Re: [net-next PATCH v4 04/13] drivers: net: dsa: qca8k: add support for cpu port 6

[Vladimir Oltean <olteanv@gmail.com>]

On Sun, Oct 10, 2021 at 01:15:47PM +0200, Ansuel Smith wrote:
> Currently CPU port is always hardcoded to port 0. This switch have 2 CPU
> port. The original intention of this driver seems to be use the
> mac06_exchange bit to swap MAC0 with MAC6 in the strange configuration
> where device have connected only the CPU port 6. To skip the
> introduction of a new binding, rework the driver to address the
> secondary CPU port as primary and drop any reference of hardcoded port.
> With configuration of mac06 exchange, just skip the definition of port0
> and define the CPU port as a secondary. The driver will autoconfigure
> the switch to use that as the primary CPU port.
> 
> Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
> ---

Does this really work? What about GLOBAL_FW_CTRL0 bit 10 (CPU_PORT_EN),
which this patch leaves alone, and whose description is:
0 = No CPU connect to switch
1 = CPU is connected to port0
If this bit is set to 1, HEAD_EN of MAC0 is set to 1

I see all of PORT0_HEADER_CTRL - PORT6_HEADER_CTRL have the option of
setting RX_HEADER_MODE and TX_HEADER_MODE.
I just have this doubt: what is port 0 supposed to do when the CPU port
is port 6? Can it be used as a regular user port, attached to an RGMII PHY?

Isn't that use case broken anyway, due to qca8k.c's broken
interpretation of RGMII delay device tree bindings (it always applies
RGMII delays on "rgmii-id", and the PHY will apply them too)?

If I were to trust the documentation, that DSA headers are enabled on
port 0 when the driver does this:

	/* Enable CPU Port */
	ret = qca8k_reg_set(priv, QCA8K_REG_GLOBAL_FW_CTRL0,
			    QCA8K_GLOBAL_FW_CTRL0_CPU_PORT_EN);

doesn't that mean that using port 0 as a user port is double-broken,
since this would implicitly enable DSA headers on it?

Or is the idea of using port 6 as the CPU port to be able to use SGMII,
which is not available on port 0? Jonathan McDowell did some SGMII
configuration for the CPU port in commit f6dadd559886 ("net: dsa: qca8k:
Improve SGMII interface handling"). If the driver supports only port 0
as CPU port, and SGMII is only available on port 6, how did he do it?

On the boards that you have with port 6 as CPU port, what is port 0 used
for? Can port 0 and 6 be CPU ports simultaneously?

I also want to understand what's the use case of this port swap. In the
QCA8337 block diagram I see a dotted-line connection between Port 0 and
the SerDes, presumably this is due to the port swap. But I don't see any
dotted line between the Port 6 and the first GMII/RGMII/MII/RMII MAC.
So what will happen to what software believes is port 6, with the swap
in place?