Skip to content
Snippets Groups Projects
README.displaying-bmps 1.21 KiB
Newer Older
  • Learn to ignore specific revisions
  • 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.