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

On Tue, Jul 15, 2014 at 6:14 PM, Alexandre Courbot [off-list ref] wrote:

On Tue, Jul 15, 2014 at 4:58 PM, Lars-Peter Clausen [off-list ref] wrote:

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On 07/15/2014 09:36 AM, Alexandre Courbot wrote:

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On Tue, Jul 15, 2014 at 4:19 PM, Arnd Bergmann [off-list ref] wrote:

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On Monday 14 July 2014 19:36:24 Mark Brown wrote:

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On Mon, Jul 14, 2014 at 08:23:55PM +0200, Arnd Bergmann wrote:

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On Monday 14 July 2014 18:18:12 Lars-Peter Clausen wrote:

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Yes. But now that you say it the gpiod_direction_output() call is
missing
from this patch.

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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?