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
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