PCI/bwctrl: Add pcie_set_target_speed() to set PCIe Link Speed

Currently, PCIe Link Speeds are adjusted by custom code rather than in a
common function provided in PCI core. The PCIe bandwidth controller
(bwctrl) introduces an in-kernel API, pcie_set_target_speed(), to set PCIe
Link Speed.

Convert Target Speed quirk to use the new API. The Target Speed quirk runs
very early when bwctrl is not yet probed for a Port and can also run later
when bwctrl is already setup for the Port, which requires the per port
mutex (set_speed_mutex) to be only taken if the bwctrl setup is already
complete.

The new API is also intended to be used in an upcoming commit that adds a
thermal cooling device to throttle PCIe bandwidth when thermal thresholds
are reached.

The PCIe bandwidth control procedure is as follows. The highest speed
supported by the Port and the PCIe device which is not higher than the
requested speed is selected and written into the Target Link Speed in the
Link Control 2 Register. Then bandwidth controller retrains the PCIe Link.

Bandwidth Notifications enable the cur_bus_speed in the struct pci_bus to
keep track PCIe Link Speed changes. While Bandwidth Notifications should
also be generated when bandwidth controller alters the PCIe Link Speed, a
few platforms do not deliver LMBS interrupt after Link Training as
expected. Thus, after changing the Link Speed, bandwidth controller makes
additional read for the Link Status Register to ensure cur_bus_speed is
consistent with the new PCIe Link Speed.

Link: https://lore.kernel.org/r/20241018144755.7875-8-ilpo.jarvinen@linux.intel.com
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
[bhelgaas: squash devm_mutex_init() error checking from
https://lore.kernel.org/r/20241030163139.2111689-1-andriy.shevchenko@linux.intel.com,
drop export of pcie_set_target_speed()]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

0 files changed, 0 insertions, 0 deletions