3 Serial issues up for discussion (was: Re: Serial core problems on embedded PPC)

[Russell King]

On Mon, Jul 29, 2002 at 07:44:08AM -0700, Tom Rini wrote:
> On Mon, Jul 29, 2002 at 10:00:10AM +0100, Russell King wrote:
> > Unless ppc and others are willing to put up with major breakage when I
> > change asm/serial.h, I don't see this getting cleaned up.  Comments on
> > this area welcome.
> 
> Well, what changes do you have in mind?

Firstly, apologies to Tom for turning this into a general discussion
mail.  At the request of Tom, this message is also CC:'d to the PPC
devel lists.

There's quite a lot in here, so please, when replying edit out stuff
your not replying to.  Thanks.

1. Serial port initialisation
-----------------------------

Firstly, one thing to bear in mind here is that, as Alan says "be nice
to make sure it was much earlier".  I guess Alan's right, so we can get
oopsen out of the the kernel relatively easily, even when we're using
framebuffer consoles.

I'm sure Alan will enlighten us with his specific reasons if required.

There have been several suggestions around on how to fix this table:

a. architectures provide a sub-module to 8250.c which contains the
   per-port details, rather than a table in serial.h.  This would
   ideally mean removing serial.h completely.  The relevant object
   would be linked into 8250.c when 8250.c is built as a module.

b. we create 8250_hub6.c, 8250_generic.c, 8250_multiport.c and friends
   each containing the parameters for the specific cards and handle it
   as above.

c. make it the responsibility of user space to tell the kernel about
   many serial ports, and leave just the ones necessary for serial
   console in the kernel.  (see issue 2 below)

d. we keep serial.h, make it 8250-compatible ports only, and change
   CONFIG_SERIAL_MULTIPORT and friends to CONFIG_SERIAL_8250_MULTIPORT
   This is the simplest and least likely to break other code.  On the
   other hand, we end up hauling the ISA table and struct old_serial_port
   into 2.6.

2. setserial API
----------------

This is actually tied closely into another issue; I'd like to get rid
of this silly idea where we're able to open serial ports that don't
exist (ie, their UART is "unknown").  This behaviour appears to be for
the benefit of setserial to allow it to modify port base addresses and
interrupt levels, etc.  Removing this facility would require a new API
for such things.  The best suggestion made so far is to do something
like:

# echo "add 0x2e8,3,autoconfig" >/dev/serialctl
# echo "remove 0x2e8" >/dev/serialctl

(or s,/dev/serialctl,/proc/tty/driver/serial, which pre-exists)

where we have "add ioport,irq,flags" and "remove ioport" (note that
mmio ports aren't covered here since they require ioremap games which
tends to be card specific!)

Why make this change?  Well, we have quite a lot of baggage being
dragged around to support configuration of an open port and being
able to open a non-existent port.  I'd really like to get rid of
this excess baggage.

3. /dev/ttyS*, /dev/ttySA*, /dev/ttyCL*, /dev/ttyAM*, etc
---------------------------------------------------------

All the above are serial ports of various types.  It has been expressed
several times that people would like to see all of them appear as
/dev/ttyS* (indeed, there was an, erm, rather heated discussion about
it a couple of years ago.)  I'm going to be neutral on this point
here.

There are several issues surrounding this:

a. The serial core.c is very almost capable of handling this abstraction,
   with one exception - a registered port can only be in one group at
   one time.  This restriction is brought about because of the way the
   tty layer handles its tty ports.

   (Handling dual registrations in two different majors gets _really_
    messy - eg, you two built-in 16550A ports and two SA1100 ports
    taking up ttyS0 to ttyS3.  You then add a 16550A PCMCIA modem,
    which becomes ttyS4.  Oh, and the SA1100 ports are also appearing
    as ttySA0 and ttySA1.  _really_ messy.  No thanks.)

b. serial consoles.  Each hardware driver handles its serial consoles
   by itself, and if you have two or more hardware drivers built in
   with serial console support, you need to be able to tell them apart
   with the console= kernel parameter.

   Again, this could be solvable if we have one "ttyS" view of everything
   (core.c would then be responsible for registering the console with
    printk.c and passing the various methods off to the relevant
    hardware).

c. People with many serial ports.  We _could_ change the device number
   allocations such that ttyS gobbles up the ttySA, ttyCL, ttyAM, etc
   device numbers so we end up with the same number of port slots
   available for those with many many serial ports in their machines.

-- 
Russell King ([e-mail])                The developer of ARM Linux
             http://www.arm.linux.org.uk/personal/aboutme.html