PCI: Indicate context lost if L1SS exit is broken during resume from system suspend

PCI: Indicate context lost if L1SS exit is broken during resume from system suspend

Per PCIe v7.0, sec 5.5.3.3.1, when exiting L1.2 due to an endpoint
asserting CLKREQ# signal, the REFCLK must be turned on within the latency
advertised in the LTR message.  This requirement applies to L1.1 as well.

On some platforms like Qcom, these requirements are satisfied during OS
runtime, but not while resuming from the system suspend. This happens
because the PCIe RC driver may remove all resource votes and turn off the
PHY analog circuitry during suspend to maximize power savings while keeping
the link in L1SS.

Consequently, when the endpoint asserts CLKREQ# to wake up, the RC driver
must restore the PHY and enable the REFCLK.  When this recovery process
exceeds the L1SS exit latency time (roughly L10_REFCLK_ON + T_COMMONMODE),
the endpoint may treat it as a fatal condition and trigger Link Down (LDn).
This results in a reset that destroys the internal device state.

So to indicate this platform limitation to the client drivers, introduce a
new flag 'pci_host_bridge::broken_l1ss_resume' and check it in
pci_suspend_retains_context(). If the flag is set by the RC driver, the API
will return 'false' indicating the client drivers that the device context
may not be retained and the drivers must be prepared for context loss.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260519-l1ss-fix-v2-2-b2c3a4bdeb15@oss.qualcomm.com