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Message-ID: <651e94ab.0c0a0220.2c147.1299@mx.google.com>
Date: Thu, 5 Oct 2023 12:49:13 +0200
From: Christian Marangi <ansuelsmth@...il.com>
To: Marek Behún <kabel@...nel.org>
Cc: "David S. Miller" <davem@...emloft.net>,
Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net v2 2/2] net: dsa: qca8k: fix potential MDIO bus
conflict when accessing internal PHYs via management frames
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@...nel.org>
Reviewed-by: Christian Marangi <ansuelsmth@...il.com>
--
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