How does the kernel assign ttySn to UARTs?

From: frowand.list@gmail.com (Frank Rowand)
To: linux-arm-kernel@lists.infradead.org
Subject: How does the kernel assign ttySn to UARTs?
Date: Mon, 8 Aug 2016 14:03:39 -0700	[thread overview]
Message-ID: <57A8F3AB.70306@gmail.com> (raw)
In-Reply-To: <CAF_dkJD5MEfNzrRqM3H_LkOmXC3XYjUYLe2=ZtzCjAS1A2TDmg@mail.gmail.com>

On 08/08/16 12:56, Patrick Doyle wrote:
> I am playing the the device tree for my (Atmel, SAMA5D2x) device and
> find myself confused by the assignment of ttySn devices to UARTs and
> FLEXCOM devices.
> 
> I started off assuming that the
> 
>     alias {
>         serial0 = &uart2;
>     };
> 
> stanza I put in my device tree resulted in ttyS0 being aliased to
> uart2 (serial at f8024000).  But when I attempted to change the alias to
> uart5 (the label for UART defined in flexcom at fc018000) I was surprised
> and confused by the fact that the console continued to spew forth on
> UART2.
> 
> So I tried removing the "alias" stanza from the device tree.  The
> console still showed up on UART2.
> 
> I wondered if it might have something to do with the order that
> devices show up in the device tree, so I swapped UART2 & UART3, but
> the console still showed up on UART2.
> 
> Just to make sure I wasn't losing my mind, I decided to comment UART2
> out completely... and then the console showed up on FLEXCOM4.  Not
> UART3, but FLEXCOM4!
> 
> Now I'm really confused.  Now it is time to ask the experts.
> 
> FWIW, my kernel command line contains "console=ttyS0", while my device
> tree enables UART2, UART3, and FLEXCOM4 (which us configured as a
> UART).  Nowhere do I enable UART0, so I am also confused has to how
> /dev/ttyS0 gets mapped to UART2 or FLEXCOM4.
> 
> Would any experts out there care to point me in the direction of some clues?

Try stdout-path in the device tree chosen node.
See Documentation/devicetree/bindings/chosen.txt

I'm not sure whether "console=ttyS0" will conflict with stdout-path.

> 
> Thanks.
> 
> --wpd
> 
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