Re: [PATCH] net: phy: micrel: reconfigure the phy on resume

Hi, Rafael, Pavel,

I recently posted a patch for re-configuring an ethernet PHY on its
.resume() callback as this PHY is in a setup with SAMA7G5 SoC that supports
a power saving mode (called backup). In this power saving mode most of the
SoC devices' power is cut of (except the RAM + its controller since the RAM
is kept in self-refresh).

This mode is mapped on standard suspend-to-mem modes (standby or mem). When
one issues:
echo mem > /sys/power/state
echo standby > /sys/power/state
the suspend()/resume() functions of each driver are saving/restoring the
content of its registers and in the platform specific PM code
(arch/arm/mach-at91/{pm.c, pm_suspend.S} the RAM is switched to
self-refresh and the CPU executes its shutdown command [1].

The power to CPU (and to other devices on the board) is delivered by a
PMIC. The PMIC regulators have (among other states) a state corresponding
to the system running state and a state corresponding to the backup state.
The PMIC regulators for each state are configured in the system
initialization phase based on device tree bindings.

On suspend, on PMIC driver is configured the next running state but the
transition to this next running state is done when the CPU executes
shutdown command [1]. There is a physical connection b/w PMIC and SoC that
asserts when CPU executes its shutdown command.

+-----+  shdn  +-----+
|PMIC |<-------| SoC |
+-----+        +-----+

So, at the end of the suspend procedure for backup mode the PMIC will
switch to the state corresponding to backup mode where all its regulators
(except the one that feeds the RAM and RAM controllers) were configured to
be turn off. Since there is a limited amount of rails on the PMIC used in
the configuration with this SoC some of the closed rails may feed other
devices on the board, e.g. an ethernet PHY, some may be on the same rail
with CPU.


+-----+  shdn  +-----+
|PMIC |<-------| SoC |
+-----+        +-----+
   | 3v3         / \
   +-------+------+
           |
           |       +---------+
           +------>| eth PHY |
                   +---------+

Up to this moment we treat this backup mode as S2R mode since the memory
was kept in self-refresh mode. Each driver was saving/restoring in/from RAM
the content of associated registers in the suspend/resume phase.

Now, SAMA7G5 is used in setups with KSZ9131 PHY in RGMII mode. For RGMII
mode user may configure skew values or DLL settings on the PHY that is
imperative for 1Gbps speeds to be reached on Ethernet link in RGMII. These
skew settings may depend on the length of TX/RX lines b/w PHY and MAC
inside SoC. These settings are passed to the PHY driver via device tree
bindings.

For this PHY I prepared a patch that re-configures these settings on PHY
driver (see at the end of the tread) on resume() function of the driver.

Heiner (in To list) proposed me to involve you (since you are the experts
on PM) in the discussion.

The questions that arries this topic (Heiner, Russel, anyone involved in
the discussion, correct me if I wrongly understood):
1/ is it OK to still treat this backup mode as a S2R mode or as a hibernate
mode? Since hibernate would treat the devices (including Ethernet PHY in
this case) as they were just powered and restore the registers content but
taking into account that in backup mode we keep the RAM in self-refresh?
2/ is it OK to have these kind of reconfiguration of one device that end up
in suspend mode with no power (in this case the Ethernet PHY) due to a
system power cut off (in this case CPU + PMIC)?

Thank you,
Claudiu Beznea

[1]
https://elixir.bootlin.com/linux/latest/source/arch/arm/mach-at91/pm_suspend.S#L159


On 13.01.2021 23:34, Heiner Kallweit wrote:

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On 13.01.2021 13:36, Claudiu.Beznea@microchip.com wrote:

quoted

On 13.01.2021 13:09, Heiner Kallweit wrote:

quoted

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On 13.01.2021 10:29, Claudiu.Beznea@microchip.com wrote:

quoted

Hi Heiner,

On 08.01.2021 18:31, Heiner Kallweit wrote:

quoted

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On 08.01.2021 16:45, Claudiu Beznea wrote:

quoted

KSZ9131 is used in setups with SAMA7G5. SAMA7G5 supports a special
power saving mode (backup mode) that cuts the power for almost all
parts of the SoC. The rail powering the ethernet PHY is also cut off.
When resuming, in case the PHY has been configured on probe with
slew rate or DLL settings these needs to be restored thus call
driver's config_init() on resume.

