Subject: [alsa-devel] [PATCH 2/4] ASoC: s3c64xx/smartq: use dynamic registration

From: [email-masked] (Rob Jones)
Date: Wed, 16 Jul 2014 09:50:02 +0100

On 16/07/14 08:51, Thierry Reding wrote:
> On Wed, Jul 16, 2014 at 04:28:33PM +0900, Alexandre Courbot wrote:
>> On Wed, Jul 16, 2014 at 4:12 PM, Thierry Reding
>> <[email-masked]> wrote:
>>> On Wed, Jul 16, 2014 at 12:00:45PM +0900, Alexandre Courbot wrote:
>>>> On Tue, Jul 15, 2014 at 6:14 PM, Alexandre Courbot <[email-masked]> wrote:
>>>>> On Tue, Jul 15, 2014 at 4:58 PM, Lars-Peter Clausen <[email-masked]> wrote:
>>>>>> On 07/15/2014 09:36 AM, Alexandre Courbot wrote:
>>>>>>>
>>>>>>> On Tue, Jul 15, 2014 at 4:19 PM, Arnd Bergmann <[email-masked]> wrote:
>>>>>>>>
>>>>>>>> On Monday 14 July 2014 19:36:24 Mark Brown wrote:
>>>>>>>>>
>>>>>>>>> On Mon, Jul 14, 2014 at 08:23:55PM +0200, Arnd Bergmann wrote:
>>>>>>>>>>
>>>>>>>>>> On Monday 14 July 2014 18:18:12 Lars-Peter Clausen wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>>> Yes. But now that you say it the gpiod_direction_output() call is
>>>>>>>>>>> missing
>>>>>>>>>>> from this patch.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> I'm lost now. The GPIO_ACTIVE_HIGH I added comes from
>>>>>>>>>> Documentation/gpio/board.txt
>>>>>>>>>> and as Linus Walleij explained to me the other day, the lookup is
>>>>>>>>>> supposed
>>>>>>>>>> to replace devm_gpio_request_one(), which in turn replaced both the
>>>>>>>>>> gpio_request and the gpio_direction_output(). Do I need to put the
>>>>>>>>>> gpiod_direction_output() back or is there another interface for that
>>>>>>>>>> when
>>>>>>>>>> registering the board gpios?
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Indeed.  If you *do* need an explicit _output() then that sounds to me
>>>>>>>>> like we either need a gpiod_get_one() or an extension to the table,
>>>>>>>>> looking at the code it seems like this is indeed the case.  We can set
>>>>>>>>> if the GPIO is active high/low, or open source/drain but there's no flag
>>>>>>>>> for the initial state.
>>>>>>>>
>>>>>>>>
>>>>>>>> (adding Alexandre and the gpio list)
>>>>>>>>
>>>>>>>> GPIO people: any guidance on how a board file should set a gpio to
>>>>>>>> output/default-high in a GPIO_LOOKUP() table to replace a
>>>>>>>> devm_gpio_request_one() call in a device driver with devm_gpiod_get()?
>>>>>>>> Do we need to add an interface extension to do this, e.g. passing
>>>>>>>> GPIOF_OUT_INIT_HIGH as the flags rather than GPIO_ACTIVE_HIGH?
>>>>>>>
>>>>>>>
>>>>>>> The way I see it, GPIO mappings (whether they are done using the
>>>>>>> lookup tables, DT, or ACPI) should only care about details that are
>>>>>>> relevant to the device layout and that should be abstracted to the
>>>>>>> driver (e.g. whether the GPIO is active low or open drain) so drivers
>>>>>>> do not need to check X conditions every time they want to drive the
>>>>>>> GPIO.
>>>>>>>
>>>>>>> Direction and initial value, on the other hand, are clearly properties
>>>>>>> that ought to be set by the driver itself. Thus my expectation here
>>>>>>> would be that the driver sets the GPIO direction and initial value as
>>>>>>> soon as it gets it using gpiod_direction_output(). In other words,
>>>>>>> there is no replacement for gpio_request_one() with the gpiod
>>>>>>> interface. Is there any use-case that cannot be covered by calling
>>>>>>> gpiod_direction_output() right after gpiod_get()? AFAICT this is what
>>>>>>> gpio_request_one() was doing anyway.
>>>>>>
>>>>>>
>>>>>> I agree with you that this is something that should be done in the driver
>>>>>> and not in the lookup table. I think that it is still a good idea to have a
>>>>>> replacement for gpio_request_one with the new GPIO descriptor API. A large
>>>>>> share of the drivers want to call either gpio_direction_input() or
