On 09/19/2018 10:27 AM, Ricardo Ribalda Delgado wrote:
Let me explain my current setup
I have a board with input and output gpios, the direction is defined
via pdata. When I run gpioinfo all the gpios are shown as input,
regardless if they are input or outputs: Eg:
root@qt5022:/tmp# ./gpioinfo
gpiochip0 - 16 lines:
line 0: "PROG_B" unused input active-high
line 1: "M0" unused input active-high
line 2: "M1" unused input active-high
line 3: "M2" unused input active-high
line 4: "DIN" unused input active-high
line 5: "CCLK" unused input active-high
line 6: unnamed unused input active-high
line 7: unnamed unused input active-high
line 8: "DONE" unused input active-high
line 9: "INIT_B" unused input active-high
line 10: unnamed unused input active-high
line 11: unnamed unused input active-high
line 12: unnamed unused input active-high
line 13: unnamed unused input active-high
line 14: unnamed unused input active-high
line 15: unnamed unused input active-high
Yes, this is a known problem that should be fixed.
That is wrong and very confusing to the user, it can also lead to a
mayor fuckup if the user decides to connect two output gpio pins
because he expects that both are input. (This is the programming port,
but I also have 24 V -high current GPIOs)
Users are expected to program the direction for every GPIO they want to
use, regardless of whatever it's set to before they open it.
There is a function in the API to tell libgpio if a gpio is out our
in. Why not use it?
Because calling that API before properly claiming the GPIO is a
programming error.
- If the configuration is hardcoded, the driver will return a fixed value
- If it is cheap to query the hardware, the driver will query the hardware,
- If it is expensive to query the hardware the driver can either
return a cached value or a fake value (current situation)
The reason why the Qualcomm driver is impacted the most is because on
ACPI platforms, the GPIO map is "sparse". That is, not every GPIO
between 0 and n-1 actually exists. So reading a GPIO that doesn't exist
is invalid.
The way to protect against that is to claim the GPIO first. If the
claim is rejected, then you know that you can't access that GPIO.
The bug is that the original code that I deleted (and that you're trying
to put back) doesn't claim the GPIO first.
From my point of view: "The get_direction callback normally triggers
a read/write to hardware, but we shouldn't be touching the hardware
for an individual GPIO until after it's been properly claimed." is
an statement specific for your platform and should be fixed in your
driver.
Either that, or I have completely missunderstund the purpouse of gpiod
:), and that could easily be the case.
It's not a platform-specific statement. It applies to all drivers. In
some drivers, the get_direction function had side-effects (like
programming muxes, IIRC) that no one really cared about but was
technically wrong.
I'm not sure how to properly fix this, but I wonder if we need some kind
of late-stage initialization where gpiolib scans all the GPIOs by
claiming them first, reading the directions, and then releasing them.
