Re: [PATCH v2 03/11] dt-bindings: arm: mediatek: add 'mediatek,pn_swap' property

[Russell King (Oracle)]

On Fri, Feb 10, 2023 at 10:34:17AM +0000, Russell King (Oracle) wrote:
> On Thu, Feb 09, 2023 at 12:30:27PM +0100, Krzysztof Kozlowski wrote:
> > On 08/02/2023 23:30, Daniel Golle wrote:
> > > Hm, none of the current PCS (or PHY) drivers are represented by a
> > > syscon node... (and maybe that's the mistake in first place?)
> > 
> > Yes.
> 
> Nos, it isn't.

To expand on this - I have no idea why you consider it a mistake that
apparently all PCS aren't represented by a syscon node.

PCS is a sub-block in an ethernet system, just the same as a MAC is a
sub-block. PCS can appear in several locations of an ethernet system,
but are generally found either side of a serial ethernet link such
as 1000base-X, SGMII, USXGMII, 10Gbase-R etc.

So, one can find PCS within an ethernet PHY - and there may be one
facing the MAC connection, and there will be another facing the media.
We generally do not need to separate these PCS from the PHY itself
because we view the PHY as one whole device.

The optional PCS on the MAC side of the link is something that we
need to know about, because this has to be configured to talk to the
PHY, or to configure and obtain negotiation results from in the case of
fibre links.

PCS on the MAC side are not a system level device, they are very much a
specific piece of ethernet hardware in the same way that the MAC is,
and we don't represent the MAC as a syscon node. There is no reason
to do so with PCS.

These PCS on the MAC side tend to be accessed via direct MMIO accesses,
or over a MDIO bus.

There's other blocks in the IEEE 802.3 ethernet layering, such as the
PMA/PMD module (which for the MAC side we tend to model with the
drivers/phy layer) - but again, these also appear in ethernet PHYs
in order to produce the electrical signals for e.g. twisted pair
ethernet.

So, to effectively state that you consider that PCS should always be
represented as a syscon node is rather naieve, and really as a DT
reviewer you should not be making such decisions, but soliciting
opinions from those who know this subject area in detail _whether_
they are some kind of system controller before making such a
decision.

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