From c08804853361bb7c6e2f9e0cdcdc0b327f71fe35 Mon Sep 17 00:00:00 2001 From: Nikita Kiryanov Date: Sun, 24 Feb 2013 21:28:43 +0000 Subject: lcd: implement a callback for splashimage On some architectures certain values of splashimage will lead to a data abort exception. Document the problem, and implement a callback for splashimage to reject such values. Cc: Anatolij Gustschin Cc: Wolfgang Denk Signed-off-by: Nikita Kiryanov Acked-by: Igor Grinberg --- doc/README.displaying-bmps | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+) create mode 100644 doc/README.displaying-bmps (limited to 'doc/README.displaying-bmps') diff --git a/doc/README.displaying-bmps b/doc/README.displaying-bmps new file mode 100644 index 0000000..3311541 --- /dev/null +++ b/doc/README.displaying-bmps @@ -0,0 +1,27 @@ +If you are experiencing hangups/data-aborts when trying to display a BMP image, +the following might be relevant to your situation... + +Some architectures cannot handle unaligned memory accesses, and an attempt to +perform one will lead to a data abort. On such architectures it is necessary to +make sure all data is properly aligned, and in many situations simply choosing +a 32 bit aligned address is enough to ensure proper alignment. This is not +always the case when dealing with data that has an internal layout such as a +BMP image: + +BMP images have a header that starts with 2 byte-size fields followed by mostly +32 bit fields. The packed struct that represents this header can be seen below: + +typedef struct bmp_header { + /* Header */ + char signature[2]; + __u32 file_size; + __u32 reserved; + __u32 data_offset; + ... etc +} __attribute__ ((packed)) bmp_header_t; + +When placed in an aligned address such as 0x80a00000, char signature offsets +the __u32 fields into unaligned addresses (in our example 0x80a00002, +0x80a00006, and so on...). When these fields are accessed by U-Boot, a 32 bit +access is generated at a non-32-bit-aligned address, causing a data abort. +The proper alignment for BMP images is therefore: 32-bit-aligned-address + 2. -- cgit v1.1