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

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Message-ID: <CAAYSxhraDF6T9URw0CpEig2VpnuTBNdZYL9uuw6vdPn0=X4zGA@mail.gmail.com>
Date:	Mon, 9 Dec 2013 16:42:23 -0800
From:	Tim Kryger <tim.kryger@...aro.org>
To:	James Hogan <james.hogan@...tec.com>
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 Fri, Dec 6, 2013 at 4:59 PM, James Hogan <james.hogan@...tec.com> wrote:

> 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.

James,

I tested tested the code you proposed (cleaned up to avoid magic
numbers) on my hardware and it works fine.

diff --git a/drivers/tty/serial/8250/8250_dw.c
b/drivers/tty/serial/8250/8250_dw.c
index 4658e3e..5f096c7 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -96,7 +96,8 @@ 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))
+                       unsigned int lcr = p->serial_in(p, UART_LCR);
+                       if ((value & ~UART_LCR_SPAR) == (lcr & ~UART_LCR_SPAR))
                                return;
                        dw8250_force_idle(p);
                        writeb(value, p->membase + (UART_LCR << p->regshift));
@@ -132,7 +133,8 @@ 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))
+                       unsigned int lcr = p->serial_in(p, UART_LCR);
+                       if ((value & ~UART_LCR_SPAR) == (lcr & ~UART_LCR_SPAR))
                                return;
                        dw8250_force_idle(p);
                        writel(value, p->membase + (UART_LCR << p->regshift));


Would you mind posting this for proper review so we can get the fix in?

Thanks,
Tim Kryger
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