Re: [PATCH] serial: imx: add rx and tx led trigger

[Arnd Bergmann <arnd@arndb.de>]

On Thursday, July 7, 2016 8:28:00 AM CEST Uwe Kleine-König wrote:
> Hello Arnd,
> 
> [adding Rob Herring to Cc]
> 
> On Wed, Jul 06, 2016 at 10:09:39PM +0200, Arnd Bergmann wrote:
> > On Wednesday, July 6, 2016 7:30:57 PM CEST Uwe Kleine-König wrote:
> > > On Wed, Jul 06, 2016 at 05:22:40PM +0200, Arnd Bergmann wrote:
> > > > On Monday, July 4, 2016 5:50:09 PM CEST Uwe Kleine-König wrote:
> > > > > On Mon, Jul 04, 2016 at 05:43:03PM +0200, Arnd Bergmann wrote:
> > > > > > On Monday, July 4, 2016 5:34:12 PM CEST Uwe Kleine-König wrote:
> > > > > > > Add support for two led triggers per UART instance that blink on
> > > > > > > transmission and reception of data respectively.
> > > > > > > 
> > > > If this is something we may want to do on other platforms as well,
> > > > we should perhaps not hardwire the name of the imx tty device in
> > > > the led trigger name.
> > > 
> > > I cannot follow. If we have several serial lines and a trigger for each
> > > of them, they must get different names. Using the device's name to
> > > distinguish them seems like a good and obvious idea.
> > 
> > The main problem I see is if someone puts the name of the trigger into
> > a dtb file, as this hardcodes the connection between the Linux driver
> > name and numbering system with the device tree binding, which are normally
> > separate.
> > 
> > If we could derive the trigger name from the "/aliases/serial%d" property
> > in DT instead, it would get a little more portable.
> 
> Alternatively we could invent a more dtish way as aliases seem to be
> frowned upon [1], something like:
> 
> 	led#0 {
> 		label = "userled";
> 		linux,default-trigger = &uart1, "tx";
> 	};
> 
> 	uart1: serial@43f90000 {
> 		...
> 		#trigger-cells = <1>;
> 	};

That looks nice, but I don't see how we could implement this in a
backwards compatible way, as we don't know whether the first cell
of the property is a phandle or a string.

> Having said that, I don't think it's a big problem if the value of
> "linux,default-trigger" is Linux-specific. Moreover, is a default
> trigger considered a hardware description?

What's more important here I think is that even in Linux, the name
of the uart is not always stable across versions or configurations,
e.g. in on OMAP we can have either ttyO%d or ttyS%d.

	Arnd