problem is smaller than it's presented.
not sure about big/small problem you are talking about. It's a problem
for when the *device* register is a 32b fixed width, which is
independent from the CPU you are running on. We also have registers that
are u16 and u64. Having fixed-width GENMASK and BIT helps avoiding
mistakes like below. Just to use one example, the diff below builds
fine on my 64b machine, yet it's obviously wrong:
$ git diff diff --git a/drivers/gpu/drm/i915/gt/intel_gt_mcr.c
b/drivers/gpu/drm/i915/gt/intel_gt_mcr.c
index 0b414eae1683..692a0ad9a768 100644
--- a/drivers/gpu/drm/i915/gt/intel_gt_mcr.c
+++ b/drivers/gpu/drm/i915/gt/intel_gt_mcr.c
@@ -261,8 +261,8 @@ static u32 rw_with_mcr_steering_fw(struct intel_gt *gt,
* No need to save old steering reg value.
*/
intel_uncore_write_fw(uncore, MTL_MCR_SELECTOR,
- REG_FIELD_PREP(MTL_MCR_GROUPID, group) |
- REG_FIELD_PREP(MTL_MCR_INSTANCEID, instance) |
+ FIELD_PREP(MTL_MCR_GROUPID, group) |
+ FIELD_PREP(MTL_MCR_INSTANCEID, instance) |
(rw_flag == FW_REG_READ ? GEN11_MCR_MULTICAST : 0));
} else if (GRAPHICS_VER(uncore->i915) >= 11) {
mcr_mask = GEN11_MCR_SLICE_MASK | GEN11_MCR_SUBSLICE_MASK;
diff --git a/drivers/gpu/drm/i915/gt/intel_gt_regs.h b/drivers/gpu/drm/i915/gt/intel_gt_regs.h
index 718cb2c80f79..c42bc2900c6a 100644
--- a/drivers/gpu/drm/i915/gt/intel_gt_regs.h
+++ b/drivers/gpu/drm/i915/gt/intel_gt_regs.h
@@ -80,8 +80,8 @@
#define GEN11_MCR_SLICE_MASK GEN11_MCR_SLICE(0xf)
#define GEN11_MCR_SUBSLICE(subslice) (((subslice) & 0x7) << 24)
#define GEN11_MCR_SUBSLICE_MASK GEN11_MCR_SUBSLICE(0x7)
-#define MTL_MCR_GROUPID REG_GENMASK(11, 8)
-#define MTL_MCR_INSTANCEID REG_GENMASK(3, 0)
+#define MTL_MCR_GROUPID GENMASK(32, 8)
+#define MTL_MCR_INSTANCEID GENMASK(3, 0)
#define IPEIR_I965 _MMIO(0x2064)
#define IPEHR_I965 _MMIO(0x2068)
If the driver didn't support 32b CPUs, this would even go unnoticed.
So, what does prevent you from using GENMASK_ULL()?
nothing is preventing me to write the wrong code, which is what we are
trying to solve. GENMASK_ULL() would generate the wrong code as that
particular register is 32b, not 64b, on the GPU.
Another point, you may teach GENMASK() to issue a warning if hi and/or lo
bigger than BITS_PER_LONG.
Which varies depending on the CPU you are building for, so it misses the
point. GENMASK_U32/GENMASK_U16/GENMASK_U8 and BIT counterparts would
emit a warning if hi is bigger than _exactly_ 32, 16 or 8, regardless
of the CPU you built the code for.
Lucas De Marchi
I still don't see the usefulness of that churn.
Lucas De Marchi
> And there are already header for bitfields with a lot of helpers
> for (similar) cases if not yours.
>
> > What would you use for printk format if you wanted to to print
> > GENMASK()?
>
> %lu, no?
--
With Best Regards,
Andy Shevchenko