PCI/ASPM: Calculate LTR_L1.2_THRESHOLD from device characteristics

PCI/ASPM: Calculate LTR_L1.2_THRESHOLD from device characteristics

Per PCIe r3.1, sec 5.5.1, LTR_L1.2_THRESHOLD determines whether we enter
the L1.2 Link state: if L1.2 is enabled and downstream devices have
reported that they can tolerate latency of at least LTR_L1.2_THRESHOLD, we
must enter L1.2 when CLKREQ# is de-asserted.

The implication is that LTR_L1.2_THRESHOLD is the time required to
transition the Link from L0 to L1.2 and back to L0, and per sec 5.5.3.3.1,
Figures 5-16 and 5-17, it appears that the absolute minimum time for those
transitions would be T(POWER_OFF) + T(L1.2) + T(POWER_ON) + T(COMMONMODE).

Therefore, compute LTR_L1.2_THRESHOLD as:

    2us T(POWER_OFF)
  + 4us T(L1.2)
  + T(POWER_ON)
  + T(COMMONMODE)
  = LTR_L1.2_THRESHOLD

Previously we set LTR_L1.2_THRESHOLD to a fixed value of 163840ns
(163.84us):

  #define LTR_L1_2_THRESHOLD_BITS     ((1 << 21) | (1 << 23) | (1 << 30))
  ((1 << 21) | (1 << 23) | (1 << 30)) = 0x40a00000
  LTR_L1.2_THRESHOLD_Value = (0x40a00000 & 0x03ff0000) >> 16 = 0xa0 = 160
  LTR_L1.2_THRESHOLD_Scale = (0x40a00000 & 0xe0000000) >> 29 = 0x2 (* 1024ns)
  LTR_L1.2_THRESHOLD = 160 * 1024ns = 163840ns

Obviously this doesn't account for the circuit characteristics of different
implementations.

Note that while firmware may enable LTR, Linux itself currently does not
enable LTR.  When L1.2 is enabled but LTR is not, LTR_L1.2_THRESHOLD is
ignored and we always enter L1.2 when it is enabled and CLKREQ# is
de-asserted.  So this patch should not have any effect unless firmware
enables LTR.

Fixes: f1f0366dd6be ("PCI/ASPM: Calculate and save the L1.2 timing parameters")
Link: https://www.coreboot.org/pipermail/coreboot-gerrit/2015-March/021134.html
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Vidya Sagar <vidyas@nvidia.com>
Cc: Kenji Chen <kenji.chen@intel.com>
Cc: Patrick Georgi <pgeorgi@google.com>
Cc: Rajat Jain <rajatja@google.com>