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    SubjectRe: [RFC PATCH v2 09/12] gpiolib: Identify GPIO descriptor arrays with direct mapping
    Date
    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?

    Thanks,
    Janusz


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