drivers/exynos4210: Wait the end of last transfer before initializing the UART

Message ID 1369223456-3701-1-git-send-email-julien.grall@linaro.org
State Superseded
Headers show

Commit Message

Julien Grall May 22, 2013, 11:50 a.m.
When early printk is enabled by the user, it's possible to loose the last
characters if the UART is initialized without waiting that the last transfer
was completed.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
---
 xen/drivers/char/exynos4210-uart.c |    6 ++++++
 1 file changed, 6 insertions(+)

Comments

Ian Campbell May 23, 2013, 11:58 a.m. | #1
On Wed, 2013-05-22 at 12:50 +0100, Julien Grall wrote:
> When early printk is enabled by the user, it's possible to loose the last
> characters if the UART is initialized without waiting that the last transfer
> was completed.

It might actually be useful to do this in early_printk, i.e. a new
early_uart_flush or so called from puts and early_puts (or even just
from the putch hook).

Any opinions?

Perhaps we should take this fix as is for 4.3 and look at the
alternatives for 4.4?


> Signed-off-by: Julien Grall <julien.grall@linaro.org>
> ---
>  xen/drivers/char/exynos4210-uart.c |    6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/xen/drivers/char/exynos4210-uart.c b/xen/drivers/char/exynos4210-uart.c
> index f7971da..13758d4 100644
> --- a/xen/drivers/char/exynos4210-uart.c
> +++ b/xen/drivers/char/exynos4210-uart.c
> @@ -104,6 +104,12 @@ static void __init exynos4210_uart_init_preirq(struct serial_port *port)
>      unsigned int divisor;
>      uint32_t ulcon;
>  
> +#ifdef EARLY_PRINTK
> +    /* Flush the UART */
> +    while ( !(exynos4210_read(uart, UTRSTAT) &  UTRSTAT_TX_EMPTY) )
> +        cpu_relax();
> +#endif
> +
>      /* reset, TX/RX disables */
>      exynos4210_write(uart, UCON, 0);
>
Ian Campbell May 23, 2013, 12:01 p.m. | #2
On Thu, 2013-05-23 at 12:58 +0100, Ian Campbell wrote:
> On Wed, 2013-05-22 at 12:50 +0100, Julien Grall wrote:
> > When early printk is enabled by the user, it's possible to loose the last
> > characters if the UART is initialized without waiting that the last transfer
> > was completed.
> 
> It might actually be useful to do this in early_printk, i.e. a new
> early_uart_flush or so called from puts and early_puts (or even just
> from the putch hook).

BTW, my concern here is crashing without flushing the buffer and
therefore missing a useful message or having a misleading log...
Julien Grall May 23, 2013, 1:40 p.m. | #3
On 05/23/2013 01:01 PM, Ian Campbell wrote:

> On Thu, 2013-05-23 at 12:58 +0100, Ian Campbell wrote:
>> On Wed, 2013-05-22 at 12:50 +0100, Julien Grall wrote:
>>> When early printk is enabled by the user, it's possible to loose the last
>>> characters if the UART is initialized without waiting that the last transfer
>>> was completed.
>>
>> It might actually be useful to do this in early_printk, i.e. a new
>> early_uart_flush or so called from puts and early_puts (or even just
>> from the putch hook).
> 
> BTW, my concern here is crashing without flushing the buffer and
> therefore missing a useful message or having a misleading log...
> 
> 


Right. The change is not complicate, so I will create early_uart_flush
and call it in early_printk.

Patch

diff --git a/xen/drivers/char/exynos4210-uart.c b/xen/drivers/char/exynos4210-uart.c
index f7971da..13758d4 100644
--- a/xen/drivers/char/exynos4210-uart.c
+++ b/xen/drivers/char/exynos4210-uart.c
@@ -104,6 +104,12 @@  static void __init exynos4210_uart_init_preirq(struct serial_port *port)
     unsigned int divisor;
     uint32_t ulcon;
 
+#ifdef EARLY_PRINTK
+    /* Flush the UART */
+    while ( !(exynos4210_read(uart, UTRSTAT) &  UTRSTAT_TX_EMPTY) )
+        cpu_relax();
+#endif
+
     /* reset, TX/RX disables */
     exynos4210_write(uart, UCON, 0);