Re: [bootwrapper PATCH 02/13] Add bit-field macros

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Jan 11, 2022 at 02:40:48PM +0000, Andre Przywara wrote:
> On Tue, 11 Jan 2022 13:06:42 +0000
> Mark Rutland <mark.rutland@arm.com> wrote:
> 
> Hi Mark,
> 
> > Arm architectural documentation typically defines bit-fields as
> > `[msb,lsb]` and single-bit fields as `[bit]`. For clarity it would be
> > helpful if we could define fields in the same way.
> > 
> > Add helpers so that we can do so, along with helper to extract/insert
> > bit-field values.
> > 
> > There should be no functional change as a result of this patch.
> > 
> > Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> > ---
> >  include/bits.h | 33 +++++++++++++++++++++++++++++++++
> >  1 file changed, 33 insertions(+)
> >  create mode 100644 include/bits.h
> > 
> > diff --git a/include/bits.h b/include/bits.h
> > new file mode 100644
> > index 0000000..8824a38
> > --- /dev/null
> > +++ b/include/bits.h
> > @@ -0,0 +1,33 @@
> > +/*
> > + * include/bits.h - helpers for bit-field definitions.
> > + *
> > + * Copyright (C) 2021 ARM Limited. All rights reserved.
> > + *
> > + * Use of this source code is governed by a BSD-style license that can be
> > + * found in the LICENSE.txt file.
> > + */
> > +#ifndef __BITS_H
> > +#define __BITS_H
> > +
> > +#ifdef __ASSEMBLY__
> > +#define UL(x)	x
> > +#define ULL(x)	x
> > +#else
> > +#define UL(x)	x##UL
> > +#define ULL(x)	x##ULL
> > +#endif
> > +
> > +#define BITS(msb, lsb) \
> 
> The kernel uses GENMASK() for this, should we follow suit here? Both
> U-Boot and Trusted Firmware decided to do so, so I consider this some kind
> of agreed naming for bitmask generation these days.

TBH, I always forget the naming of GENMASK(), and chose `BITS()` to more
clearly align with `BIT()`, and also the way the architecture documentation
speaks about "bits [msb:lsb]".

I'm not wedded to the naming, but IMO `GENMASK()` isn't any better, even if
that's what linux uses. Regardless of the specific names, I'd like the
single-bit and multi-bit helpers to clearly align naming-wise.

For now I'd prefer to stick with `BIT()` and `BITS()`.

> > +((~ULL(0) >> (63 - msb)) & (~ULL(0) << lsb))
> > +
> > +#define BIT(b)	BITS(b, b)
> > +
> > +#define BITS_LSB(bits)	(__builtin_ffsll(bits) - 1)
> 
> Shall there be some comment explaining the functionality and arguments? Or
> maybe use "mask" instead of the more ambiguous "bits" name here?
> TBH I needed to read the implementation of the next macro to understand
> what it does.

If there's any confusion here I think we need comments regardless, since
neither `bits` nor `mask` imply contiguity, which is the important factor. I'll
add some comments with examples.

I'm happy to also rename the `bits` parameter to `mask`.

> > +
> > +#define BITS_EXTRACT(val, bits) \
> 
> Same here, having BITS_EXTRACT(val, mask) looks more readable to me.

I'll do as above hree, and likewise for the cases below.

Thanks,
Mark.

> 
> Cheers,
> Andre
> 
> > +	(((val) & (bits)) >> BITS_LSB(bits))
> > +
> > +#define BITS_INSERT(bits, val) \
> > +	(((val) << BITS_LSB(bits)) & (bits))
> > +
> > +#endif
> 

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