[PATCH net-next 01/10] net: stmmac: rework mac_interface and phy_interface documentation

[Russell King (Oracle)]

Based on new research, it has come to light that the comment that I
added in a014c35556b9 ("net: stmmac: clarify difference between
"interface" and "phy_interface"") is not fully correct.

Update the comment to properly describe the difference between the two.

All of the DTS files in the kernel tree do not mention the "mac-mode"
property, which results in mac_interface and phy_interface being the
same. Also, none of the platform glue drivers set mac_interface to
anything but PHY_INTERFACE_MODE_NA. This means that for all the
platforms known to mainline, mac_interface is either the same as
phy_interface, or it is PHY_INTERFACE_MODE_NA.

Thus, updating the definition for mac_interface in stmmac.h has no
material effect on current uses known to mainline, but the change opens
the door to cleaning up all uses.

Signed-off-by: Russell King (Oracle) <rmk+kernel at armlinux.org.uk>
---
 include/linux/stmmac.h | 30 ++++++++++++++++++++++--------
 1 file changed, 22 insertions(+), 8 deletions(-)

diff --git a/include/linux/stmmac.h b/include/linux/stmmac.h
index e284f04964bf..f14f34ec6d5e 100644
--- a/include/linux/stmmac.h
+++ b/include/linux/stmmac.h
@@ -190,18 +190,32 @@ struct plat_stmmacenet_data {
 	int bus_id;
 	int phy_addr;
 	/* MAC ----- optional PCS ----- SerDes ----- optional PHY ----- Media
-	 *       ^                               ^
-	 * mac_interface                   phy_interface
+	 *                          ^            ^
+	 *                    mac_interface phy_interface
 	 *
-	 * mac_interface is the MAC-side interface, which may be the same
-	 * as phy_interface if there is no intervening PCS. If there is a
-	 * PCS, then mac_interface describes the interface mode between the
-	 * MAC and PCS, and phy_interface describes the interface mode
-	 * between the PCS and PHY.
+	 * The Synopsys dwmac core only covers the MAC and an optional
+	 * integrated PCS. Where the integrated PCS is used with a SerDes,
+	 * e.g. for 1000base-X or Cisco SGMII, the connection between the
+	 * PCS and SerDes will be TBI.
+	 *
+	 * Where the Synopsys dwmac core has been instantiated with multiple
+	 * interface modes, these are selected via core-external configuration
+	 * which is sampled when the dwmac core is reset. How this is done is
+	 * platform glue specific, but this defines the interface used from
+	 * the Synopsys dwmac core to the rest of the SoC.
+	 *
+	 * Where PCS other than the optional integrated Synopsys dwmac PCS
+	 * is used, this counts as "the rest of the SoC" in the above
+	 * paragraph.
+	 *
+	 * Thus, mac_interface is of little use inside the stmmac code;
+	 * please do not use unless there is a definite requirement, and
+	 * make sure to gain review feedback first.
 	 */
 	phy_interface_t mac_interface;
 	/* phy_interface is the PHY-side interface - the interface used by
-	 * an attached PHY.
+	 * an attached PHY or SFP etc. This is equivalent to the interface
+	 * that phylink uses.
 	 */
 	phy_interface_t phy_interface;
 	struct stmmac_mdio_bus_data *mdio_bus_data;
-- 
2.47.3