Re: [PATCH 5/5] Serial: UART driver changes for Cavium OCTEON.

Shinya Kuribayashi
2008-11-28T03:28:00Z

Hi David,

David Daney wrote:
> Cavium UART implementation won't work with the standard 8250 driver
> as-is. Define a new uart_config (PORT_OCTEON) and use it to enable
> special handling required by the OCTEON's serial port. Two new bug
> types are defined.

You just added one new bug type, not two. ;-)

> diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
> index 3bb8e30..63dbbb6 100644
> --- a/drivers/serial/8250.c
> +++ b/drivers/serial/8250.c
> @@ -279,6 +279,14 @@ static const struct serial8250_config uart_config[] = {
> .fcr = UART_FCR_ENABLE_FIFO | UART_FCR_R_TRIG_10,
> .flags = UART_CAP_FIFO,
> },
> + [PORT_OCTEON] = {
> + .name = "OCTEON",
> + .fifo_size = 64,
> + .tx_loadsz = 64,
> + .bugs = UART_BUG_TIMEOUT,
> + .fcr = UART_FCR_ENABLE_FIFO | UART_FCR_R_TRIG_10,
> + .flags = UART_CAP_FIFO,
> + },
> };
>
> #if defined (CONFIG_SERIAL_8250_AU1X00)

I see your point. Then let's look at this commit:

|commit 40b36daad0ac704e6d5c1b75789f371ef5b053c1
|Author: Alex Williamson <alex.williamson@hp.com>
|Date: Wed Feb 14 00:33:04 2007 -0800
|
| [PATCH] 8250 UART backup timer
|
| The patch below works around a minor bug found in the UART of the remote
| management card used in many HP ia64 and parisc servers (aka the Diva
| UARTs). The problem is that the UART does not reassert the THRE interrupt
| if it has been previously cleared and the IIR THRI bit is re-enabled. This
| can produce a very annoying failure mode when used as a serial console,
| allowing a boot/reboot to hang indefinitely until an RX interrupt kicks it
| into working again (ie. an unattended reboot could stall).
|
| To solve this problem, a backup timer is introduced that runs alongside the
| standard interrupt driven mechanism. This timer wakes up periodically,
| checks for a hang condition and gets characters moving again. This backup
| mechanism is only enabled if the UART is detected as having this problem,
| so systems without these UARTs will have no additional overhead.
|
| This version of the patch incorporates previous comments from Pavel and
| removes races in the bug detection code. The test is now done before the
| irq linking to prevent races with interrupt handler clearing the THRE
| interrupt. Short delays and syncs are also added to ensure the device is
| able to update register state before the result is tested.
|
| Aristeu says:
|
| this was tested on the following HP machines and solved the problem:
| rx2600, rx2620, rx1600 and rx1620s.
|
| hpa says:
|
| I have seen this same bug in soft UART IP from "a major vendor."
|
| Signed-off-by: Alex Williamson <alex.williamson@hp.com>
| Cc: "H. Peter Anvin" <hpa@zytor.com>
| Cc: Russell King <rmk@arm.linux.org.uk>
| Acked-by: Aristeu Sergio Rozanski Filho <aris@cathedrallabs.org>
| Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
| Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

AFAICS this patch tries to handle the same problem, and added reasonable
framework with serial8250_backup_timeout, etc. And now we have UART_BUG
_THRE for this issue. Then it might be better to use existing codes as
much as possible.

Just random thoughts,

- could we dynamically probe this bug in serial8250_startup as well?

- is there any possibility this hardware works with an existing
serial8250_backup_timeout, not doing this below:

> @@ -1714,7 +1722,7 @@ static void serial8250_timeout(unsigned long data)
> unsigned int iir;
>
> iir = serial_in(up, UART_IIR);
> - if (!(iir & UART_IIR_NO_INT))
> + if (!(iir & UART_IIR_NO_INT) || (up->bugs & UART_BUG_TIMEOUT))
> serial8250_handle_port(up);
> mod_timer(&up->timer, jiffies + poll_timeout(up->port.timeout));
> }

I'm not an UART expert, any feedback is highly appreciated.

Thanks,

--
Shinya Kuribayashi
NEC Electronics