Re: MSI to memory?

[Helge Hafting <helge.hafting@hist.no>]

Roland Dreier wrote:

>    Helge> Won't that put a bad load on the bus?  Someone else might
>    Helge> need it: * Another cpu in a smp system * Any device doing
>    Helge> bus-master transfers, even in a UP system
>
>Actually with MSI, the PCI device writes directly to a host address.
>In the proposed usage in this mail thread, the address is in host
>memory, so there's no bus load to poll the memory.  Presumably the
>memory will be pulled into cache for the duration of the poll loop, so
>there's not even any memory bandwidth consumed.  (Of course this only
>works on an architecture where PCI DMA is cache coherent)
>  
>
I see.  Still, this should only be used when we expect a
short wait only, similiar to situations when we use a spinlock.
Otherwise, the cpu could be put to better use by scheduling.

Helge Hafting