linux-kernel - Re: [PATCH 1/3] TTY: add support for "tty slave" devices.

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Date:	Sat, 13 Dec 2014 22:22:59 +0000
From:	Grant Likely <grant.likely@...aro.org>
To:	Sebastian Reichel <sre@...nel.org>
Cc:	NeilBrown <neilb@...e.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mark Rutland <mark.rutland@....com>,
	Jiri Slaby <jslaby@...e.cz>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] TTY: add support for "tty slave" devices.

On Sat, Dec 13, 2014 at 5:46 PM, Sebastian Reichel <sre@...nel.org> wrote:
> Hi,
>
> On Fri, Dec 12, 2014 at 11:59:20AM +0000, Grant Likely wrote:
>> [...]
>> > --- a/Documentation/devicetree/bindings/serial/of-serial.txt
>> > +++ b/Documentation/devicetree/bindings/serial/of-serial.txt
>> > @@ -39,6 +39,10 @@ Optional properties:
>> >    driver is allowed to detect support for the capability even without this
>> >    property.
>> >
>> > +Optional child node:
>> > +- a platform device listed as a child node will be probed and
>> > +  powered-on whenever the tty is in use (open).
>> > +
>>
>> The biggest concern I have is what happens to nodes that already have
>> child devices that /don't/ match this use case? It is possible that some
>> UART nodes already have a child node used to store other data. There are
>> two ways to handle this; 1) add a new bool property that indicates the
>> child nodes are tty slave devices, or 2) Make each uart driver
>> explicitly enable the feature so that driver authors can check if it is
>> a problem for that device. I personally would suggest #1 because then it
>> can be enabled in generic code.
>
> maybe simple depend on the compatible value? If the UART node has
> child nodes to store other random data it should not have a
> compatible value?

That's not a given. It is entirely possible that drivers have used
compatible in the child nodes.

However, I may be stirring up trouble for nothing. We could enable it
for all child nodes that have a compatible value, and then blacklist
any parent nodes that want to use a different behaviour. If someone
complains then we will fix it.

g.
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