net: dsa: microchip: fix register write order in ksz8_ind_write8()

[ Upstream commit b7fb772 ]

This bug was noticed while re-implementing parts of the kernel
driver in userspace using spidev. The goal was to enable some
of the errata workarounds that Microchip describes in their
errata sheet [1].

Both the errata sheet and the regular datasheet of e.g. the KSZ8795
imply that you need to do this for indirect register accesses:
- write a 16-bit value to a control register pair (this value
  consists of the indirect register table, and the offset inside
  the table)
- either read or write an 8-bit value from the data storage
  register (indicated by REG_IND_BYTE in the kernel)

The current implementation has the order swapped. It can be
proven, by reading back some indirect register with known content
(the EEE register modified in ksz8_handle_global_errata() is one of
these), that this implementation does not work.

Private discussion with Oleksij Rempel of Pengutronix has revealed
that the workaround was apparantly never tested on actual hardware.

[1] https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ProductDocuments/Errata/KSZ87xx-Errata-DS80000687C.pdf

Signed-off-by: Tobias Jakobi (Compleo) <tobias.jakobi.compleo@gmail.com>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Fixes: 7b6e623 ("net: dsa: microchip: ksz8795: handle eee specif erratum")
Link: https://lore.kernel.org/r/20240304154135.161332-1-tobias.jakobi.compleo@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/net/dsa/microchip/ksz8795.c

@@ -49,9 +49,9 @@ static int ksz8_ind_write8(struct ksz_device *dev, u8 table, u16 addr, u8 data)
 	mutex_lock(&dev->alu_mutex);
 
 	ctrl_addr = IND_ACC_TABLE(table) | addr;
-	ret = ksz_write8(dev, regs[REG_IND_BYTE], data);
+	ret = ksz_write16(dev, regs[REG_IND_CTRL_0], ctrl_addr);
 	if (!ret)
-		ret = ksz_write16(dev, regs[REG_IND_CTRL_0], ctrl_addr);
+		ret = ksz_write8(dev, regs[REG_IND_BYTE], data);
 
 	mutex_unlock(&dev->alu_mutex);