Re: [PATCH 2/4] gpio: Introduce ->get_multiple callback

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

On Sun, Aug 27, 2017 at 7:34 PM, Lukas Wunner <lukas@xxxxxxxxx> wrote:
> On Wed, Aug 23, 2017 at 09:38:21AM +0200, Linus Walleij wrote:
>> On Mon, Aug 21, 2017 at 3:12 PM, Lukas Wunner <lukas@xxxxxxxxx> wrote:
>> > SPI-attached GPIO controllers typically read out all inputs in one go.
>> > If callers desire the values of multipe inputs, ideally a single readout
>> > should take place to return the desired values.  However the current
>> > driver API only offers a ->get callback but no ->get_multiple (unlike
>> > ->set_multiple, which is present).  Thus, to read multiple inputs, a
>> > full readout needs to be performed for every single value (barring
>> > driver-internal caching), which is inefficient.
>> >
>> > In fact, the lack of a ->get_multiple callback has been bemoaned
>> > repeatedly by the gpio subsystem maintainer:
>> > http://www.spinics.net/lists/linux-gpio/msg10571.html
>> > http://www.spinics.net/lists/devicetree/msg121734.html
>> >
>> > Introduce the missing callback.  Add corresponding consumer functions
>> > such as gpiod_get_array_value().  Amend linehandle_ioctl() to take
>> > advantage of the newly added infrastructure.
>>
>> Could you please patch
>> Documentation/gpio/consumer.txt
>> to mention the new functions as well?
>
> Missed that, will fix in v2.  There is an oddity in that document
> in that it claims gpiod_get_value() / gpiod_set_value() do not return
> errors.  The kerneldoc in gpiolib.c and driver.h says otherwise,
> it allows the get functions to return a negative errno.  Now what?

Probably errors in the documentation.

Feel free to correct it.

> At least with SPI-attached GPIO controllers, the transmission is never
> guaranteed to succeed, so errors can occur both for input and output
> GPIOs.  The MAX3191x is input-only and does pass SPI errors back to
> the caller.  Output drivers such as gpio-74x164.c silently ignore
> SPI errors, which is arguably a problem.

This has historical reasons.

>> Maybe we should have a section in Documentation/gpio/driver.txt,
>> that doc is a bit incomplete right now.
>
> First time I've seen that file. :-)  I used the kerneldoc as guidance
> when writing the gpio-max3191x.c driver and found it sufficient, so I'm
> not sure how driver.txt could be expanded?

Don't worry about it if you feel it's superfluous.

Yours,
Linus Walleij
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