When would the SoC enter this backup mode?

It could enter in this mode based on request for standby or suspend-to-mem:
echo mem > /sys/power/state
echo standby > /sys/power/state

What I didn't mentioned previously is that the RAM remains in self-refresh
while the rest of the SoC is powered down.

This leaves the question which driver sets backup mode in the SoC.

From Linux point of view the backup mode is a standard suspend-to-mem PM
mode. The only difference is in SoC specific PM code
(arch/arm/mach-at91/pm_suspend.S) where the SoC shutdown command is
executed at the end and the fact that we save the address in RAM of
cpu_resume() function in a powered memory. Then, the resume is done with
the help of bootloader (it configures necessary clocks) and jump the
execution to the previously saved address, resuming Linux.

quoted

Whatever/whoever wakes the SoC later would have to take care that basically
everything that was switched off is reconfigured (incl. calling phy_init_hw()).

For this the bootloader should know the PHY settings passed via DT (skew
settings or DLL settings). The bootloader runs from a little SRAM which, at
the moment doesn't know to parse DT bindings and the DT parsing lib might
be big enough that the final bootloader size will cross the SRAM size.

quoted

So it' more or less the same as waking up from hibernation. Therefore I think
the .restore of all subsystems would have to be executed, incl. .restore of
the MDIO bus.

I see your point. I think it has been implemented like a standard
suspend-to-mem PM mode because the RAM remains in self-refresh whereas in
hibernation it is shut of (as far as I know).

quoted

Having said that I don't think that change belongs into the
PHY driver.
Just imagine tomorrow another PHY type is used in a SAMA7G5 setup.
Then you would have to do same change in another PHY driver.

I understand this. At the moment the PM code for drivers in SAMA7G5 are
saving/restoring in/from RAM the registers content in suspend/resume()
functions of each drivers and I think it has been chosen like this as the
RAM remains in self-refresh. Mapping this mode to hibernation will involve
saving the content of RAM to a non-volatile support which is not wanted as
this increases the suspend/resume time and it wasn't intended.

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And would it suspend the
MDIO bus before cutting power to the PHY?

SAMA7G5 embeds Cadence macb driver which has a integrated MDIO bus. Inside
macb driver the bus is registered with of_mdiobus_register() or
mdiobus_register() based on the PHY devices present in DT or not. On macb
suspend()/resume() functions there are calls to
phylink_stop()/phylink_start() before cutting/after enabling the power to
the PHY.

quoted

I'm asking because in mdio_bus_phy_restore() we call phy_init_hw()
already (that calls the driver's config_init).

As far as I can see from documentation the .restore API of dev_pm_ops is
hibernation specific (please correct me if I'm wrong). On transitions to
backup mode the suspend()/resume() PM APIs are called on the drivers.

I'm not a Linux PM expert, to me it seems your use case is somewhere in the
middle between s2r and hibernation. I *think* the assumption with s2r is
that one component shouldn't simply cut the power to another component,
and the kernel has no idea about it.

My personal point of view:
If a driver cuts power to another component in s2r, it should take care that
this component is properly re-initialized once power is back.
Otherwise I would miss to see why we need different callbacks resume and restore.

It may be worth to involve the following people/list:

HIBERNATION (aka Software Suspend, aka swsusp)
M:      "Rafael J. Wysocki" [off-list ref]
M:      Pavel Machek [off-list ref]
L:      linux-pm@vger.kernel.org

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Thank you,
Claudiu Beznea

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Signed-off-by: Claudiu Beznea <redacted>
---
 drivers/net/phy/micrel.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/phy/micrel.c b/drivers/net/phy/micrel.c
index 3fe552675dd2..52d3a0480158 100644
--- a/drivers/net/phy/micrel.c
+++ b/drivers/net/phy/micrel.c

@@ -1077,7 +1077,7 @@ static int kszphy_resume(struct phy_device *phydev)
       */
      usleep_range(1000, 2000);

-     ret = kszphy_config_reset(phydev);
+     ret = phydev->drv->config_init(phydev);
      if (ret)
              return ret;