cramfs: implement uncompressed and arbitrary data block positioning

Two new capabilities are introduced here:

- The ability to store some blocks uncompressed.

- The ability to locate blocks anywhere.

Those capabilities can be used independently, but the combination
opens the possibility for execute-in-place (XIP) of program text segments
that must remain uncompressed, and in the MMU case, must have a specific
alignment.  It is even possible to still have the writable data segments
from the same file compressed as they have to be copied into RAM anyway.

This is achieved by giving special meanings to some unused block pointer
bits while remaining compatible with legacy cramfs images.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Chris Brandt <chris.brandt@renesas.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 30 insertions, 1 deletions

diff --git a/fs/cramfs/README b/fs/cramfs/README
index 9d4e7ea311f4..d71b27e0ff15 100644
--- a/fs/cramfs/README
+++ b/fs/cramfs/README
@@ -49,17 +49,46 @@ same as the start of the (i+1)'th <block> if there is one).  The first
 <block> immediately follows the last <block_pointer> for the file.
 <block_pointer>s are each 32 bits long.
 
+When the CRAMFS_FLAG_EXT_BLOCK_POINTERS capability bit is set, each
+<block_pointer>'s top bits may contain special flags as follows:
+
+CRAMFS_BLK_FLAG_UNCOMPRESSED (bit 31):
+	The block data is not compressed and should be copied verbatim.
+
+CRAMFS_BLK_FLAG_DIRECT_PTR (bit 30):
+	The <block_pointer> stores the actual block start offset and not
+	its end, shifted right by 2 bits. The block must therefore be
+	aligned to a 4-byte boundary. The block size is either blksize
+	if CRAMFS_BLK_FLAG_UNCOMPRESSED is also specified, otherwise
+	the compressed data length is included in the first 2 bytes of
+	the block data. This is used to allow discontiguous data layout
+	and specific data block alignments e.g. for XIP applications.
+
+
 The order of <file_data>'s is a depth-first descent of the directory
 tree, i.e. the same order as `find -size +0 \( -type f -o -type l \)
 -print'.
 
 
 <block>: The i'th <block> is the output of zlib's compress function
-applied to the i'th blksize-sized chunk of the input data.
+applied to the i'th blksize-sized chunk of the input data if the
+corresponding CRAMFS_BLK_FLAG_UNCOMPRESSED <block_ptr> bit is not set,
+otherwise it is the input data directly.
 (For the last <block> of the file, the input may of course be smaller.)
 Each <block> may be a different size.  (See <block_pointer> above.)
+
 <block>s are merely byte-aligned, not generally u32-aligned.
 
+When CRAMFS_BLK_FLAG_DIRECT_PTR is specified then the corresponding
+<block> may be located anywhere and not necessarily contiguous with
+the previous/next blocks. In that case it is minimally u32-aligned.
+If CRAMFS_BLK_FLAG_UNCOMPRESSED is also specified then the size is always
+blksize except for the last block which is limited by the file length.
+If CRAMFS_BLK_FLAG_DIRECT_PTR is set and CRAMFS_BLK_FLAG_UNCOMPRESSED
+is not set then the first 2 bytes of the block contains the size of the
+remaining block data as this cannot be determined from the placement of
+logically adjacent blocks.
+
 
 Holes
 -----