This patch adds workaround for TI J721E errata i2183
(https://www.ti.com/lit/er/sprz455a/sprz455a.pdf).
PCIe fails to link up if SERDES lanes not used by PCIe are assigned to
another protocol. For example, link training fails if lanes 2 and 3 are
assigned to another protocol while lanes 0 and 1 are used for PCIe to
form a two lane link. This failure is due to an incorrect tie-off on an
internal status signal indicating electrical idle.

Status signals going from SERDES to PCIe Controller are tied-off when a
lane is not assigned to PCIe. Signal indicating electrical idle is
incorrectly tied-off to a state that indicates non-idle. As a result,
PCIe sees unused lanes to be out of electrical idle and this causes
LTSSM to exit Detect.Quiet state without waiting for 12ms timeout to
occur. If a receiver is not detected on the first receiver detection
attempt in Detect.Active state, LTSSM goes back to Detect.Quiet and
again moves forward to Detect.Active state without waiting for 12ms as
required by PCIe base specification. Since wait time in Detect.Quiet is
skipped, multiple receiver detect operations are performed back-to-back
without allowing time for capacitance on the transmit lines to
discharge. This causes subsequent receiver detection to always fail even
if a receiver gets connected eventually.

The workaround only works for 1-lane PCIe configuration. This workaround
involves enabling receiver detect override by setting TX_RCVDET_OVRD_PREG_j
register of the lane running PCIe to 0x2. This causes SERDES to indicate
successful receiver detect when LTSSM is in Detect.Active state, whether a
receiver is actually present or not. If the receiver is present, LTSSM
proceeds to link up as expected. However if receiver is not present, LTSSM
will time out in Polling.Configuration substate since the expected training
sequence packets will not be received.
Signed-off-by: Swapnil Jakhade <sjakhade@cadence.com>
Link: https://lore.kernel.org/r/20220303055026.24899-1-sjakhade@cadence.comSigned-off-by: Vinod Koul <vkoul@kernel.org>