[U-Boot] [PATCH 1/5] Add bmp_layout module for accessing BMP header data

[Wolfgang Denk]

Dear Albert ARIBAUD,

In message <20130204222628.545da91e at lilith> you wrote:
> 
> The point about md not checking alignment is indeed valid: one should
> know that a md.l requires a 4-byte-aligned address or it will abort.

Or, one might do this _intentionally_ for some testing purposes.
For me it is of utmost importance that U-Boot does not come in my way
in such things.  It is supposed to do _exactly_ what I ask it to do,
even if this appears to be a stupid thing.

> OTOH, a cautious user may think that to ensure proper alignment, a BMP
> should be loaded on a 4-byte boundary, but IIUC that it precisely what
> will cause the load to fail, due to the sole 4-byte field of the BMP
> header being misaligned by two bytes.

Sole 4 byte field?  The bitmap file header has actually two 32-bit
entries: file_size, and data_offset. [The "reserved" entry as we are
using it should actually be two 16 bit entries, at least according to
[1]).

Yes, struct bmp_header is a packed array with non-natural order of
entries; see also section "Bitmap file header" at [1]; we may consider
this a really stupid thing to do, but we have to live with this fact.

[1] http://en.wikipedia.org/wiki/BMP_file_format

As I understand the problem comes from the fact, that this issue has
long been undetected, because the PTB that were/are responsible for
GCC on ARM had decided that any access to apacked structure would be
silently broken down into byte accesses, no matter what the actual
data type was.  While more recent versions of GCC started actually
attempting 16 or 32 bit accesses, which failed on some systems.

> So if we leave BMP loading as it is now, the load address will need to
> be 16-bit-but-not-32-bit-aligned, which is complicated enough to
> require documentation.

Indeed, this should be documented.  And eventually the bmp command
should print a warning message if it sees other alignment.

> Or, the BMP struct could be prepended with two bytes so that the
> load address alignment requirement becomes a simple 4-byte boundary,
> which most users are... bound... to choose naturally.

That would violate the "standard" defining the BMP header structure.


Best regards,

Wolfgang Denk

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