Re: [PATCH 2/2] dt-bindings: gpio: Add binding for gpio-mockup

[Rob Herring <robh@kernel.org>]

On Tue, Sep 18, 2018 at 01:25:17PM +0200, Vincent Whitchurch wrote:
> On Sun, Sep 16, 2018 at 06:24:52PM -0500, Rob Herring wrote:
> > On Thu, Sep 06, 2018 at 02:47:23PM +0200, Linus Walleij wrote:
> > > On Thu, Sep 6, 2018 at 1:09 PM Bartosz Golaszewski <brgl@bgdev.pl> wrote:
> > > 
> > > > Do we even need DT bindings for this? Device tree bindings mean that
> > > > we commit to a stable interface so that once a device with given DT
> > > > blob is released, it will work on every future kernel. Also: DT should
> > > > only include information about actual HW, not operating system
> > > > concepts. Meanwhile this is for testing purposes only and it won't end
> > > > up in any actual DT source file upstream.
> > > 
> > > Hm that relates to another discussion I have with the DT maintainers
> > > about a virtualized display panel.
> > > 
> > > I do not know how the DT maintainers feel about supporting things
> > > in the kernel that uses DT infrastructure and specially tailored
> > > device trees but include elements with no formal device tree
> > > bindings. Would be interesting to hear their thoughts on this.
> > 
> > I have one usecase in mind where I think it is valid, but for this case 
> > in particular, I think it would be better to just provide a sysfs 
> > interface. If there's no GPIO binding connections to this mockup 
> > controller, there's no real need for DT. OTOH, if this device allowed 
> > building a test framework for all the other GPIO based drivers and 
> > bindings such as LED, PWM, bitbanged buses, etc.
> 
> My original usecase for this was for leds-gpio.  I used the DT bindings
> added by this patch and hooked up a leds-gpio via DT in order to develop
> userspace.  Once the hardware with the GPIO expander arrived, the mockup
> device in the DT was replaced with the expander.  I don't see how a
> sysfs interface would allow the same thing.

Okay, I'm fine with the binding. I'm also okay with not documenting this 
at all as this should never be in any stable or upstream dts file.

Rob