Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 03 Dec 2003 18:44:41 -0500 | From | Timothy Miller <> | Subject | Re: Fix locking in input |
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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. :)
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