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

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Date:	Fri, 06 Dec 2013 16:59:16 -0800 (PST)
From:	James Hogan <james.hogan@...tec.com>
To:	Tim Kryger <tim.kryger@...aro.org>
Cc:	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	linux-serial@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Patch Tracking <patches@...aro.org>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Gregory Clement <gregory.clement@...e-electrons.com>,
	Lior Amsalem <alior@...vell.com>,
	Jason Cooper <jason@...edaemon.net>,
	ARM Kernel List <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] serial: 8250_dw: Improve unwritable LCR workaround

On Friday 06 December 2013 16:31:38 Tim Kryger wrote:
> On Fri, Dec 6, 2013 at 3:51 PM, James Hogan <james.hogan@...tec.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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