WLAN from SDIO to SPI: battle plan

[Joerg Reisenweber]

Am Mi  27. August 2008 schrieb Andy Green:
> Somebody in the thread at some point said:
> | HI:
> |   Yes. it works after remove "__attribute__((__packed__))"
> |
> | #arm-angstrom-linux-gnueabi-objdump -d ./cpu/arm920t/s3c24x0/spi.o
> |   ....
> |   d0:	e59d2004 	ldr	r2, [sp, #4]
> |   d4:	e3a0304f 	mov	r3, #79	; 0x4f
> |   d8:	e582300c 	str	r3, [r2, #12]
> |   dc:	e59d2004 	ldr	r2, [sp, #4]
> |   e0:	e3a03001 	mov	r3, #1	; 0x1
> |   e4:	e5823008 	str	r3, [r2, #8]
> |   e8:	e59d2004 	ldr	r2, [sp, #4]
> |   ec:	e283301e 	add	r3, r3, #30	; 0x1e
> |   f0:	e5823000 	str	r3, [r2]
> |   f4:	e3a01000 	mov	r1, #0	; 0x0
> |   f8:	e2811001 	add	r1, r1, #1	; 0x1
> |   fc:	e59d2004 	ldr	r2, [sp, #4]
> |  100:	e3a030ff 	mov	r3, #255	; 0xff
> |
> |   I see the same problem, if use "__packed__" attribute,  GCC
> | will compile with byte-access code which may takes more
> |  instruction. Say, update a counter, and it can't keep it ATOMIC access,
> | may produce race, or data corrupt.
> 
> You need __packed__ if the start of your struct pointer may not be
> aligned to 32-bit boundary, but you want to access 32-bit items in
> there.  This happens in network processing code for example, the struct
> representing the packet payload can easily be at funny alignment due to
> snipped headers from earlier protocols.
> 
> You don't need it for dealing with a bunch of 32-bit registers always
> found on 32-bit boundaries.

I remember when writing TTY-driver code in assembler for BS2000 back in 1985, 
there were all sorts of PACK and ALIGN16 and ALIGN32 directives to cope with 
that. There was good reason they were there.
Think in COBOL-M there were similar options for data and linkage section.

Just a nearly OT annotation ;-) Sorry couldn't resist as these low-level 
machine interfaces were my favorite battlefield
cheers
jOERG
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