Re: [PATCH 01/17] bitfield: Add non-constant field_{prep,get}() helpers

From: Alex Elder <elder@ieee.org>
To: Johannes Berg <johannes@sipsolutions.net>,
	Geert Uytterhoeven <geert+renesas@glider.be>,
	Tony Lindgren <tony@atomide.com>,
	Russell King <linux@armlinux.org.uk>,
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	Alexandre Belloni <alexandre.belloni@bootlin.com>,
	Ludovic Desroches <ludovic.desroches@microchip.com>,
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	Jonathan Cameron <jic23@kernel.org>,
	Lars-Peter Clausen <lars@metafoo.de>,
	Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>,
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Subject: Re: [PATCH 01/17] bitfield: Add non-constant field_{prep,get}() helpers
Date: Mon, 22 Nov 2021 19:52:43 -0600	[thread overview]
Message-ID: <5936f811-fa48-33e9-2a1a-66c68f74aa5e@ieee.org> (raw)
In-Reply-To: <01b44b38c087c151171f8d45a2090474c2559306.camel@sipsolutions.net>

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
> 