PXA/8250 UART Conflicts

Dmitry Artamonow mad_soft at inbox.ru
Thu May 13 17:40:58 EDT 2010

On 17:12 Tue 11 May     , Russell King - ARM Linux wrote:
> On Tue, May 11, 2010 at 11:00:36AM -0400, Michael Cashwell wrote:
> > I'd like the tty layer to just assign the minor numbers in sequence,
> > first come first served. But I don't see how to get that. Or is there
> > some platform way to influence the tty range requested?
> It's just plain not supported.  You can't have two different tty drivers
> using the same namespace and have it work.
> Unfortunately, earlier on it was the opinion that "ttyS" means "serial
> driver" and therefore "we shall use the ttyS namespace for our SoC
> driver".  And this is the resulting mess that it causes.

Yup, that's a long standing bug (and still not fixed). I remember I first
happened to stumble upon it about three years ago and it's already been old
back then. In fact it's a general PITA on any PXA board with PCMCIA/CF
slot (handhelds, for example, but not only) when you trying to use some
serial PCMCIA peripherals (e.g. GPSes, wired and wireless modems,
RS232 converters, maybe even bluetooth dongles). With 8250 enabled as a module 
and PXA serial built-in it simply doesn't works.

Openembedded guys for ages have patched kernel with hacks like these to solve
this problem:

Though, personally I'm preferring solution with moving PXA ports into
another namespace (be it ttySA, or ttyPXA, or whatever). So maybe it's
time to finally make a first step and apply something like Russell's patch now,
and then gradually deprecate usage of old ttyS namespace for PXA, or maybe
even left it as a non-default config option (so users who can't reconfigure
their userspace/bootloader can still use newer kernels with old behaviour).

I'd like to hear some comments on the subject from Eric (adding him to Cc:)

Best regards,
Dmitry "MAD" Artamonow

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