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On Fri, 4 Sep 1998, Inaky Perez Gonzalez wrote:
> >>>>> "B" == B James Phillippe <bryan@terran.org> writes:
>
> B> Hello Kernel Gurus, I'm wondering what is the best way to align a
> B> chunk of kernel memory (from kmalloc, say) on a particular byte
> B> boundary. 16 bytes, for instance.
>
> I had the same problem; however, I noticed everything was
> using a power-of-two chunk allocation scheme, so I did a dirty
> function [__usb_kmalloc_align]:
My minimal expectation from a kernel memory allocator is to provide
at least alignment on the lowest power of 2 greater than the requested
size. This can be achieved by using power of 2 sized actual memory
chunks in the allocator.
Such a scheme wastes 25% memory in average but provides lots of
goodnesses and is simple to implement.
Linux-2.0 kernel allocator does not provide such power of 2 alignments.
[ 5 minutes code removed ]
> The point is, if the size of the object is smaller then the
> alignment, ask for a chunk sized as the alignment, else the size. The
> rounding to the next power of two will do the same when
> allocating.
If you want to align a object on a power of 2 greater than the lowest
power of 2 greater than the object size, then you just want to waste
memory, in my opinion. Do you really need that ?
> I've successfully allocated _everyone_ of them, even under
> heavy load, but I am not sure it will always success. So I implemented
> a workaround for the case it failed.
>
> If not aligned, it will recursively allocate more chunks,
> until one falls into the wanted alignment; then it will free them and
> let you with the aligned one. It's ugly, but it works. There's a depth
> limit, just in case ...
>
> It's dirty, could be optimized, etc, etc ... but hey, I did it
> in five minutes :)
Most of things we did in five minutes or less generally donnot smell good.
:-))
BTW, I have attached the allocator code I use in latest experimental
sym53c8xx driver for linux-2.0, if this can help.
Seems Linux-2.1 provides the expected power of 2 alignment, but drivers
have to work for linux-2.0 too.
Regards,
Gerard.
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