ARM: big performance waste in memcpy_{from,to}io

[Russell King - ARM Linux]

On Fri, Nov 13, 2009 at 12:32:41PM +0100, Hubert Feurstein wrote:
> The memcpy_{to,from}io-function don't has to care about the bus-width of the 
> attached peripheral, because this is already handled correctly by the static 
> memory controller of your arm-derivate (Of course this one has to be 
> configured correctly to the peripherals bus width). In the rare case where you 
> have to take care about that it is anyway a bad idea to use a memcpy_xxio-
> function.

I believe there are SoCs where using 32-bit reads on lesser-width buses
generate faults.  There are certainly SoCs which do abort reads/writes
to certain peripherals which aren't the right size.

> Just want to ask the community if there is a really good reason why this 
> bottle neck is still in the ARM kernel?

No one has been bothered for the last 15 years to optimise it.

> @Russell: What's your opinion on that?

The problem is whether optimising it will end up breaking existing users.