>>>>>> gpio_direction_output() right after requesting the GPIO. Combining both the
>>>>>> requesting and the configuration of the GPIO into one function call makes
>>>>>> the code a bit shorter and also simplifies the error handling. Even more so
>>>>>> if e.g. the GPIO is optional. This was one of the main reasons why
>>>>>> gpio_request_one was introduced, see the commit[1] that added it.
>>>>>
>>>>> I am not opposed to it as a convenience function. Note that since the
>>>>> open-source and open-drain flags are already handled by the lookup
>>>>> table, the only flags it should handle are those related to direction,
>>>>> value, and (maybe) sysfs export.
>>>>
>>>> Problem is, too much convenience functions seems to ultimately kill convenience.
>>>>
>>>> The canonical way to request a GPIO is by providing a (device,
>>>> function, index) triplet to gpiod_get_index(). Since most functions
>>>> only need one GPIO, we have gpiod_get(device, function) which is
>>>> basically an alias to gpiod_get_index(device, function, 0) (note to
>>>> self: we should probably inline it).
>>>>
>>>> On top of these comes another set of convenience functions,
>>>> gpiod_get_optional() and gpiod_get_index_optional(), which return NULL
>>>> instead of -ENOENT if the requested GPIO mapping does not exist. This
>>>> is useful for the common case where a driver can work without a GPIO.
>>>>
>>>> Of course these functions all have devm counterparts, so we currently
>>>> have 8 (devm_)gpiod_get(_index)(_optional) functions.
>>>>
>>>> If we are to add functions with an init flags parameter, we will end
>>>> with 16 functions. That starts to be a bit too much to my taste, and
>>>> maybe that's where GPIO consumers should sacrifice some convenience to
>>>> preserve a comprehensible GPIO API.
>>>>
>>>> There might be other ways to work around this though. For instance, we
>>>> could replace the _optional functions by a GPIOF_OPTIONAL flag to be
>>>> passed to a more generic function that would also accept direction and
>>>> init value flags. Actually I am not seeing any user of the _optional
>>>> variant in -next, so maybe we should just do this. Thierry, since you
>>>> introduced the _optional functions, can we get your thoughts about
>>>> this?
>>>
>>> I personally prefer explicit naming of the functions rather than putting
>>> a bunch of flags into some parameter. If you're overly concerned about
>>> the amount of convenience functions, perhaps the _index variants can be
>>> left out for gpiod_get_one(). I'd argue that if drivers want to deal
>>> with that level of detail anyway, they may just as well add the index
>>> explicitly when calling the function.
>>>
>>> While we're at it, gpiod_get_one() doesn't sound like a very good name.
>>> All other variants only request "one" as well. Perhaps something like
>>> gpiod_get_with_flags() would be a better name.
>>>
>>> Then again, maybe rather than add a new set of functions we should bite
>>> the bullet and change gpiod_get() (and variants) to take an additional
>>> flags parameter. There aren't all that many users yet (I count 26
>>> outside of drivers/gpio), so maybe now would still be a good time to do
>>> that.
>>
>> That sounds reasonable indeed. And preferable to getting an aneurysm
>> after trying to spell devm_gpiod_get_index_optional_with_flags().
>>
>> This also makes the most sense since most GPIO users will want to set
>> a direction and value right after obtaining one. So if there is no
>> objection I will probably start refactoring gpiod_get() this week.
>
> Sounds good to me.
>

In light of this, should I hold off starting on devm_gpiod_get_array()
as discussed on here last week?

> Thierry

-- 
Rob Jones
Codethink Ltd
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