Re: [PATCH v2] bitfield: Add FIELD_MODIFY() helper

[Jie Luo]

On 4/11/2025 12:18 AM, Yury Norov wrote:
> On Thu, Apr 10, 2025 at 09:10:48PM +0800, Luo Jie wrote:
> > Add a helper for replacing the contents of bitfield in memory
> > with the specified value.
> >
> > Even though a helper xxx_replace_bits() is available, it is not
> > well documented, and only reports errors at the run time, which
> > will not be helpful to catch possible overflow errors due to
> > incorrect parameter types used.
> >
> > Add the helper FIELD_MODIFY() to the FIELD_XXX family of bitfield
> > macros. It is functionally similar as xxx_replace_bits(), and in
> > addition adds the compile time type checking.
> >
> > FIELD_MODIFY(&reg, REG_FIELD_C, c) is the wrapper of the code below.
> > reg &= ~REG_FIELD_C;
> > reg |= FIELD_PREP(REG_FIELD_C, c);
> >
> > Signed-off-by: Luo Jie <quic_luoj@xxxxxxxxxxx>
> > ---
> >
> > The new added macro FIELD_MODIFY() is expected to be used by the
> > following Ethernet PPE driver as link.
> > https://lore.kernel.org/linux-arm-msm/20250209-qcom_ipq_ppe-v3-0-453ea18d3271@xxxxxxxxxxx/
> >
> > Changes in v2:
> > - Update the documented example for FIELD_MODIFY().
> > - Improve the commit message to describe the need for the change.
> >
> >   include/linux/bitfield.h | 23 ++++++++++++++++++++---
> >   1 file changed, 20 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> > index 63928f173223..421c7701a18d 100644
> > --- a/include/linux/bitfield.h
> > +++ b/include/linux/bitfield.h
> > @@ -7,8 +7,9 @@
> >   #ifndef _LINUX_BITFIELD_H
> >   #define _LINUX_BITFIELD_H
> >   
> > -#include <linux/build_bug.h>
> >   #include <asm/byteorder.h>
> > +#include <linux/build_bug.h>
> > +#include <linux/typecheck.h>
>
> Don't change the headers order: linux first, asm next.

Understand, I will fix this order in the next version.

> >   
> >   /*
> >    * Bitfield access macros
> > @@ -38,8 +39,7 @@
> >    *	  FIELD_PREP(REG_FIELD_D, 0x40);
> >    *
> >    * Modify:
> > - *  reg &= ~REG_FIELD_C;
> > - *  reg |= FIELD_PREP(REG_FIELD_C, c);
> > + *  FIELD_MODIFY(REG_FIELD_C, &reg, c);
> >    */
> >   
> >   #define __bf_shf(x) (__builtin_ffsll(x) - 1)
> > @@ -156,6 +156,23 @@
> >   		(typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask));	\
> >   	})
> >   
> > +/**
> > + * FIELD_MODIFY() - modify a bitfield element
> > + * @_mask: shifted mask defining the field's length and position
> > + * @_reg_p: pointer to the memory that should be updated
> > + * @_val: value to store in the bitfield
> > + *
> > + * FIELD_MODIFY() modifies the set of bits in @_reg_p specified by @_mask,
> > + * by replacing them with the bitfield value passed in as @_val.
> > + */
>
> Please inspect the codebase and convert existing opencoded FIELD_MODIFY()s.
> I don't ask you to convert every driver out there, but core kernel files
> should be clear.
>
> The first good candidate for you is __tlbi_level() in arm64. You may want
> to use Coccinelle to automate the search.
>
> Thanks,
> Yury

OK, thanks for the suggestions. I will use the tool to find and convert
the opencoded FIELD_MODIFY() instances.

> > +#define FIELD_MODIFY(_mask, _reg_p, _val)				\
> > +	({								\
> > +		typecheck_pointer(_reg_p);				\
> > +		__BF_FIELD_CHECK(_mask, *(_reg_p), _val, "FIELD_MODIFY: "); \
> > +		*(_reg_p) &= ~(_mask);					\
> > +		*(_reg_p) |= ((_val) << __bf_shf(_mask)) & (_mask);	\
> > +	})
> > +
> >   extern void __compiletime_error("value doesn't fit into mask")
> >   __field_overflow(void);
> >   extern void __compiletime_error("bad bitfield mask")
> > --
> > 2.34.1