On 04/14/2013 04:32 AM, Mauro Carvalho Chehab wrote:
Em Sat, 13 Apr 2013 17:37:26 +0300
Antti Palosaari <crope@xxxxxx> escreveu:
On 04/13/2013 05:25 PM, Mauro Carvalho Chehab wrote:
Em Sat, 13 Apr 2013 16:15:39 +0300
Antti Palosaari <crope@xxxxxx> escreveu:
On 04/13/2013 12:48 PM, Frank Schäfer wrote:
Patch 1 removes the unneeded and broken gpio register caching code.
Patch 2 adds the gpio register defintions for the em25xx/em276x/7x/8x
and patch 3 finally adds a new helper function for gpio ports with separate
registers for read and write access.
I have nothing to say directly about those patches - they looked good at
the quick check. But I wonder if you have any idea if it is possible to
use some existing Kernel GPIO functionality in order to provide standard
interface (interface like I2C). I did some work last summer in order to
use GPIOLIB and it is used between em28xx-dvb and cxd2820r for LNA
control. Anyhow, I was a little bit disappointed as GPIOLIB is disabled
by default and due to that there is macros to disable LNA when GPIOLIB
is not compiled.
I noticed recently there is some ongoing development for Kernel GPIO. I
haven't looked yet if it makes use of GPIO interface more common...
I have conflicting opinions myself weather we should use gpiolib or not.
I don't mind with the fact that GPIOLIB is disabled by default. If all
media drivers start depending on it, distros will enable it to keep
media support on it.
I never took the time to take a look on what methods gpiolib provides.
Maybe it will bring some benefits. I dunno.
Compare to benefits of I2C bus. It offers standard interface. Also it
offers userspace debug interface - like I2C also does.
I2C benefit is that the same I2C driver can be used by several different
drivers. GPIO code, on the other hand, is on most cases[1] specific to a
given device.
That is same for GPIO - it offers standard interface between modules for
GPIO "bus".
I used it to control LNA, which is connected to demodulator (cxd2820r)
GPIO. It is bridge which gets LNA API commands and GPIO is property of
demod. Some interface is needed in order to deliver data between bridge
and demod in that case.
[1] Ok, if you're using a GPIO pin to carry some protocol inside it, like
UART, RC, etc, then I can see a benefit on using a bus type of solution.
Just looking at the existing drivers (almost all has some sort of GPIO
config), GPIO is just a single register bitmask read/write. Most drivers
need already bitmask read/write operations. So, in principle, I can't
foresee any code simplification by using a library.
Use of lib interface is not very practical inside of module, however it
could be used. Again, as compared to I2C there is some bridge drivers
which do some I2C access using I2C interface, even bridge could do it
directly (as it offers I2C adapter). I think it is most common to do it
directly to simplify things.
Also, from a very pragmatic view, changing (almost) all existing drivers
to use gpiolib is a big effort.
It is not needed to implement for all driver as one go.
However, for that to happen, one question should be answered: what
benefits would be obtained by using gpiolib?
Obtain GPIO access between modules using standard interface and offer
handy debug interface to switch GPIOs from userspace.
It is known that enabling both analog and digital demods at the same time
can melt some devices. So, it is risky to allow userspace to touch
the GPIOs that enable such chips.
(ok, there are also other forms to melt such devices in userspace
if the user has CAP_SYS_ADMIN)
Do you need eyeglasses? I said it is debug interface. It needs root
privileges in order to setup and use.
I can say I could surely break more devices via I2C debug interface than
GPIO debug interface in case both are implemented by every driver. Just
sent garbage writes to well known eeprom addresses and kaboom. Your
device is bricked. It is totally stupid to say you could brick your
device using debug functionality - yes you can, but it is very unlikely
someone does it as a mistake.
You could ask why we use Kernel I2C library as we could do it directly
:) Or clock framework. Or SPI, is there SPI bus modeled yet?
As I said, i2c allowed code re-usage. Probably, the clock framework and
SPI also can be used for that.
With regards to GPIO, at least currently, I can only see its usage
justified, in terms of code reuse, for remote controllers.
Maybe better to read Kernel GPIO documentation. There is few points
mentioned why to use it and what are advantages.
regards
Antti
--
http://palosaari.fi/
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