Re: [PATCH] devicetree - document using aliases to set spi bus number.

[Geert Uytterhoeven]

On Wed, May 25, 2016 at 8:44 PM, Rob Herring <robh@xxxxxxxxxx> wrote:
> On Tue, May 24, 2016 at 06:41:41PM +0100, Mark Rutland wrote:
>> On Tue, May 24, 2016 at 06:39:20PM +0200, Christer Weinigel wrote:
>> > Document how to use devicetree aliases to assign a stable
>> > bus number to a spi bus.
>> >
>> > Signed-off-by: Christer Weinigel <christer@xxxxxxxxxxx>
>> >
>> > ---
>> >
>> > Trivial documentation change.
>> >
>> > Not having used devicetree that much it was surprisingly hard to
>> > figure out how to assign a stable bus number to a spi bus.  Add a
>> > simple example that shows how to do that.
>> >
>> > Mark Cced as the SPI maintainer.  Or should trivial documentation
>> > fixes like this be addressed to someone else?
>> >
>> >   /Christer
>> >
>> >  Documentation/devicetree/bindings/spi/spi-bus.txt | 10 ++++++++++
>> >  1 file changed, 10 insertions(+)
>> >
>> > diff --git a/Documentation/devicetree/bindings/spi/spi-bus.txt b/Documentation/devicetree/bindings/spi/spi-bus.txt
>> > index 42d5954..c35c4c2 100644
>> > --- a/Documentation/devicetree/bindings/spi/spi-bus.txt
>> > +++ b/Documentation/devicetree/bindings/spi/spi-bus.txt
>> > @@ -94,3 +94,13 @@ SPI example for an MPC5200 SPI bus:
>> >                     reg = <1>;
>> >             };
>> >     };
>> > +
>> > +Normally SPI buses are assigned dynamic bus numbers starting at 32766
>> > +and counting downwards.  It is possible to assign the bus number
>> > +statically using devicetee aliases.  For example, on the MPC5200 the
>> > +"spi@f00" device above is connected to the "soc" bus.  To set its
>> > +bus_num to 1 add an aliases entry like this:
>>
>> As Mark Brown pointed out, this is very Linux-specific (at least in the
>> wording of the above).
>>
>> Generally, aliases are there to match _physical_ identifiers (e.g. to
>> match physical labels for UART0, UART1, and on).

Indeed.

> While there may be some correlation to physical identifiers, the reality
> is aliases are used for mapping to Linux numbering. Their primary use
> has been to avoid breaking existing userspace and kernel command lines
> when converting to DT. The reality is that matters on very few platforms
> and can be solved in other ways for new platforms. For serial port
> console, that means using stdout-path for example.

Fortunately the SPI aliases are optional, while for serial devices they
are mandatory in most (all?) drivers. But this is more a relic of the aging
serial subsystem, which still relies on allocating arrays of NR_MAX_PORTS at
driver registration time, and thus can't support uart_port.line counting down
from 32767 for dynamic numbers like SPI does.
(BTW, I'd really like to see that fixed ;-)

Aliases are also an obstacle for DT overlays:
  1. You can't update the aliases from an overlay (I did post a patch to
     implement that, but it's ugly and buggy, but I still use that as I need to
     load DT overlays for serial port testing),
  2. They're almost a guarantee for conflicts (does your overlay provide spi2,
     or does mine?).

So yes, try to stay away from aliases if you can.
There's nothing a udev rule can't solve, can it?

> For SPI, I think we should use "label" which reflects a name that is
> defined by the h/w design and is meaningful to the user. Then perhaps
> the device becomes "/dev/spi/by-name/<label>/spidev.0" or simply
> "/dev/spidev-<label>.0".

I guess using the node name is not an option (say "use standard node names")?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds