linux-kernel - Re: [PATCH] bitfield.h: add FIELD_MAX

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Date: Mon, 20 May 2024 15:26:37 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Yury Norov <yury.norov@...il.com>
Cc: Michal Schmidt <mschmidt@...hat.com>, Rasmus Villemoes
 <linux@...musvillemoes.dk>, linux-kernel@...r.kernel.org, Alex Elder
 <elder@...aro.org>, "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] bitfield.h: add FIELD_MAX_CONST

On Mon, 20 May 2024 12:29:54 -0700 Yury Norov wrote:
> > A simplified example of what I actually want to use in a driver:
> >   #define DATA_SIZE_M GENMASK(3, 0)
> >   #define MAX_DATA_SIZE FIELD_MAX_CONST(DATA_SIZE_M)
> >   static void f(void) {
> >   	char buf[MAX_DATA_SIZE];

You need a "+ 1" somewhere here, right?

> >   }
> > 
> > In the implementation, reuse the existing compile-time checks from
> > FIELD_PREP_CONST.
> > 
> > Signed-off-by: Michal Schmidt <mschmidt@...hat.com>  
> 
> Hi Michal,
> 
> So... FIELD_MAX() already requires the _mask to be a const value.
> Now you add a FIELD_MAX_CONST(), which makes it more confusing.

+1, I think this is doing too much in general.