Re: [libgpiod] feature request: output state read and sustain

[Kent Gibson <warthog618@xxxxxxxxx>]

On Wed, Jun 29, 2022 at 01:17:54PM +0200, Jiří Prchal wrote:
> 
> 
> On 29. 06. 22 12:47, Kent Gibson wrote:
> > On Wed, Jun 29, 2022 at 12:27:13PM +0200, Andy Shevchenko wrote:
> > > On Wed, Jun 29, 2022 at 11:27 AM Jiří Prchal <jiri.prchal@xxxxxxxxxxx> wrote:
> > > > On 29. 06. 22 9:23, Kent Gibson wrote:
> > > > > On Tue, Jun 28, 2022 at 03:08:20PM +0200, Jiří Prchal wrote:
> > > 
> > > > > > using new libgpiod / chardev driver, is there any way to get state of
> > > > > > output? I mean one process sets it for example to 1 and another process
> > > > > > reads this output state for example to show that on web page.
> > > 
> > > I'm not sure it's guaranteed to read output back. Some (b0rken?) GPIO
> > > chips may not allow this on H/W level and when reading they always
> > > will get the value of Input Buffer (now imagine if the line is
> > > configured as Output with Input being disconnected from the physical
> > > pin).
> > > 
> > 
> > Agreed.  Userspace should know the value they set the output to, and so
> > have no need to read it back.  GPIO is not NVM.
> Not NVM, bat RAM and it keeps their data untill reset, after reset it has specific value (usually 0)
> I haven't seen HW without possibility of reading back output register, but I don't say there couldn't be such one.
> 

No, no it is not RAM.

And I have seen hardware where you can't read back the output.
And so has Andy apparently.
That is the problem - there is lots of hardware out there and we are
trying to provide a consistent interface to inconsistent hardware.

Cheers,
Kent.