BugLink: https://bugs.launchpad.net/bugs/1818813

[ Upstream commit 84fb6c7f ]

It was noticed that unbinding and rebinding the KSZ8851 ethernet
resulted in the driver reporting "failed to read device ID" at probe.
Probing the reset line with a 'scope while repeatedly attempting to
bind the driver in a shell loop revealed that the KSZ8851 RSTN pin is
constantly held at zero, meaning the device is held in reset, and
does not respond on the SPI bus.

Experimentation with the startup delay on the regulator set to 50ms
shows that the reset is positively released after 20ms.

Schematics for this board are not available, and the traces are buried
in the inner layers of the board which makes tracing where the RSTN pin
extremely difficult.  We can only guess that the RSTN pin is wired to a
reset generator chip driven off the ethernet supply, which fits the
observed behaviour.

Include this delay in the regulator startup delay - effectively
treating the reset as a "supply stable" indicator.

This can not be modelled as a delay in the KSZ8851 driver since the
reset generation is board specific - if the RSTN pin had been wired to
a GPIO, reset could be released earlier via the already provided support
in the KSZ8851 driver.

This also got confirmed by Peter Ujfalusi <peter.ujfalusi@ti.com> based
on Blaze schematics that should be very close to SDP4430:

TPS22902YFPR is used as the regulator switch (gpio48 controlled):
Convert arm boot_lock to raw The VOUT is routed to TPS3808G01DBV.
(SCH Note: Threshold set at 90%. Vsense: 0.405V).

According to the TPS3808 data sheet the RESET delay time when Ct is
open (this is the case in the schema): MIN/TYP/MAX: 12/20/28 ms.
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
[tony@atomide.com: updated with notes from schematics from Peter]
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>