[PATCH v2] powerpc: Add reloc_offset() to font bitmap pointer used for bootx_printf()

[Finn Thain]

Since Linux v6.7, booting using BootX on an Old World PowerMac produces
an early crash. Stan Johnson writes, "the symptoms are that the screen
goes blank and the backlight stays on, and the system freezes (Linux
doesn't boot)."

Further testing revealed that the failure can be avoided by disabling
CONFIG_BOOTX_TEXT. Bisection revealed that the regression was caused by
a change to the font bitmap pointer that's used when btext_init() begins
painting characters on the display, early in the boot process.

Christophe Leroy explains, "before kernel text is relocated to its final
location ... data is addressed with an offset which is added to the
Global Offset Table (GOT) entries at the start of bootx_init()
by function reloc_got2(). But the pointers that are located inside a
structure are not referenced in the GOT and are therefore not updated by
reloc_got2(). It is therefore needed to apply the offset manually by using
PTRRELOC() macro."

Cc: Cedar Maxwell <cedarmaxwell at mac.com>
Cc: Stan Johnson <userm57 at yahoo.com>
Cc: "Dr. David Alan Gilbert" <linux at treblig.org>
Cc: Benjamin Herrenschmidt <benh at kernel.crashing.org>
Cc: stable at vger.kernel.org
Link: https://lists.debian.org/debian-powerpc/2025/10/msg00111.html
Link: https://lore.kernel.org/linuxppc-dev/d81ddca8-c5ee-d583-d579-02b19ed95301@yahoo.com/
Reported-by: Cedar Maxwell <cedarmaxwell at mac.com>
Closes: https://lists.debian.org/debian-powerpc/2025/09/msg00031.html
Bisected-by: Stan Johnson <userm57 at yahoo.com>
Tested-by: Stan Johnson <userm57 at yahoo.com>
Fixes: 0ebc7feae79a ("powerpc: Use shared font data")
Suggested-by: Christophe Leroy <christophe.leroy at csgroup.eu>
Signed-off-by: Finn Thain <fthain at linux-m68k.org>
---
Changed since v1:
 - Improved commit log entry to better explain the need for PTRRELOC().
---
 arch/powerpc/kernel/btext.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/btext.c b/arch/powerpc/kernel/btext.c
index 7f63f1cdc6c3..ca00c4824e31 100644
--- a/arch/powerpc/kernel/btext.c
+++ b/arch/powerpc/kernel/btext.c
@@ -20,6 +20,7 @@
 #include <asm/io.h>
 #include <asm/processor.h>
 #include <asm/udbg.h>
+#include <asm/setup.h>
 
 #define NO_SCROLL
 
@@ -463,7 +464,7 @@ static noinline void draw_byte(unsigned char c, long locX, long locY)
 {
 	unsigned char *base	= calc_base(locX << 3, locY << 4);
 	unsigned int font_index = c * 16;
-	const unsigned char *font	= font_sun_8x16.data + font_index;
+	const unsigned char *font = PTRRELOC(font_sun_8x16.data) + font_index;
 	int rb			= dispDeviceRowBytes;
 
 	rmci_maybe_on();
-- 
2.49.1