Re: [RFC v2 1/2] PCI-Express Non-Transparent Bridge Support

On Tue, Jul 31, 2012 at 07:45:29AM -0600, Bjorn Helgaas wrote:

On Mon, Jul 30, 2012 at 12:15 PM, Jon Mason [off-list ref] wrote:

quoted

On Mon, Jul 30, 2012 at 10:50:13AM -0600, Bjorn Helgaas wrote:

quoted

On Sun, Jul 29, 2012 at 6:26 PM, Jon Mason [off-list ref] wrote:

quoted

A PCI-Express non-transparent bridge (NTB) is a point-to-point PCIe bus
connecting 2 systems, providing electrical isolation between the two subsystems.
A non-transparent bridge is functionally similar to a transparent bridge except
that both sides of the bridge have their own independent address domains.  The
host on one side of the bridge will not have the visibility of the complete
memory or I/O space on the other side of the bridge.  To communicate across the
non-transparent bridge, each NTB endpoint has one (or more) apertures exposed to
the local system.  Writes to these apertures are mirrored to memory on the
remote system.  Communications can also occur through the use of doorbell
registers that initiate interrupts to the alternate domain, and scratch-pad
registers accessible from both sides.

The NTB device driver is needed to configure these memory windows, doorbell, and
scratch-pad registers as well as use them in such a way as they can be turned
into a viable communication channel to the remote system.  ntb_hw.[ch]
determines the usage model (NTB to NTB or NTB to Root Port) and abstracts away
the underlying hardware to provide access and a common interface to the doorbell
registers, scratch pads, and memory windows.  These hardware interfaces are
exported so that other, non-mainlined kernel drivers can access these.
ntb_transport.[ch] also uses the exported interfaces in ntb_hw.[ch] to setup a
communication channel(s) and provide a reliable way of transferring data from
one side to the other, which it then exports so that "client" drivers can access
them.  These client drivers are used to provide a standard kernel interface
(i.e., Ethernet device) to NTB, such that Linux can transfer data from one
system to the other in a standard way.

Signed-off-by: Jon Mason <redacted>
---
 MAINTAINERS                 |    6 +
 drivers/Kconfig             |    2 +
 drivers/Makefile            |    1 +
 drivers/ntb/Kconfig         |   13 +
 drivers/ntb/Makefile        |    3 +
 drivers/ntb/ntb_hw.c        | 1178 ++++++++++++++++++++++++++++++++++++
 drivers/ntb/ntb_hw.h        |  206 +++++++
 drivers/ntb/ntb_regs.h      |  150 +++++
 drivers/ntb/ntb_transport.c | 1387 +++++++++++++++++++++++++++++++++++++++++++
 include/linux/ntb.h         |   92 +++

Where will drivers for non-Intel NTBs fit in this hierarchy?  It seems
a bit presumptuous to claim the generic "ntb" names just for Intel
devices.

I've tried to make it all generic enough that non-Intel NTBs should plug in with
minimal changes to ntb_hw.c.  If their design is too divergent, then a slight
redesign of ntb_hw.c might be necessary.  But from what I've seen of other
designs on the internet, they appear to be extremely similar.  The transport and
client drivers were written with the hardware abstracted away as much as
possible to prevent the need to modify it for different hardware.  If there is
anything which is Intel hardware specific, I'd be happy to change it to make it
more generic.

That makes sense from a technical point of view, but I think it's
going to cause maintenance issues.  For example, assume PLX NTB
support is added.  Will PLX be happy about having to convince you to
accept changes?  Will Intel be happy about having to release a new
driver for their hardware just to incorporate a PLX bug fix?  Will
users of PLX hardware accept a new driver release that only benefits
Intel users?

Firstly, I don't believe this is an issue.  The amount of hardware
specific code is small (<150 loc), and the design of the PLX hardware
appears to be the same as Intel (from what I've read on the internet).

Assuming Intel is me, I am always happy to do whatever I can to help
PLX or any other NTB vendor.  I am willing to make internal releases
(which I am assuming is the issue you are referring to) for any change
to the driver.

If necessary, refactoring the driver shouldn't be a significant issue.
However, I would prefer to do it if/when another NTB driver wants to
be added.  PLX (and those who are using their NTB hardware) hasn't
pushed an NTB driver yet, and may never (as their design docs on the
web are from '05).  Why complicate the driver for them if they never
come?

Thanks,
Jon