[RFC PATCH 1/5] net: dsa: mv88e6xxx: introduce support for two chips using direct smi addressing

[Rasmus Villemoes <rasmus.villemoes@prevas.dk>]

The 88e6250 (as well as 6220, 6071, 6070, 6020) do not support
multi-chip (indirect) addressing. However, one can still have two of
them on the same mdio bus, since the device only uses 16 of the 32
possible addresses, either addresses 0x00-0x0F or 0x10-0x1F depending
on the ADDR4 pin at reset [since ADDR4 is internally pulled high, the
latter is the default].

In order to prepare for supporting the 88e6250 and friends, introduce
mv88e6xxx_info::dual_chip to allow having a non-zero sw_addr while
still using direct addressing.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
---
 drivers/net/dsa/mv88e6xxx/chip.c | 10 +++++++---
 drivers/net/dsa/mv88e6xxx/chip.h |  5 +++++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/net/dsa/mv88e6xxx/chip.c b/drivers/net/dsa/mv88e6xxx/chip.c
index c078c791f481..f66daa77774b 100644
--- a/drivers/net/dsa/mv88e6xxx/chip.c
+++ b/drivers/net/dsa/mv88e6xxx/chip.c
@@ -62,6 +62,10 @@ static void assert_reg_lock(struct mv88e6xxx_chip *chip)
  * When ADDR is non-zero, the chip uses Multi-chip Addressing Mode, allowing
  * multiple devices to share the SMI interface. In this mode it responds to only
  * 2 registers, used to indirectly access the internal SMI devices.
+ *
+ * Some chips use a different scheme: Only the ADDR4 pin is used for
+ * configuration, and the device responds to 16 of the 32 SMI
+ * addresses, allowing two to coexist on the same SMI interface.
  */
 
 static int mv88e6xxx_smi_read(struct mv88e6xxx_chip *chip,
@@ -87,7 +91,7 @@ static int mv88e6xxx_smi_single_chip_read(struct mv88e6xxx_chip *chip,
 {
 	int ret;
 
-	ret = mdiobus_read_nested(chip->bus, addr, reg);
+	ret = mdiobus_read_nested(chip->bus, addr + chip->sw_addr, reg);
 	if (ret < 0)
 		return ret;
 
@@ -101,7 +105,7 @@ static int mv88e6xxx_smi_single_chip_write(struct mv88e6xxx_chip *chip,
 {
 	int ret;
 
-	ret = mdiobus_write_nested(chip->bus, addr, reg, val);
+	ret = mdiobus_write_nested(chip->bus, addr + chip->sw_addr, reg, val);
 	if (ret < 0)
 		return ret;
 
@@ -4548,7 +4552,7 @@ static struct mv88e6xxx_chip *mv88e6xxx_alloc_chip(struct device *dev)
 static int mv88e6xxx_smi_init(struct mv88e6xxx_chip *chip,
 			      struct mii_bus *bus, int sw_addr)
 {
-	if (sw_addr == 0)
+	if (sw_addr == 0 || chip->info->dual_chip)
 		chip->smi_ops = &mv88e6xxx_smi_single_chip_ops;
 	else if (chip->info->multi_chip)
 		chip->smi_ops = &mv88e6xxx_smi_multi_chip_ops;
diff --git a/drivers/net/dsa/mv88e6xxx/chip.h b/drivers/net/dsa/mv88e6xxx/chip.h
index 546651d8c3e1..5759fffbd39c 100644
--- a/drivers/net/dsa/mv88e6xxx/chip.h
+++ b/drivers/net/dsa/mv88e6xxx/chip.h
@@ -123,6 +123,11 @@ struct mv88e6xxx_info {
 	 * when it is non-zero, and use indirect access to internal registers.
 	 */
 	bool multi_chip;
+	/* Dual-chip Addressing Mode
+	 * Some chips respond to only half of the 32 SMI addresses,
+	 * allowing two to coexist on the same SMI interface.
+	 */
+	bool dual_chip;
 	enum dsa_tag_protocol tag_protocol;
 
 	/* Mask for FromPort and ToPort value of PortVec used in ATU Move
-- 
2.20.1