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

[Rob Herring <robh@kernel.org>]

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.

The case I have in mind is where you want to attach a fake DT node to a 
non-DT device for the purposes of defining other downstream devices. For 
example, an FTDI USB to serial chip with downstream SPI, UART, I2C, 
GPIO, etc. devices attached. This could be on an x86 which doesn't have 
a base DT.

Rob