[PATCH v1 3/3] doc: at91sam9263-ek: add notes

[Sam Ravnborg <sam@ravnborg.org>]

Add notes from working on bootstrap support

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
---
 .../boards/at91/microchip-at91sam9263-ek.rst       | 122 +++++++++++++++++++++
 1 file changed, 122 insertions(+)

diff --git a/Documentation/boards/at91/microchip-at91sam9263-ek.rst b/Documentation/boards/at91/microchip-at91sam9263-ek.rst
index 74ddb3cdb..8022cdd67 100644
--- a/Documentation/boards/at91/microchip-at91sam9263-ek.rst
+++ b/Documentation/boards/at91/microchip-at91sam9263-ek.rst
@@ -8,3 +8,125 @@ Building barebox:
 .. code-block:: sh
 
   make ARCH=arm at91sam9263ek_defconfig
+
+Notes while working on at91sam9263ek bootstrap support
+
+The at91sam9263 have support for a boot program,
+like the other members in the Atmel at91 series.
+
+The boot program (ROMBOOT) will try to load the
+boot program from DataFlash, SD Card, NAND Flash and USB
+
+SD Card is the first to try.
+It looks for a file named BOOT.BIN in the first
+partition in a FAT16/32 filesystem.
+
+To generate the SD Card image I used genimage:
+(https://github.com/pengutronix/genimage)
+Onle 2 GB SD card works, 4 GB did not work. ROMBOOT do not
+support high capacity SD cards.
+
+Configuration file:
+
+.. code-block:: none
+
+  image boot.vfat {
+	name = "boot"
+	vfat {
+		/*
+		 * RomBOOT in the at91sam9263 does not recognize
+		 * the default FAT partition created by mkdosfs.
+		 * -n BOOT - Set volume label to "BOOT"
+		 * -F 16   - Force the partition to FAT 16
+		 * -D 0    - Set drive number to 0
+		 * -R 1 -a - Reserve only one sector
+		 * The combination of "-D 0" AND "-R 1 -a"
+		 * is required with mkdosfs version 4.1
+		 */
+		extraargs = "-n BOOT -F 16 -D 0 -R 1 -a"
+		file BOOT.BIN    { image = "barebox.bin" } // barebox.bin from root of barebox dir 
+		file barebox.bin { image = "barebox-at91sam9263ek.img" }
+		file zImage      { image = "zImage" }
+	}
+
+	size = 16M
+  }
+
+  image rootfs.ext4 {
+	ext4 {
+		label = "root"
+	}
+	mountpoint = "/"
+	size = 1500M
+  }
+
+  image SD {
+	hdimage {}
+
+	partition boot {
+		partition-type = 0xc
+		bootable = "true"
+		image = "boot.vfat"
+	}
+
+	partition root {
+		image = "rootfs.ext4"
+		partition-type = 0x83
+	}
+
+  }
+
+ROMBOOT will load the BOOT.BIN file to internal SRAM that
+starts at 0x300000. Maximum size 0x12000 (72 KiB).
+When loaded ROMBOOT will remap like this:
+
+.. code-block:: none
+
+  0x00000000           0x00000000
+    Internal ROM   =>     Internal SRAM
+
+  0x00300000          0x00400000
+    Internal SRAM  =>     Internal ROM
+
+It is not documented but assumed that ROMBOOT uses the
+MMU to remap the addresses.
+There seems not to be a dedicated remapping feature that is used.
+
+Note: For DataFlash and NAND Flash the image is validated.
+The first 28 bytes must be valid load PC or PC relative addressing.
+Vector 0x6 must include the size of the image (in bytes).
+This validation is (according to datasheet) not done for SD Card boots.
+
+barebox related notes when trying to make it work with PBL enabled
+
+To let barebox detect the SD card early use: CONFIG_MCI_STARTUP=y
+
+When PBL (and MULTI_IMAGE) are enabled then barebox creates
+a binary with the following structure:
+
+.. code-block:: none
+
+  +----------------------+
+  | PBL (PreBootLoader)  |
+  +----------------------+
+  | piggy.o              |
+  |+--------------------+|
+  ||barebox second stage||
+  |+--------------------+|
+  +----------------------+
+
+The PBL contains code from the sections .text_head_entry*, .text_bare_init* and .text*
+
+``.text_head_entry*:``
+This is the reset vector and exception vectors. Must be the very first in the file
+
+``.text_bare_init*:``
+Everything in this section, and , is checked at link time.
+Size most be less than BAREBOX_MAX_BARE_INIT_SIZE / ARCH_BAREBOX_MAX_BARE_INIT_SIZE
+
+at91 specify the size of the two sections in exception vector 6 (see above),
+if CONFIG_AT91_LOAD_BAREBOX_SRAM is defined.
+I think this is because some at91 variants have only very limited SRAM size,
+and we copy only a minimal part to the SRAM. The remaining part is then
+executed in-place.
+For at91sam9263 we have a large SRAM so there is room for the full bootstrap binary.
-- 
2.12.0


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