[PATCH] serial: DCC(JTAG) serial and console emulation support

[Daniel Walker]

On Wed, 2010-10-13 at 21:45 +0200, Arnd Bergmann wrote:
> On Wednesday 13 October 2010 20:08:35 Daniel Walker wrote:
> > On Wed, 2010-10-13 at 19:44 +0200, Arnd Bergmann wrote:
> > > On Wednesday 13 October 2010 18:17:03 Daniel Walker wrote:
> > > > > I think you would be much better off making it a "hvc" driver, where
> > > > > you just need to provide a read character and write character function
> > > > > and an optional interrupt handler but otherwise have the common hvc
> > > > > code take care of polling the hardware and talking to the tty layer.
> > > > 
> > > > I don't know what the "hvc" driver is "Hypervisor Virtual Console"
> > > > maybe?
> > > 
> > > Yes, it originally was used only on hypervisors that had simple
> > > read/write type consoles, but has now turned into a generic facility
> > > that is used by a number of consoles that don't look like classic
> > > serial ports.
> > > 
> > > > Can you give any sort of example driver which does what you
> > > > suggesting?
> > > 
> > > Look at drivers/char/hvc_tile.c for the simplest case or
> > > drivers/char/hvc_vio.c for one that uses interrupts.
> > 
> > I found it independently actually .. It looks like there's at least two
> > problems. This jtag driver has a status register which flags when RX is
> > available, and TX is possible. I'm not sure this status register fits
> > into the model.
> 
> I think that is how they all work. The read/write functions simply return
> the number of characters transferred, which may be zero if the output
> is busy or the input is empty.
> 
> > The other thing is that we have a ttyJ registered for
> > this driver, and it would be nice to use that over something like ttyHVC
> > (I'm not sure if that name is correct, just a guess).
> 
> It's hvc0, but I don't see this as a problem because the driver was never
> upstream before -- you don't really get to complain about compatibility
> with out-of-tree code :-)

I'm complaining about naming issues, not compatibility.

> Seriously, I don't think you need it, but if you really do, we can probably
> find a way to work around this by changing the base hvc driver.

I think we would have to add something that allows different major/minor
numbers, because something that includes the name "Hypervisor" shouldn't
be used for something general.. /dev/hvcX is also registered for "IBM
iSeries/pSeries virtual console" (according to www.lanana.org) which is
confusing ..

Daniel

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