[alsa-devel] [PATCH 1/4] dt-bindings: Document the dmas and dma-names properties for bcm2835-analog-audio

[Michael Zoran]

On Tue, 2017-03-21 at 07:53 -0500, Rob Herring wrote:
> On Mon, Mar 20, 2017 at 12:07 PM, Michael Zoran <mzoran at crowfest.net>
> wrote:
> > On Mon, 2017-03-20 at 11:50 -0500, Rob Herring wrote:
> > > On Sat, Mar 11, 2017 at 10:38:17PM -0800, Michael Zoran wrote:
> > > > Add documentation for the DMA properties required when using
> > > > the
> > > > PWM controller of the bcm2835 for analog audio output.
> > > > 
> > > > Signed-off-by: Michael Zoran <mzoran at crowfest.net>
> > > > ---
> > > >  Documentation/devicetree/bindings/pwm/pwm-bcm2835.txt | 9
> > > > ++++++++-
> > > >  1 file changed, 8 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/Documentation/devicetree/bindings/pwm/pwm-
> > > > bcm2835.txt
> > > > b/Documentation/devicetree/bindings/pwm/pwm-bcm2835.txt
> > > > index cf573e85b11d..08fc06fc083a 100644
> > > > --- a/Documentation/devicetree/bindings/pwm/pwm-bcm2835.txt
> > > > +++ b/Documentation/devicetree/bindings/pwm/pwm-bcm2835.txt
> > > > @@ -1,14 +1,21 @@
> > > >  BCM2835 PWM controller (Raspberry Pi controller)
> > > > 
> > > >  Required properties:
> > > > -- compatible: should be "brcm,bcm2835-pwm"
> > > > +- compatible: should be "brcm,bcm2835-pwm" or "brcm,bcm2835-
> > > > analog-audio"
> > > 
> > > Compatibles are not for selecting usage modes. You should
> > > describe
> > > the
> > > connection to the pwm that makes it an audio output rather than
> > > change
> > > the compatible.
> > > 
> > 
> > Cool, that's good to know that someone let me know after all this
> > time.
> 
> Well, there are more submitters than reviewers and maintainers and we
> have travel and vacation (aka lives).
> 
> >  And please tell how if DT nodes are an exact documentation of the
> > hardware how are virtualized DT nodes like firmware getting into
> > the
> > DTs?
> 
> DT is software's view of the hardware. Firmware provides interfaces
> to
> the OS just like a h/w device does. And just like h/w devices, the
> firmware is not discoverable and needs to be described. Originally,
> DT
> was the firmware's interface to the OS with OpenFirmware.
> 
> Rob

OK, I understand better know.  So it sounds like what I really want to
do is be able to keep the existing DT as much as possible but load an
alternative driver.   Presumably at the request of the end user some
how.

I'm not sure something like that is supported in the linux kernel.  For
this specific example, I don't know of any reason I couldn't write a
replacement driver for the existing PWM control that does both audio
and standard PWM, but I've heard replacing existing drivers is really
accepted and having alternative drivers isn't something that's
supported.