[openib-general] [PATCH] kDAPL: cleanup dat/ a bit more

[Grant Grundler]

On Sat, Jun 04, 2005 at 04:33:12PM -0700, Caitlin Bestler wrote:
...
> The Quadrics patch informed zero copy devices
> of new mappings, but does so when the device
> is not actively doing work. That latter piece of
> information is not available for an RDMA device.

It certainly is.

Anyway, I agree with Christoph.

> For normal devices the OS has a reasonable
> expectation on when the device is performing
> DMA, because it is doing so in response to a
> request that the OS knows about. For an RDMA
> device a DMA transfer can be in progress as
> as a result of an RDMA Read -- something the
> host processor knows nothing about.

That's definitely not true. Host maps all DMA.
No one can DMA to/from the host without registering the
memory for a DMA mapping. The host does NOT know when
any given DMA mapping is active - but that's true for
FC/SCSI devices today too.

The device driver must take reasonable measures
to quiesce that DMA mapping.  Maybe send a signal
to the task that owns the DMA mapping?
Ie warn the task, wait a bit, then kill the task with
extreme prejudice if the resources it's sitting on
aren't free yet. I don't expect Hotplug support
to be simple to implement. But it's definitely possible.

grant