netdev - RGMII timing calibration (on 12nm Amlogic SoCs)

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Date:   Fri, 25 Sep 2020 23:47:18 +0200
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     netdev@...r.kernel.org, linux-amlogic@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux@...linux.org.uk, peppe.cavallaro@...com,
        alexandre.torgue@...com, joabreu@...opsys.com, davem@...emloft.net,
        kuba@...nel.org
Subject: RGMII timing calibration (on 12nm Amlogic SoCs) - integration into dwmac-meson8b

Hello,

Amlogic's 12nm SoC generation requires some RGMII timing calibration
within the Ethernet controller glue registers.
This calibration is only needed for the RGMII modes, not for the
(internal) RMII PHY.
With "incorrect" calibration settings Ethernet speeds up to 100Mbit/s
will still work fine, but no data is flowing on 1Gbit/s connections
(similar to when RX or TX delay settings are incorrect).

A high-level description of this calibration (the full code can be
seen in [0] and [1]):
- there are sixteen possible calibration values: [0..15]
- switch the Ethernet PHY to loopback mode
- for each of the sixteen possible calibration values repeat the
following steps five times:
-- write the value to the calibration register
-- construct an Ethernet loopback test frame with protocol 0x0808
("Frame Relay ARP")
-- add 256 bytes of arbitrary data
-- use the MAC address of the controller as source and destination
-- send out this data packet
-- receive this data packet
-- compare the contents and remember if the data is valid or corrupted
- disable loopback mode on the Ethernet PHY
- find the best calibration value by getting the center point of the
"longest streak"
- write this value to the calibration register

My question is: how do I integrate this into the dwmac-meson8b (stmmac
based) driver?
I already found some interesting and relevant bits:
- stmmac_selftests.c uses phy_loopback() and also constructs data
which is sent-out in loopback mode
- there's a serdes_powerup callback in struct plat_stmmacenet_data
which is called after register_netdev()
- I'm not sure if there's any other Ethernet driver doing some similar
calibration (and therefore a way to avoid some code-duplication)


Any recommendations/suggestions/ideas/hints are welcome!
Thank you and best regards,
Martin


[0] https://github.com/khadas/u-boot/blob/4752efbb90b7d048a81760c67f8c826f14baf41c/drivers/net/designware.c#L707
[1] https://github.com/khadas/linux/blob/khadas-vims-4.9.y/drivers/net/ethernet/stmicro/stmmac/dwmac-meson.c#L466