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

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Date:   Wed, 24 Nov 2021 15:07:47 +0100
From:   Johannes Berg <johannes@...solutions.net>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        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>,
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        Ping-Ke Shih <pkshih@...ltek.com>,
        Kalle Valo <kvalo@...eaurora.org>,
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        Linus Walleij <linus.walleij@...aro.org>,
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        alsa-devel@...a-project.org
Subject: Re: [PATCH 01/17] bitfield: Add non-constant field_{prep,get}()
 helpers

On Wed, 2021-11-24 at 05:59 -0800, Jakub Kicinski wrote:
> 
> FWIW I never found the be/le versions useful. Most of the time the data
> comes from bus accessors which swap or is unaligned so you have to do
> be/le_get_unaligned, which swaps. Plus if you access/set multiple
> fields you'd swap them one by one which seems wasteful.

Oh, we use them all the time in wifi!

I'm not sure I'm too concerned about wasteful - actually in wifi most of
the time it's little endian to start with, which matches the CPU for all
practical uses of wifi (**), and often we just access one field or so.
And anyway if we extract more than a single bit we need to swap anyway,
and I hope if it's just a single bit the compiler will optimize since
the one side is a constant? But whatever ...

(**) I had a fight with big-endian ARM a few years ago just to get wifi
tested on big-endian ...


> Right now it seems the uppercase macros are more prevalent.
> 

Not in my world ;-)

$ git grep FIELD_GET -- ... | wc -l
20
$ git grep le32_get_bits -- ... | wc -l
44
$ git grep le16_get_bits -- ... | wc -l
12
$ git grep u8_get_bits -- ... | wc -l
17

:-)

johannes