ASPM is supported by Qcom host controllers/bridges on most of the recent
platforms and so the devices tested so far. But for enabling ASPM by
default (without using Kconfig, kernel command-line or sysfs), BIOS has
to enable ASPM on both host bridge and downstream devices during boot.

Unfortunately, none of the BIOS available on Qcom platforms enables
ASPM. Due to this, the platforms making use of Qcom SoCs draw high power
during runtime.

To fix this power draw issue, users have to enable ASPM using Kconfig,
kernel command-line, sysfs or the BIOS has to start enabling ASPM.

The latter may happen in the future, but that won't address the issue on
current platforms. Also, asking users to enable a feature to get the power
management right would provide an unpleasant out-of-the-box experience.

So the apt solution is to enable ASPM in the controller driver itself. And
this is being accomplished by calling pci_enable_link_state() in the newly
introduced host_post_init() callback for all the devices connected to the
bus. This function enables all supported link low power states for both
host bridge and the downstream devices.

Due to limited testing, ASPM is only enabled for platforms making use of
ops_1_9_0 callbacks.

[kwilczynski: commit log]
Link: https://lore.kernel.org/linux-pci/20231010155914.9516-3-manivannan.sadhasivam@linaro.orgSigned-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>