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SubjectRe: [RFC PATCH v2 09/12] gpiolib: Identify GPIO descriptor arrays with direct mapping
On Tue, 07 Aug 2018 18:50:22 +0200
Janusz Krzysztofik <jmkrzyszt@gmail.com> wrote:

> Hi Linus,
>
> On Tuesday, August 7, 2018 1:29:43 AM CEST Linus Walleij wrote:
> > Hi Janusz!
> >
> > On Tue, Aug 7, 2018 at 12:29 AM Janusz Krzysztofik <jmkrzyszt@gmail.com>
> wrote:
> >
> > > Certain GPIO array lookup results may map directly to GPIO pins of a
> > > single GPIO chip in hardware order. If that condition is recognized
> > > and handled efficiently, significant performance gain of get/set array
> > > functions may be possible.
> > >
> > > While processing a request for an array of GPIO descriptors, verify if
> > > the descriptors just collected represent consecutive pins of a single
> > > GPIO chip. Pass that information with the array to the caller so it
> > > can benefit from enhanced performance as soon as bitmap based get/set
> > > array functions which can make efficient use of that are available.
> > >
> > > Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
> > (...)
> > > This function returns a struct gpio_descs which contains an array of
> > > -descriptors::
> > > +descriptors. It may also contain a valid descriptor of a single GPIO
> chip in
> > > +case the array strictly matches pin hardware layout of the chip::
> > >
> > > struct gpio_descs {
> > > unsigned int ndescs;
> > > struct gpio_desc *desc[];
> > > + struct gpio_chip *chip;
> >
> > This must be motivated: if the only purpose is to indicate to the consumer
> that
> > all GPIOs are on the same chip, why not just have a
> >
> > bool all_on_same_chip;
> >
> > That you set to true if these are all on the same chip?
>
> My approach would probably save one or two instructions per get/set call, but
> I'm not stuck to it and will be happy to find a better solution.
>
> How about folding the chip descriptor inside an additional structure, private
> to drivers, with internals not revealed to consumers?

Or just get the chip from gpio_descs->desc[0]->gdev->chip when
->all_on_same_chip is true...

That adds 2 dereferencing though.

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