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

Ezequiel Garcia
2013-11-26T18:36:10Z

Hello,

On Tue, Oct 01, 2013 at 10:18:08AM -0700, Tim Kryger wrote:
> When configured with UART_16550_COMPATIBLE=NO or in versions prior to
> the introduction of this option, the Designware UART will ignore writes
> to the LCR if the UART is busy. The current workaround saves a copy of
> the last written LCR and re-writes it in the ISR for a special interrupt
> that is raised when a write was ignored.
>
> Unfortunately, interrupts are typically disabled prior to performing a
> sequence of register writes that include the LCR so the point at which
> the retry occurs is too late. An example is serial8250_do_set_termios()
> where an ignored LCR write results in the baud divisor not being set and
> instead a garbage character is sent out the transmitter.
>
> Furthermore, since serial_port_out() offers no way to indicate failure,
> a serious effort must be made to ensure that the LCR is actually updated
> before returning back to the caller. This is difficult, however, as a
> UART that was busy during the first attempt is likely to still be busy
> when a subsequent attempt is made unless some extra action is taken.
>
> This updated workaround reads back the LCR after each write to confirm
> that the new value was accepted by the hardware. Should the hardware
> ignore a write, the TX/RX FIFOs are cleared and the receive buffer read
> before attempting to rewrite the LCR out of the hope that doing so will
> force the UART into an idle state. While this may seem unnecessarily
> aggressive, writes to the LCR are used to change the baud rate, parity,
> stop bit, or data length so the data that may be lost is likely not
> important. Admittedly, this is far from ideal but it seems to be the
> best that can be done given the hardware limitations.
>
> Lastly, the revised workaround doesn't touch the LCR in the ISR, so it
> avoids the possibility of a "serial8250: too much work for irq" lock up.
> This problem is rare in real situations but can be reproduced easily by
> wiring up two UARTs and running the following commands.
>
> # stty -F /dev/ttyS1 echo
> # stty -F /dev/ttyS2 echo
> # cat /dev/ttyS1 &
> [1] 375
> # echo asdf > /dev/ttyS1
> asdf
>
> [ 27.700000] serial8250: too much work for irq96
> [ 27.700000] serial8250: too much work for irq96
> [ 27.710000] serial8250: too much work for irq96
> [ 27.710000] serial8250: too much work for irq96
> [ 27.720000] serial8250: too much work for irq96
> [ 27.720000] serial8250: too much work for irq96
> [ 27.730000] serial8250: too much work for irq96
> [ 27.730000] serial8250: too much work for irq96
> [ 27.740000] serial8250: too much work for irq96
>
> Signed-off-by: Tim Kryger <tim.kryger@linaro.org>
> Reviewed-by: Matt Porter <matt.porter@linaro.org>
> Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
> ---
>
> Changes in v2:
> - Rebased on tty-next
> - Updated commit messsage to mention UART_16550_COMPATIBLE
> - Removed potentially unnecessary read of LSR and MSR
> - Only attempt workaround when LCR write is ignored
>
> drivers/tty/serial/8250/8250_dw.c | 41 ++++++++++++++++++++++++++++++---------
> 1 file changed, 32 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
> index d04a037..4658e3e 100644
> --- a/drivers/tty/serial/8250/8250_dw.c
> +++ b/drivers/tty/serial/8250/8250_dw.c
> @@ -57,7 +57,6 @@
>
> struct dw8250_data {
> u8 usr_reg;
> - int last_lcr;
> int last_mcr;
> int line;
> struct clk *clk;
> @@ -77,17 +76,33 @@ static inline int dw8250_modify_msr(struct uart_port *p, int offset, int value)
> return value;
> }
>
> +static void dw8250_force_idle(struct uart_port *p)
> +{
> + serial8250_clear_and_reinit_fifos(container_of
> + (p, struct uart_8250_port, port));
> + (void)p->serial_in(p, UART_RX);
> +}
> +
> static void dw8250_serial_out(struct uart_port *p, int offset, int value)
> {
> struct dw8250_data *d = p->private_data;
>
> - if (offset == UART_LCR)
> - d->last_lcr = value;
> -
> if (offset == UART_MCR)
> d->last_mcr = value;
>
> writeb(value, p->membase + (offset << p->regshift));
> +
> + /* Make sure LCR write wasn't ignored */
> + if (offset == UART_LCR) {
> + int tries = 1000;
> + while (tries--) {
> + if (value == p->serial_in(p, UART_LCR))
> + return;
> + dw8250_force_idle(p);
> + writeb(value, p->membase + (UART_LCR << p->regshift));
> + }
> + dev_err(p->dev, "Couldn't set LCR to %d\n", value);
> + }
> }
>
> static unsigned int dw8250_serial_in(struct uart_port *p, int offset)
> @@ -108,13 +123,22 @@ static void dw8250_serial_out32(struct uart_port *p, int offset, int value)
> {
> struct dw8250_data *d = p->private_data;
>
> - if (offset == UART_LCR)
> - d->last_lcr = value;
> -
> if (offset == UART_MCR)
> d->last_mcr = value;
>
> writel(value, p->membase + (offset << p->regshift));
> +
> + /* Make sure LCR write wasn't ignored */
> + if (offset == UART_LCR) {
> + int tries = 1000;
> + while (tries--) {
> + if (value == p->serial_in(p, UART_LCR))
> + return;
> + dw8250_force_idle(p);
> + writel(value, p->membase + (UART_LCR << p->regshift));
> + }
> + dev_err(p->dev, "Couldn't set LCR to %d\n", value);
> + }
> }
>
> static unsigned int dw8250_serial_in32(struct uart_port *p, int offset)
> @@ -132,9 +156,8 @@ static int dw8250_handle_irq(struct uart_port *p)
> if (serial8250_handle_irq(p, iir)) {
> return 1;
> } else if ((iir & UART_IIR_BUSY) == UART_IIR_BUSY) {
> - /* Clear the USR and write the LCR again. */
> + /* Clear the USR */
> (void)p->serial_in(p, d->usr_reg);
> - p->serial_out(p, UART_LCR, d->last_lcr);
>
> return 1;
> }

Since v3.13-rc1, this commit seems to have introduced some oddities on
some of our boards. See this log snippet:

Serial: 8250/16550 driver, 4 ports, IRQ sharing disabled
����R�console [ttyS0] enabled
console [ttyS0] enabled
bootconsole [earlycon0] disabled
bootconsole [earlycon0] disabled
dw-apb-uart d0012100.serial: Couldn't set LCR to 191
dw-apb-uart d0012100.serial: Couldn't set LCR to 191
dw-apb-uart d0012100.serial: Couldn't set LCR to 224
dw-apb-uart d0012100.serial: Couldn't set LCR to 224
d0012100.serial: ttyS1 at MMIO 0xd0012100 (irq = 18, base_baud = 15625000) is a 16550A

This behavior appear in at least Armada 370 and Armada XP boxes.

I confirm reverting this commit fixes the issue and things get back to normal.
Here's the complete kernel log: sprunge.us/gMdL

Ideas?
--
Ezequiel García, Free Electrons
Embedded Linux, Kernel and Android Engineering
http://free-electrons.com
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