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Date: Fri, 10 Mar 2017 09:41:39 -0600
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Christophe LEROY <christophe.leroy@c-s.fr>
Cc: Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>, Scott Wood <oss@buserror.net>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] powerpc: sysdev: cpm1: Optimise gpio bit calculation
Message-ID: <20170310154139.GM31469@gate.crashing.org>
References: <20170309094206.A832167992@localhost.localdomain> <87a88tle0y.fsf@concordia.ellerman.id.au> <534d8795-ce21-9af6-498d-22597213a3a2@c-s.fr> <20170310130630.GH31469@gate.crashing.org> <906c025b-47d0-f906-2613-291f350bd48d@c-s.fr> <20170310143241.GJ31469@gate.crashing.org> <777e903f-b816-5efa-2ece-e4e16fab27c6@c-s.fr>
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On Fri, Mar 10, 2017 at 03:41:23PM +0100, Christophe LEROY wrote:
> >>>>gpio_get() and gpio_set() are used extensively by some GPIO based
> >>>>drivers like SPI, NAND, so it may be worth it as it doesn't impair
> >>>>readability (if anyone prefers, we could write  (1 << 31) >> i  instead
> >>>>of  0x80000000 >> i )
> >>>
> >>>1 << 31 is undefined behaviour, of course.
> >>
> >>Shall it be 1U << 31 ?
> >
> >Sure, that works.  "1 << (31 - i)" is most readable (but it doesn't yet
> >generate the code you want).
> 
> Euh .... I'm a bit lost. Do you mean the form we have today is the 
> driver is wrong ?

Heh, yes.  But is't okay with GCC, so don't worry about it.

The point is that "0x80000000 >> i" is less readable.


Segher