WLAN from SDIO to SPI: battle plan

Andy Green andy at openmoko.com
Wed Aug 27 17:21:17 CEST 2008

Hash: SHA1

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.

- -Andy
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