Re: [PATCH v2] serial: 8250_dw: Improve unwritable LCR workaround

James Hogan
2013-12-07T00:59:18Z

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On Friday 06 December 2013 16:31:38 Tim Kryger wrote:
> On Fri, Dec 6, 2013 at 3:51 PM, James Hogan <james.hogan@imgtec.com> wrote:
> > On Friday 06 December 2013 23:29:02 James Hogan wrote:
> >> So it looks like the LCR does always change immediately for me in this
> >> case
> >> (obviously it hasn't hit the BUSY case), but not all the bits can be
> >> written. In particular bit 5 and bit 7 at the least. If I do this (sorry
> >> for whitespace munging):
> >>
> >> diff --git a/drivers/tty/serial/8250/8250_dw.c
> >> b/drivers/tty/serial/8250/8250_dw.c
> >> index 4658e3e..722d448 100644
> >> --- a/drivers/tty/serial/8250/8250_dw.c
> >> +++ b/drivers/tty/serial/8250/8250_dw.c
> >> @@ -96,7 +96,7 @@ static void dw8250_serial_out(struct uart_port *p, int
> >> offset, int value)
> >>
> >> if (offset == UART_LCR) {
> >>
> >> int tries = 1000;
> >> while (tries--) {
> >>
> >> - if (value == p->serial_in(p, UART_LCR))
> >> + if (value & ~0xa0 == p->serial_in(p, UART_LCR) &
> >> ~0xa0)>>
> >> return;
> >>
> >> dw8250_force_idle(p);
> >> writeb(value, p->membase + (UART_LCR <<
> >> p->regshift));
> >>
> >> @@ -132,7 +132,7 @@ static void dw8250_serial_out32(struct uart_port *p,
> >> int offset, int value)
> >>
> >> if (offset == UART_LCR) {
> >>
> >> int tries = 1000;
> >> while (tries--) {
> >>
> >> - if (value == p->serial_in(p, UART_LCR))
> >> + if (value & ~0xa0 == p->serial_in(p, UART_LCR) &
> >> ~0xa0)>
> > My appologies, that should have had some more brackets (I should have
> > retested after cleaning up my debugging). I.e.
> > + if ((value & ~0xa0) == (p->serial_in(p, UART_LCR)
> > & ~0xa0))
> >
> > Cheers
> > James
>
> James,
>
> Thanks for the information. This is really helpful.
>
> You are right about bit 5 being a problem. Its behavior differs
> between IP versions 3.00a and 3.14c per the docs.
>
> As for bit 7, when it doesn't change this indicates the UART was busy
> and we need to do the workaround.
>
> Would you mind testing it again using a mask of ~0x20?

It appears to work with ~0x20 too, and the workaround isn't getting hit (only
tested boot and logging in - nothing fancy). I think having the printks in
this code with the console directed at the serial must have caused
resursion/busy problems somehow.

Cheers
James

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