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

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Message-ID: <5936f811-fa48-33e9-2a1a-66c68f74aa5e@ieee.org>
Date:   Mon, 22 Nov 2021 19:52:43 -0600
From:   Alex Elder <elder@...e.org>
To:     Johannes Berg <johannes@...solutions.net>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        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>,
        Joel Stanley <joel@....id.au>,
        Ping-Ke Shih <pkshih@...ltek.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Magnus Damm <magnus.damm@...il.com>,
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        Keerthy <j-keerthy@...com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Amit Kucheria <amitk@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>,
        Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-omap@...r.kernel.org,
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Subject: Re: [PATCH 01/17] bitfield: Add non-constant field_{prep,get}()
 helpers

On 11/22/21 10:32 AM, 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?
> 
> We have the upper-case (constant) versions, and already
> {u32,...}_get_bits()/etc.

I've used these a lot (and personally prefer the lower-case ones).

Your new macros don't do anything to ensure the field mask is
of the right form, which is basically:  (2 ^ width - 1) << shift

I really like the property that the field mask must be constant.

That being said, I've had to use some strange coding patterns
in order to adhere to the "const only" rule in a few cases.
So if you can come up with a satisfactory naming scheme I'm
all for it.

					-Alex



> 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()?
> 
> johannes
>