Re: MSI to memory?

From: Helge Hafting
Date: Thu Jul 08 2004 - 04:02:28 EST


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
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