linux-kernel - Re: [RESEND PATCH v4 1/8] bitops: Introduce the for_each_set

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Date:   Thu, 4 Oct 2018 19:30:28 +0900
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        linus.walleij@...aro.org, akpm@...ux-foundation.org,
        linux-gpio@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [RESEND PATCH v4 1/8] bitops: Introduce the for_each_set_clump
 macro

On Tue, Oct 02, 2018 at 11:21:42AM +0300, Andy Shevchenko wrote:
> On Tue, Oct 02, 2018 at 09:42:48AM +0200, Rasmus Villemoes wrote:
> > On 2018-10-02 03:13, William Breathitt Gray wrote:
> 
> > The cover letter says
> > 
> >   The clump_size argument can be an arbitrary number of bits and is not
> >   required to be a multiple of 2.
> > 
> > by which I assume you mean "power of 2", but either way, the above code
> > does not seem to take into account the case where bits_offset +
> > clump_size straddles a word boundary, so it wouldn't work for a
> > clump_size that does not divide BITS_PER_LONG.
> 
> E.g. 3 bits in a clump? Hmm...
> 
> Why would we need that? I mean some real use case?

GPIOs in hardware may be routed to devices logically in groups of I/O
lines, yet must still be accessed via the word-sized registers on the
operating machine.

For example, suppose a GPIO card is used to control a set of shower
devices. The card supports 4 shower devices, each device controlled by 3
lines of I/O: enable, hot-cold selection, high-low pressure selection.
In this case, a operating machine would still have to access the GPIO
lines via the I/O registers (e.g. 8-bit port I/O); but with a macro
handling a clump size of 3-bits, we can loop logically by each shower
device which is much simpler from a driver perspective.

William Breathitt Gray