[openib-general] Re: [PATCH 21 of 53] ipath - use phys_to_virt instead of bus_to_virt

[Roland Dreier]

    Caitlin> True, but how does that constrain the local interfaces by
    Caitlin> which the driver is informed of the set of pages that
    Caitlin> back a given memory region? The driver must still
    Caitlin> ultimately provide dma accessible addresses to the
    Caitlin> device. RDMA just changes the timing of the steps, albeit
    Caitlin> radically, but not what the steps are.

It's only a problem for "reserved L_Key" types of things, where the
device is supposed to just use the address given in a work request
without translating it.  No translation means that work requests have
to contain "bus addresses" -- addresses that are what the device would
put on the bus to access memory.  But if a device needs to simulate
DMA in software, then it really needs a kernel virtual address, not a
bus address.  But it's pretty ugly to have to put that knowledge in
every consumer.

 - R.