linux-kernel - Re: [PATCH 01/17] bitfield: Add non-constant field

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Message-ID: <12825803045d1cec0df968f72a9ef2724a2548fb.camel@sipsolutions.net>
Date:   Tue, 23 Nov 2021 17:21:41 +0100
From:   Johannes Berg <johannes@...solutions.net>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Tony Lindgren <tony@...mide.com>,
        Russell King <linux@...linux.org.uk>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Paul Walmsley <paul@...an.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Ludovic Desroches <ludovic.desroches@...rochip.com>,
        Tero Kristo <kristo@...nel.org>,
        Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Lorenzo Bianconi <lorenzo.bianconi83@...il.com>,
        Benoit Parrot <bparrot@...com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Andrew Jeffery <andrew@...id.au>,
        Ulf Hansson <ulf.hansson@...aro.org>,
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        Ping-Ke Shih <pkshih@...ltek.com>,
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Subject: Re: [PATCH 01/17] bitfield: Add non-constant field_{prep,get}()
 helpers

On Tue, 2021-11-23 at 09:30 +0100, Geert Uytterhoeven wrote:
> > We have the upper-case (constant) versions, and already
> > {u32,...}_get_bits()/etc.
> 
> These don't work for non-const masks.

Obviously, I know that. Still, just saying.

I'm actually in the opposite camp to you I guess - I much prefer the
typed versions (u32_get_bits() and friends) over the FIELD_GET() macros
that are more magic.

Mostly though that's because the typed ones also have le32_/be32_/...
variants, which are tremendously useful, and so I prefer to use them all
across. In fact, I have considered in the past to just remove the upper-
case macros entirely but ... no time I guess.

> > Also, you're using __ffs(), which doesn't work for 64-bit on 32-bit
> > architectures (afaict), so that seems a bit awkward.
> 
> That's a valid comment. Can be fixed by using a wrapper macro
> that checks if typeof(mask) == u64, and uses an __ffs64() version when
> needed.

You can't really do a typeof()==something, but you can check the size,
so yeah, that could be done.

> > Maybe we can make {u32,...}_get_bits() be doing compile-time only checks
> > if it is indeed a constant? The __field_overflow() usage is already only
> > done if __builtin_constant_p(v), so I guess we can do the same with
> > __bad_mask()?
> 
> Are all compilers smart enough to replace the division by
> field_multiplier(field) by a shift?

In the constant case they are, but you'd have to replace
field_multiplier() with the __ffs(), including the size check discussed
above. Then it's no longer a constant, and then I'm not so sure it would
actually be able to translate it, even if it's "1<<__ffs64(...)". I
guess you can check, or just change it to not use the division and
multiplication, but shifts/masks instead manually?

IOW - I would much prefer to make the type_get_bits() and friends work
for non-constant masks.

In fact, you have e.g. code in drivers/usb/chipidea/udc.c that does
things like cpu_to_le32(mul << __ffs(...)) - though in those cases it's
actually constant today, so you could already write it as
le32_encode_bits(...).

johannes