[PATCH v5 1/2] efi_loader: Avoid overwriting previous outputs on console screen clearing

[Jan Kiszka]

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 | 14 +++++++++++++-
 2 files changed, 22 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..faa605e88d1 100644
--- a/lib/efi_loader/efi_console.c
+++ b/lib/efi_loader/efi_console.c
@@ -461,10 +461,22 @@ 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, screen_rows, screen_columns;
+
+		/* Avoid overwriting previous outputs on streaming consoles */
+		screen_rows = efi_cout_modes[efi_con_mode.mode].rows;
+		screen_columns = efi_cout_modes[efi_con_mode.mode].columns;
+		printf(ESC "[%d;1H",
+		       (int)screen_rows + 1, (int)screen_columns + 1);
+		for (row = 1; row < screen_rows; row++)
+			printf("\n");
+	}
+
 	/*
 	 * The Linux console wants both a clear and a home command. The video
 	 * uclass does not support <ESC>[H without coordinates, yet.
-- 
2.35.3