[PATCH net-next v3 2/8] net: mdio: realtek-rtl9300: Add polling documentation

[Markus Stockhausen <markus.stockhausen@gmx.de>]

Add a detailed explanation how the hardware polling unit in the
Realtek Otto switches works. This simplifies developing future
patches and reviewing them.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
---
 drivers/net/mdio/mdio-realtek-rtl9300.c | 68 +++++++++++++++++++++++++
 1 file changed, 68 insertions(+)

diff --git a/drivers/net/mdio/mdio-realtek-rtl9300.c b/drivers/net/mdio/mdio-realtek-rtl9300.c
index 892ed3780a65..562f9c7f2895 100644
--- a/drivers/net/mdio/mdio-realtek-rtl9300.c
+++ b/drivers/net/mdio/mdio-realtek-rtl9300.c
@@ -35,6 +35,74 @@
  *
  * The driver works out the mapping based on the MDIO bus described in device tree and phandles on
  * the ethernet-ports property.
+ *
+ * The devices have a hardware polling unit that runs in the background without any CPU load. It
+ * constantly scans the MDIO bus and the attached PHYs and updates the MAC status registers.
+ *
+ * How does the polling work?
+ *
+ * Each device has a SMI_POLL_CTRL register. A per-port bitmask decides if the hardware polling of
+ * the associated bus/address is active or not. The hardware runs a tight loop over this and for
+ * each set polling bit it issues a status check for the PHY. Attaching a logic analyzer to the
+ * MDIO bus of an RTL8380 and RTL8393 gives the following commands (in kernel notation):
+ *
+ *	RTL8380				RTL8393
+ *	---------------------------	---------------------------
+ *	phy_write(phy, 31, 0x0);	phy_read(phy, 0);
+ *	phy_write(phy, 13, 0x7);	phy_read(phy, 1);
+ *	phy_write(phy, 14, 0x3c);	phy_read(phy, 4);
+ *	phy_write(phy, 13, 0x8007);	phy_read(phy, 5);
+ *	phy_read(phy, 14);		phy_read(phy, 6);
+ *	phy_write(phy, 13, 0x7);	phy_read(phy, 9);
+ *	phy_write(phy, 14, 0x3d);	phy_read(phy, 10);
+ *	phy_write(phy, 13, 0x8007);	phy_read(phy, 15);
+ *	phy_read(phy, 14);		phy_write(phy, 13, 0x7);
+ *	phy_read(phy, 9);		phy_write(phy, 14, 0x3c);
+ *	phy_read(phy, 10);		phy_write(phy, 13, 0x4007);
+ *	phy_read(phy, 15);		phy_read(phy, 14);
+ *	phy_read(phy, 0);		phy_write(phy, 13, 0x7);
+ *	phy_read(phy, 1);		phy_write(phy, 14, 0x3d);
+ *	phy_read(phy, 4);		phy_write(phy, 13, 0x4007);
+ *	phy_read(phy, 5);		phy_read(phy, 14);
+ *	phy_read(phy, 6);
+ *
+ * From the above snippets it polls MDIO_AN_EEE_ADV and MDIO_AN_EEE_LPABLE via C45 over C22. As
+ * of now it is unclear how the hardware decides if a PHY supports these registers. After one PHY
+ * status is read, the polling engine goes over to the next PHY. Basically the bus is always busy
+ * and the MAC status is updated in real-time.
+ *
+ * How does MDIO access from kernel work?
+ *
+ * When issuing MDIO accesses via an MMIO based interface the final write to the command register
+ * sets a "run command now" bit. Between two polling sequences for different PHYs the hardware
+ * checks if a user command needs to run and sends it onto the bus. Afterwards it simply continues
+ * its polling work. Inspecting the command sequence for a paged read on the logic analyzer gives:
+ *
+ *	RTL8380				RTL8393
+ *	---------------------------	---------------------------
+ *	phy_write(phy, 31, page);	phy_write(phy, 31, page);
+ *	phy_write(phy, reg, value);	phy_write(phy, reg, value);
+ *					phy_write(phy, 31, 0);
+ *
+ * What does this mean?
+ *
+ * There are slight differences in polling and PHY access between the models but the challenge
+ * stays the same. On the one hand that greatly simplifies the MAC layer, on the other hand it
+ * has some implications for the kernel PHY subsystem.
+ *
+ * - Without the polling and a proper MAC status, some of the link handling features do not work.
+ *   Especially an unpopulated MAC_LINK_STS register cancels operations to other MAC registers.
+ * - The Realtek page register 31 is magically modified in the background so that polling will
+ *   read the right data. On the RTL838x polling simply resets it to zero. Other devices seem
+ *   to track the page access "magically" in the background.
+ * - A C45 over C22 kernel access sequence is most likely to fail because chances are high that
+ *   the polling engine overwrites registers 13/14 in between.
+ * - PHY firmware loading can have issues. Especially if a PHY is designed to expect a clean
+ *   sequence of registers and values without deviation.
+ * - An access to one PHY will need to wait for the next free slot of the polling engine.
+ *
+ * Conclusion: The Realtek MDIO bus driver PHY access must know and handle any interference that
+ * arises from the above described hardware polling.
  */
 
 #include <linux/bitfield.h>
-- 
2.54.0