[RFC] possible removal of omap-serial

[balbi@ti.com (Felipe Balbi)]

On Thu, Mar 20, 2014 at 09:45:42PM -0500, Robert Nelson wrote:
> On Thu, Mar 20, 2014 at 9:36 PM, Greg KH <gregkh@linuxfoundation.org> wrote:
> > On Thu, Mar 20, 2014 at 08:37:29PM -0500, Felipe Balbi wrote:
> >> On Thu, Mar 20, 2014 at 08:35:57PM -0500, Felipe Balbi wrote:
> >> > Hi,
> >> >
> >> > On Thu, Mar 20, 2014 at 05:12:28PM -0700, Greg KH wrote:
> >> > > On Thu, Mar 20, 2014 at 06:52:10PM -0500, Felipe Balbi wrote:
> >> > > > Hi folks,
> >> > > >
> >> > > > I've been toying with the idea of removing
> >> > > > drivers/tty/serial/omap-serial.c since that's, to put it bluntly, an
> >> > > > ungly copy of 8250 driver.
> >> > > >
> >> > > > The original concern was wrt suspend/resume but I think it'd be a far
> >> > > > better approach to implement runtime PM in 8250 and write a rather small
> >> > > > 8250-omap.c glue (much like 8250-acorn.c or 8250-dw.c) just to get the
> >> > > > OMAP-specific details out of the way.
> >> > > >
> >> > > > The question I have is: omap-serial.c calls the serial devnodes ttyO\d,
> >> > > > instead of ttyS\d so removing omap-serial.c would have a direct impact
> >> > > > in userland. I wonder if it's an acceptable "regression" considering
> >> > > > we'd be able to reuse 8250 gaining proper Flow Control support, proper
> >> > > > DMA support, years and years of bug-fixes, etc.
> >> > >
> >> > > Breaking device node names is a contentious issue for serial ports, I
> >> > > don't think you can do that :(
> >> >
> >> > would an upstream udev rule creating a symbolic link from ttyO to ttyS
> >> > be enough ?
> >> >
> >> > I didn't test this yet but I guess this is enough (?)
> >> >
> >> > KERNEL=="ttyO[0-9]", GROUP="dialout", SYMLINK+="ttyS"
> >>
> >> or actually it should be to other way around, ttyS would be the real
> >> device:
> >>
> >> KERNEL=="ttyS[0-9]", GROUP="dialout", SYMLINK+="ttyO"
> >
> > As udev rules don't ship with the kernel, this might be tough to do :(
> >
> > Might be easier to make the 8250 driver handle different "names" like
> > Alan said.

I'll see if I find a way to avoid that or at least see if we find any
other way of creating a symlink... In any case, just switching back to
8250, even if just maintaining ttyO name, is already a big benefit.

> On the support side, I'm not looking forward to this for beagle/panda
> users.  We've already converted them once from ttySx -> ttyOx back in
> 2.6.33/2.6.34? days.  That was an irc/email/u-boot/kernel nightmare...

that's exactly why we're talking about ways to maintain backwards
compatibility here. But I'm more interested in finding a way to switch
over to ttyS and have a symlink to ttyO, that way a simple debootstrap
(or any other ARM distro minimal rootfs) would work out of the box,
without any changes, just like in "normal" systems.

-- 
balbi
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