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Diffstat (limited to 'doc/README.displaying-bmps')
| -rw-r--r-- | doc/README.displaying-bmps | 27 |
1 files changed, 27 insertions, 0 deletions
diff --git a/doc/README.displaying-bmps b/doc/README.displaying-bmps new file mode 100644 index 0000000000..331154166d --- /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. |