[PATCH v4 1/2] efi_loader: Avoid overwriting previous outputs on console screen clearing

[Jan Kiszka]

On 18.01.23 22:02, Heinrich Schuchardt wrote:
> 
> 
> Am 18. Januar 2023 21:58:42 MEZ schrieb Jan Kiszka <jan.kiszka at siemens.com>:
>> On 18.01.23 21:56, Heinrich Schuchardt wrote:
>>>
>>>
>>> Am 18. Januar 2023 21:44:58 MEZ schrieb Jan Kiszka <jan.kiszka at siemens.com>:
>>>> 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. Still, there
>>>> were remaining concerns about side effects, so this is provided as an
>>>> opt-in feature.
>>>>
>>>> Signed-off-by: Jan Kiszka <jan.kiszka at siemens.com>
>>>> ---
>>>> lib/efi_loader/Kconfig       |  9 +++++++++
>>>> lib/efi_loader/efi_console.c | 10 +++++++++-
>>>> 2 files changed, 18 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/lib/efi_loader/Kconfig b/lib/efi_loader/Kconfig
>>>> index b498c72206f..b2c3250b4ad 100644
>>>> --- a/lib/efi_loader/Kconfig
>>>> +++ b/lib/efi_loader/Kconfig
>>>> @@ -124,6 +124,15 @@ config EFI_SET_TIME
>>>> 	  Provide the SetTime() runtime service at boottime. This service
>>>> 	  can be used by an EFI application to adjust the real time clock.
>>>>
>>>> +config EFI_SCROLL_ON_CLEAR_SCREEN
>>>> +	bool "Avoid overwriting previous output on clear screen"
>>>> +	help
>>>> +	  Instead of moving the cursor home when the console screen should be
>>>> +	  cleared, emit blank new lines so that previous output is scrolled
>>>> +	  out of sight rather than overwritten. This allows on serial consoles
>>>> +	  to capture complete boot logs (except for interactive menus etc.)
>>>> +	  and can ease debugging related issues.
>>>> +
>>>> config EFI_HAVE_CAPSULE_SUPPORT
>>>> 	bool
>>>>
>>>> diff --git a/lib/efi_loader/efi_console.c b/lib/efi_loader/efi_console.c
>>>> index 4d08dd3763a..e40c129d1b2 100644
>>>> --- a/lib/efi_loader/efi_console.c
>>>> +++ b/lib/efi_loader/efi_console.c
>>>> @@ -461,10 +461,18 @@ 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)
>>>> {
>>>> +	if (CONFIG_IS_ENABLED(EFI_SCROLL_ON_CLEAR_SCREEN)) {
>>>> +		unsigned int row;
>>>> +
>>>> +		/* Avoid overwriting previous outputs on streaming consoles */
>>>
>>> Thanks for making the behavior configurable.
>>>
>>> We cannot assume that the cursor is on the last line. 
>>>
>>> Assume that the cursor is on the first line. Then the screen will still be deleted.
>>>
>>> Especially in GRUB the cursor often is in the upper half of the screen
>>>
>>> The cursor should be moved to the right lower corner before scrolling.
>>
>> Can do, though logging interactive menus was not in scope (and can't be
>> solved this way anyway).
>>
>> Jan
>>
> 
> 
> E.g. you can edit a menu entry in GRUB and press F10 and want to know what the entry looked like.
> 

Yes, that special case could be caught (not very relevant in embedded
practice, though), but any change you did before that is likely not
readably logged. That's why I don't consider interactive menus in scope
here.

Nevertheless, patch update is under test now.

Jan

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