Re: b43 : unaligned access on mips

[matthieu castet <castet.matthieu@free.fr>]

Hi Michael,

Michael Buesch wrote:
> On Tuesday 02 June 2009 00:00:12 matthieu castet wrote:
>> matthieu castet wrote:
>>> Hi,
>>>
>>> b43_generate_plcp_hdr generate unaligned access on mips with gcc [1] 
>>> from openwrt.
>>>
>>> A small testcase [2] show that &plcp->data is access as a 32 bit aligned 
>>> variable (see the "lw      $2,0($4)" and "sw      $2,0($4)").
>>> I don't know enough mips to know if it is a gcc bug (ignoring the packed 
>>> attribute) or something missing in b43 code.
>> For example using "plcp->data" instead "*data" produce the correct code.
> 
> Uhm, I'm not sure. This code has been in the driver as-is forever and I don't
> see any unaligned access issues on mips (I checked a month or so ago).
Where does this pointer come from ? May be other code change change this 
pointer alignement?
Which toolchain did you use ?
Could you try to build the testcase I posted and see which code it 
generate ?


> The plcp data structure is also __attribute__((packed)), so there can't be any
> unaligned accesses, as gcc is required to _expect_ unaligned accesses on structures
> with packed attribute. So it is required to do byte accesses on architectures where
> alignment matters.
> So I don't think there's an issue in the code.
> 
But the code doesn't use the structure to access the data.
It use by an extra pointer "data", and I believe gcc loose the packed 
info (use byte accesses) when we do the "u32 *data = &plcp->data".


Matthieu