[PATCH v3 1/3] efi_loader: Avoid overwriting previous outputs on console screen clearing

[Heinrich Schuchardt]

On 11/4/22 09:06, Jan Kiszka wrote:
> From: Jan Kiszka <jan.kiszka at siemens.com>
>
> Before clearing the screen, ensure that no previous output of firmware
> or UEFI programs will be overwritten on serial devices or other
> streaming consoles. This helps generating complete boot logs.
>
> Tested regarding multi-output against qemu-x86_defconfig.
>
> Signed-off-by: Jan Kiszka <jan.kiszka at siemens.com>
> ---
>   lib/efi_loader/efi_console.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/lib/efi_loader/efi_console.c b/lib/efi_loader/efi_console.c
> index 4d08dd3763a..6ce0fcc168d 100644
> --- a/lib/efi_loader/efi_console.c
> +++ b/lib/efi_loader/efi_console.c
> @@ -461,10 +461,16 @@ static efi_status_t EFIAPI efi_cout_set_attribute(
>   }
>
>   /**
> - * efi_cout_clear_screen() - clear screen
> + * efi_clear_screen() - clear screen
>    */
>   static void efi_clear_screen(void)
>   {
> +	unsigned int row;
> +
> +	/* Avoid overwriting previous outputs on streaming consoles */
> +	for (row = 1; row < efi_cout_modes[efi_con_mode.mode].rows; row++)
> +		printf("\n");
> +

Scrolling on a framebuffer is a very expensive operations: For each of
the 135 lines of an UHD display you have to copy 32 MiB. This would
create a delay of multiple seconds on a slow device.

So keep the patch for your debugging purposed. But we cannot merge it
into upstream.

Best regards

Heinrich

>   	/*
>   	 * The Linux console wants both a clear and a home command. The video
>   	 * uclass does not support <ESC>[H without coordinates, yet.