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

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Message-ID: <20211122171739.03848154@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Mon, 22 Nov 2021 17:17:39 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Tony Lindgren <tony@...mide.com>,
        Russell King <linux@...linux.org.uk>,
        Rajendra Nayak <rnayak@...eaurora.org>,
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        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>,
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Subject: Re: [PATCH 01/17] bitfield: Add non-constant field_{prep,get}()
 helpers

On Mon, 22 Nov 2021 17:32:43 +0100 Johannes Berg wrote:
> On Mon, 2021-11-22 at 16:53 +0100, Geert Uytterhoeven wrote:
> > The existing FIELD_{GET,PREP}() macros are limited to compile-time
> > constants.  However, it is very common to prepare or extract bitfield
> > elements where the bitfield mask is not a compile-time constant.
> 
> I'm not sure it's really a good idea to add a third API here?

+1

> We have the upper-case (constant) versions, and already
> {u32,...}_get_bits()/etc.
> 
> Also, you're using __ffs(), which doesn't work for 64-bit on 32-bit
> architectures (afaict), so that seems a bit awkward.
> 
> 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()?

Either that or add decomposition macros. Are compilers still really bad
at passing small structs by value?