Re: [PATCH net v2 2/2] net: dsa: qca8k: fix potential MDIO bus conflict when accessing internal PHYs via management frames

[Christian Marangi <ansuelsmth@gmail.com>]

On Wed, Oct 04, 2023 at 11:19:04AM +0200, Marek Behún wrote:
> Besides the QCA8337 switch the Turris 1.x device has on it's MDIO bus
> also Micron ethernet PHY (dedicated to the WAN port).
> 
> We've been experiencing a strange behavior of the WAN ethernet
> interface, wherein the WAN PHY started timing out the MDIO accesses, for
> example when the interface was brought down and then back up.
> 
> Bisecting led to commit 2cd548566384 ("net: dsa: qca8k: add support for
> phy read/write with mgmt Ethernet"), which added support to access the
> QCA8337 switch's internal PHYs via management ethernet frames.
> 
> Connecting the MDIO bus pins onto an oscilloscope, I was able to see
> that the MDIO bus was active whenever a request to read/write an
> internal PHY register was done via an management ethernet frame.
> 
> My theory is that when the switch core always communicates with the
> internal PHYs via the MDIO bus, even when externally we request the
> access via ethernet. This MDIO bus is the same one via which the switch
> and internal PHYs are accessible to the board, and the board may have
> other devices connected on this bus. An ASCII illustration may give more
> insight:
> 
>            +---------+
>       +----|         |
>       |    | WAN PHY |
>       | +--|         |
>       | |  +---------+
>       | |
>       | |  +----------------------------------+
>       | |  | QCA8337                          |
> MDC   | |  |                        +-------+ |
> ------o-+--|--------o------------o--|       | |
> MDIO    |  |        |            |  | PHY 1 |-|--to RJ45
> --------o--|---o----+---------o--+--|       | |
>            |   |    |         |  |  +-------+ |
> 	   | +-------------+  |  o--|       | |
> 	   | | MDIO MDC    |  |  |  | PHY 2 |-|--to RJ45
> eth1	   | |             |  o--+--|       | |
> -----------|-|port0        |  |  |  +-------+ |
>            | |             |  |  o--|       | |
> 	   | | switch core |  |  |  | PHY 3 |-|--to RJ45
>            | +-------------+  o--+--|       | |
> 	   |                  |  |  +-------+ |
> 	   |                  |  o--|  ...  | |
> 	   +----------------------------------+
> 
> When we send a request to read an internal PHY register via an ethernet
> management frame via eth1, the switch core receives the ethernet frame
> on port 0 and then communicates with the internal PHY via MDIO. At this
> time, other potential devices, such as the WAN PHY on Turris 1.x, cannot
> use the MDIO bus, since it may cause a bus conflict.
> 
> Fix this issue by locking the MDIO bus even when we are accessing the
> PHY registers via ethernet management frames.
> 
> Fixes: 2cd548566384 ("net: dsa: qca8k: add support for phy read/write with mgmt Ethernet")
> Signed-off-by: Marek Behún <kabel@kernel.org>

Reviewed-by: Christian Marangi <ansuelsmth@gmail.com>

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