Re: [patch 21/32] NTB/msi: Convert to msi_on_each_desc()

Logan Gunthorpe
2021-12-01T18:46:47Z

On 2021-12-01 11:14 a.m., 'Jason Gunthorpe' via linux-ntb wrote:
> On Wed, Dec 01, 2021 at 06:35:35PM +0100, Thomas Gleixner wrote:
>> On Wed, Dec 01 2021 at 09:00, Jason Gunthorpe wrote:
>>> On Wed, Dec 01, 2021 at 11:16:47AM +0100, Thomas Gleixner wrote:
>>>> Looking at the device slices as subdevices with their own struct device
>>>> makes a lot of sense from the conceptual level.
>>>
>>> Except IMS is not just for subdevices, it should be usable for any
>>> driver in any case as a general interrupt mechiansm, as you alluded to
>>> below about ethernet queues. ntb seems to be the current example of
>>> this need..
>>
>> But NTB is operating through an abstraction layer and is not a direct
>> PCIe device driver.
>
> I'm not sure exactly how NTB seems to be split between switchtec and
> the ntb code, but since the ntbd code seems to be doing MMIO touches,
> it feels like part of a PCIe driver?

Eh, sort of. NTB has lots of layers. At the top you'll find ntb_netdev
which is an network interface. Below that is ntb_transport() which is a
generic queue pair. Below that is the hardware driver itself (ie
switchtec) through the abstraction layer. The switchtec driver is split
in two: the main driver which just allows for information and
administration of the switch itself and switchtec_ntb which is the
module that provides the NTB abstractions to twiddle its registers.

ntb_transport() doesn't directly do MMIO touches (as it doesn't know
what the underlying hardware registers are). Except for the memory
windows which are usually setup to be a specific BAR (or parts of a
BAR). ntb_transport will do MMIO writes to those specific BAR address
which correspond to writing into buffers on the peer.

Logan