PCI/bwctrl: Re-add BW notification portdrv as PCIe BW controller

This mostly reverts the commit b4c7d207 ("PCI/LINK: Remove bandwidth
notification"). An upcoming commit extends this driver building PCIe
bandwidth controller on top of it.

PCIe bandwidth notifications were first added in the commit e8303bb7
("PCI/LINK: Report degraded links via link bandwidth notification") but
later had to be removed. The significant changes compared with the old
bandwidth notification driver include:

1) Don't print the notifications into kernel log, just keep the Link
   Speed cached in struct pci_bus updated. While somewhat unfortunate,
   the log spam was the source of complaints that eventually lead to
   the removal of the bandwidth notifications driver (see the links
   below for further information).

2) Besides the Link Bandwidth Management Interrupt, also enable Link
   Autonomous Bandwidth Interrupt to cover the other source of bandwidth
   changes.

3) Handle Link Speed updates robustly. Refresh the cached Link Speed
   when enabling Bandwidth Notification Interrupts, and solve the race
   between Link Speed read and LBMS/LABS update in
   pcie_bwnotif_irq_thread().

4) Use concurrency safe LNKCTL RMW operations.

5) The driver is now called PCIe bwctrl (bandwidth controller) instead
   of just bandwidth notifications because of increased scope and
   functionality within the driver.

6) Coexist with the Target Link Speed quirk in pcie_failed_link_retrain().
   Provide LBMS counting API for it.

7) Tweaks to variable/functions names for consistency and length reasons.

Bandwidth Notifications enable the cur_bus_speed in the struct pci_bus to
keep track PCIe Link Speed changes.

[bhelgaas: This is based on previous work by Alexandru Gagniuc
<mr.nuke.me@gmail.com>; see e8303bb7 ("PCI/LINK: Report degraded links
via link bandwidth notification")]

Link: https://lore.kernel.org/r/20241018144755.7875-7-ilpo.jarvinen@linux.intel.com
Link: https://lore.kernel.org/all/20190429185611.121751-1-helgaas@kernel.org/
Link: https://lore.kernel.org/linux-pci/20190501142942.26972-1-keith.busch@intel.com/
Link: https://lore.kernel.org/linux-pci/20200115221008.GA191037@google.com/


Suggested-by: Lukas Wunner <lukas@wunner.de> # Building bwctrl on top of bwnotif
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
[bhelgaas: squash fix to drop IRQF_ONESHOT and convert to hardirq handler:
https://lore.kernel.org/r/20241115165717.15233-1-ilpo.jarvinen@linux.intel.com

]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>