Re: Fix locking in input

[Timothy Miller <miller@techsource.com>]

Manfred Spraul wrote:
> Russell King wrote:
> 
>> On Sun, Nov 23, 2003 at 11:24:02PM +0100, Manfred Spraul wrote:
>>  
>>
>>> I think one platform (early ARM?) cannot access bytes directly, and 
>>> implement the access with read 16-bit, change 8-bit, write back 16 
>>> bit.   
>>
>>
>> Nope.
>>
> It seems it's Alpha:
> 
> On Thu, 28 Dec 2000, Linus wrote:
> 
>> FreeBSD doesn't try to be portable any more, but Linux does, and there 
>> are
>> architectures where 8- and 16-bit accesses aren't *atomic* but have to be
>> done with read-modify-write cycles.
>>
>> And even for fields like "age", where we don't care whether the age 
>> itself
>> is 100% accurate, we _do_ care that the fields close-by don't get strange
>> effects from updating "age". We used to have exactly this problem on 
>> alpha
>> back in the 2.1.x timeframe.
>>


This is MOSTLY true, but not entirely.  As I recall, Alpha has two 
addressing modes: Sparse and Dense.

The mode you're referring to is dense addressing where 32-bit and 64-bit 
accesses are atomic, but 16 and 8 bit accesses require RMW.

Sparse mode allows for atomic 8 and 16 bit accesses, but the access size 
is specified in the address using a rather bizarre binary encoding that 
doesn't look much like a regular address value.  You would probably not 
want to use this unless you absolutely HAD to use some I/O device that 
required atomic byte or word accesses.

I'm sure someone else will (or already has) pipe(d) in with a more 
accurate explanation.  :)