I am attempting to get the 8520.c driver's serial console working with
a 16550A UART implementation, and have run into what I consider to be
a bug: In short, the proper 'port.type' for this serial port is not
set until the module init (serial8250_init) is called, so the FCR is
set incorrectly during serial8250_console_init for any port type which
is different than UNKNOWN.
The exact problem is that the FCR is being set to '0x0' for a port
type of 'UNKNOWN', when for my specific 16550A, it should be set to
'0xC1' - and this makes my screen fill with empty characters instead
of the printk output I need. It appears that after some time (once
the kernel actually calls serial8250_init) the problem clears up, but
I still lose a large section of the output before this point.
The proper flags are sitting there in the uart_config array for the
proper type, and this type is properly deduced by the 'autoconfig'
routine that eventually gets called by serial8250_init. The problem
is that the autoconfig is isn't called by serial8250_console_init
which of course happens much earlier than serial8250_init.
I have a workaround that works for me, telling the
serial8250_set_termios() routine to only set the FCR if the type is
not UNKNOWN, changing
if (up->port.type != PORT_16750 ) {
to
if (up->port.type != PORT_16750 && up->port.type != PORT_UNKNOWN) {
This will leave the FCR at whatever it was at boot time, which for me
is correct.
Would this be a "good enough" fix in general, or would there be a
better way of doing this?
I suppose the other way would be to duplicate some of what
serial8250_init does (like call 'autoconfig') in
serial8250_console_init, but I haven't tried this yet... maybe it
depends on too much kernel magic to be called as early as
'console_init'?
Any suggestions on the best way to fix this would be great, I'd be
happy to develop this more, test a bit, and submit a patch.
--
Jim Ramsay
"Me fail English? That's unpossible!"
On Monday 30 May 2005 4:38 pm, Jim Ramsay wrote:
> I am attempting to get the 8520.c driver's serial console working with
> a 16550A UART implementation, and have run into what I consider to be
> a bug: In short, the proper 'port.type' for this serial port is not
> set until the module init (serial8250_init) is called, so the FCR is
> set incorrectly during serial8250_console_init for any port type which
> is different than UNKNOWN.
>
> The exact problem is that the FCR is being set to '0x0' for a port
> type of 'UNKNOWN', when for my specific 16550A, it should be set to
> '0xC1' - and this makes my screen fill with empty characters instead
> of the printk output I need.
Shouldn't a 16550A UART work correctly with FCR==0x0, i.e., with FIFOs
disabled? Is your UART broken?
Serial console output is always polled, one character at a time, so
you shouldn't need FIFOs until later.