ntb_hw_plx: Workaround read-only scratchpad registers

On several systems we've noticed that when NTB link goes down, the
Physical Layer User Test Pattern registers we use as additional
scratchpad registers (that is explicitly allowed by the chip specs)
become read-only for about 100us. I see no explanation for this in
the chip specs, neither why it was not seen before, may be a race.
Since we do need these registers, workaround it by repeating writes
until we succeed or 1ms timeout expire.

MFC after: 1 week

sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

ntb: Remove unused devclass arguments to DRIVER_MODULE.

newbus: Move from bus_child_{pnpinfo,location}_src to bus_child_{pnpinfo,location} with sbuf

Now that the upper layers all go through a layer to tie into these
information functions that translates an sbuf into char * and len. The
current interface suffers issues of what to do in cases of truncation,
etc. Instead, migrate all these functions to using struct sbuf and these
issues go away. The caller is also in charge of any memory allocation
and/or expansion that's needed during this process.

Create a bus_generic_child_{pnpinfo,location} and make it default. It
just returns success. This is for those busses that have no information
for these items. Migrate the now-empty routines to using this as
appropriate.

Document these new interfaces with man pages, and oversight from before.

Reviewed by: jhb, bcr
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D29937

Make ntb(4) send bus_get_dma_tag() requests to parent buses passing real
bus' child pointers instead of grandchilds.

DMAR does not like requests from devices not parented directly by PCI.

MFC after: 2 weeks

Add driver for NTB in AMD SoC.

This patch is the driver for NTB hardware in AMD SoCs (ported from Linux)
and enables the NTB infrastructure like Doorbells, Scratchpads and Memory
window in AMD SoC. This driver has been validated using ntb_transport and
if_ntb driver already available in FreeBSD.

Submitted by: Rajesh Kumar <rajesh1.kumar@amd.com>
MFC after: 1 month
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D18774

Make NTB drivers report more info via NewBus methods.

MFC after: 12 days

Once more refactor KPI between NTB hardware and consumers.

New design allows hardware resources to be split between several consumers.
For example, one BAR can be dedicated for remote memory access, while other
resources can be used for packet transport for virtual Ethernet interface.
And even without resource split, this code allows to specify which consumer
driver should attach the hardware.

From some points this makes the code even closer to Linux one, even though
Linux does not provide the described flexibility.

NewBus'ify NTB subsystem.

This follows NTB subsystem modularization in Linux, tuning it to FreeBSD
native NewBus interfaces. This change allows to support different types
of hardware with different drivers, support multiple NTB instances in a
system, ntb_transport module use for needs other then if_ntb, etc.

Sponsored by: iXsystems, Inc.