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

[Jan Kiszka]

On 07.11.22 17:29, Heinrich Schuchardt wrote:
> 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.

Thanks for explaining this - now. I didn't observe this delay in QEMU,
but maybe that wasn't representative

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

It's in production - we can't afford shipping without it.

To avoid that this will be the last patch between full upstream and our
device one day: Do you see a way to confine overwrite protection to real
streaming device, UARTs and such?

Thanks,
Jan

-- 
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