[U-Boot] [PATCH 2/2] console: Fix pre-console flushing via cfb_console being very slow

[Ian Campbell <ijc+uboot@hellion.org.uk>]

On Tue, 2015-05-05 at 13:21 +0200, Hans de Goede wrote:
> On my A10 OlinuxIno Lime I noticed a huge (5+ seconds) delay coming from
> console_init_r. This turns out to be caused by the preconsole buffer flushing
> to the cfb_console. The Lime only has a 16 bit memory bus and that is already
> heavy used to scan out the 1920x1080 framebuffer.
> 
> The problem is that print_pre_console_buffer() was printing the buffer once
> character at a time and the cfb_console code then ends up doing a cache-flush
> for touched display lines for each character.
> 
> This commit fixes this by first building a 0 terminated buffer and then
> printing it in one puts() call, avoiding unnecessary cache flushes.
> 
> This changes the time for the flush from 5+ seconds to not noticable.
> 
> The downside of this approach is that the pre-console buffer needs to fit
> on the stack, this is not that much to ask since we are talking about plain
> text here. This commit also adjusts the sunxi CONFIG_PRE_CON_BUF_SZ to
> actually fit on the stack. Sunxi currently is the only user of the pre-console
> code so no other boards need to be adjusted.
> 
> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> ---
>  README                         |  3 +++
>  common/console.c               | 40 ++++++++++++++++++++++------------------

I think you need an ack from the maintainer of this file more than you
do from me. I'm happy for you to enable this option if that maintainer
is happy for it to exist in the first place.

>  include/configs/sunxi-common.h |  2 +-
>  3 files changed, 26 insertions(+), 19 deletions(-)
> [...]
> diff --git a/include/configs/sunxi-common.h b/include/configs/sunxi-common.h
> index c78141e..5d77e71 100644
> --- a/include/configs/sunxi-common.h
> +++ b/include/configs/sunxi-common.h
> @@ -402,7 +402,7 @@ extern int soft_i2c_gpio_scl;
>  
>  /* Enable pre-console buffer to get complete log on the VGA console */
>  #define CONFIG_PRE_CONSOLE_BUFFER
> -#define CONFIG_PRE_CON_BUF_SZ		(1024 * 1024)
> +#define CONFIG_PRE_CON_BUF_SZ		4096 /* Aprox 2 80*25 screens */

"Approx".