arm64: booti: allow to place kernel image anywhere in physical memory

At first, the ARM64 Linux booting requirement recommended that the kernel image be placed text_offset bytes from 2MB aligned base near the start of usable system RAM because memory below that base address was unusable at that time.

This requirement was relaxed by Linux commit a7f8de168ace ("arm64:
allow kernel Image to be loaded anywhere in physical memory"). Since then, the bit 3 of the flags field indicates the tolerance of the kernel physical placement. If this bit is set, the 2MB aligned base may be anywhere in physical memory. For details, see Documentation/arm64/booting.txt of Linux.

The booti command should be also relaxed. If the bit 3 is set, images->ep is respected, and the image is placed at the nearest bootable location. Otherwise, it is relocated to the start of the system RAM to keep the original behavior.

Signed-off-by Masahiro Yamada <[email protected]>

---

Changes in v2:
- Use le64_to_cpu() for correct endian-ness
- Check the bit 3

 cmd/booti.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

cmd/booti.c +3 -1

index bff87a8..8f3507d 100644
@@ -11,6 +11,8 @@
 #include <image.h>
 #include <lmb.h>
 #include <mapmem.h>
+#include <linux/kernel.h>
+#include <linux/sizes.h>
 
 DECLARE_GLOBAL_DATA_PTR;
 

@@ -20,7 +22,7 @@ struct Image_header {
 	uint32_t	code1;		/* Executable code */
 	uint64_t	text_offset;	/* Image load offset, LE */
 	uint64_t	image_size;	/* Effective Image size, LE */
-	uint64_t	res1;		/* reserved */
+	uint64_t	flags;		/* Kernel flags, LE */
 	uint64_t	res2;		/* reserved */
 	uint64_t	res3;		/* reserved */
 	uint64_t	res4;		/* reserved */

@@ -51,10 +53,18 @@ static int booti_setup(bootm_headers_t *images)
 	}
 
 	/*

cmd/booti.c static int booti_setup(bootm_headers_t *images) · line 62

Quoted 12 lines

      /*

cmd/booti.c +6 -3

-      * If we are not at the correct run-time location, set the new
-      * correct location and then move the image there.
+      * If bit 3 of the flags field is set, the 2MB aligned base of the
+      * kernel image can be anywhere in physical memory, so respect
+      * images->ep.  Otherwise, relocate the image to the base of RAM
+      * since memory below it is not accessible via the linear mapping.
       */
-     dst = gd->bd->bi_dram[0].start + le64_to_cpu(ih->text_offset);
+     if (le64_to_cpu(ih->flags) & BIT(3))
+             dst = images->ep - le64_to_cpu(ih->text_offset);

I take it this is a pre-correction for the ALIGN() below?

There's one last wrinkle to take care of here, if we want to boot a kernel older than commit a2c1d73b94ed49f5 (i.e. v3.16). Until then, the text_offset was of unknown endianness.

As mentiond in the Linux documentation, you can detect this based on the image_size field, e.g.

        uint64_t text_offset;

        /*
         * Prior to Linux commit a2c1d73b94ed49f5, the text_offset field
         * is of unknown endianness. In these cases, the image_size
         * field is zero, and we can assume a fixed value of 0x80000.
         */
        if (le64_to_cpu(ih->image_size) == 0)
                text_offset = 0x80000;
        else
                text_offset = (le64_to_cpu(ih->text_offset));

... then you can reuse that text_offset value for both cases above.