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    SubjectRe: [PATCH v2 1/2] gpio: Add driver for Zynq GPIO controller
    Hi Linus,

    On Tue, 2014-07-08 at 11:34AM +0200, Linus Walleij wrote:
    > On Mon, Jul 7, 2014 at 6:08 PM, Sören Brinkmann
    > <soren.brinkmann@xilinx.com> wrote:
    >
    > >> So: what is the usecase for these GPIOs?
    > >
    > > Yea, in this case it was a button. I have to look at these drivers. It's
    > > very likely that they cover what I want. But this case is trivial. I
    > > really don't do anything but enabling the IRQ by writing to the edge
    > > attribute and press the push-button connected to that GPIO line.
    >
    > In case of a system using device tree it is very trivial to add a gpio
    > key binding. After compiling in the gpio keys driver this small
    > snipper type is all that is really needed in most cases:
    >
    > /* User key mapped in as "escape" */
    > gpio-keys {
    > compatible = "gpio-keys";
    > user-button {
    > label = "user_button";
    > gpios = <&gpio0 3 0x1>;
    > linux,code = <1>; /* KEY_ESC */
    > gpio-key,wakeup;
    > };
    > };

    Thanks for the example. I think for my cases these drivers are exactly
    the right thing.

    >
    >
    > > But as a general note: I think we have quite some customers trying to do
    > > GPIO in userspace.
    >
    > For what? I mean the use cases. Usually it is a bad idea, and
    > as shown above, just using the right existing device driver with
    > device tree is much easier, also for an end user, given they know
    > how to alter DTs and compile in kernel modules.
    >
    > > With Zynq's FPGA portion, a lot of things come down
    > > to make signals accessible in Linux and a lot of people do not want or
    > > need a full blown kernel driver and use GPIO. The request for 'how to
    > > handle GPIO IRQs in userspace' is pretty common. Often this gets passed
    > > on to UIO though.
    >
    > The short answer is don't handle GPIO IRQs in userspace.
    >
    > Userspace drivers is generally a bad idea and should not be written.
    > The kernel is intended to handle hardware.
    >
    > The above is doubly true for anything involving IRQs. Just think
    > of what IRQs are. They are one of the reasons why we have a
    > kernel and not just write all software on a system from scratch
    > ourselves.

    I fully agree and you don't have to convince me. But to a lot of our
    customers that are used to use FPGAs, SOCs and Linux are pretty new. You
    see a lot of scary stuff. Accessing /dev/mem seems to be a lot of
    people's big hammer solution for everything. Then I always perceive it
    as great progress if things like the sysfs interface are used instead.

    Thanks,
    Sören

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