Re: [PATCH v3 2/4] net: stmmac: dwmac-qcom-ethqos: Mask PHY mode if configured with rgmii-id

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Tue, Jan 21, 2025 at 01:47:55PM +0100, Krzysztof Kozlowski wrote:
> On 21/01/2025 08:54, Yijie Yang wrote:
> > The Qualcomm board always chooses the MAC to provide the delay instead of
> > the PHY, which is completely opposite to the suggestion of the Linux
> > kernel.

You still need to explain why it's preferable to match this in the mainline
kernel. Does it not work when you use the phylib maintainers suggestion
(if that is so, you need to state as much.)

> How does the Linux kernel suggest it?

It's what phylib maintainers prefer, as documented in many emails from
Andrew Lunn and in Documentation/networking/phy.rst:

"Whenever possible, use the PHY side RGMII delay for these reasons:

* PHY devices may offer sub-nanosecond granularity in how they allow a
  receiver/transmitter side delay (e.g: 0.5, 1.0, 1.5ns) to be specified. Such
  precision may be required to account for differences in PCB trace lengths

* PHY devices are typically qualified for a large range of applications
  (industrial, medical, automotive...), and they provide a constant and
  reliable delay across temperature/pressure/voltage ranges

* PHY device drivers in PHYLIB being reusable by nature, being able to
  configure correctly a specified delay enables more designs with similar delay
  requirements to be operated correctly
"

> > The usage of phy-mode in legacy DTS was also incorrect. Change the
> > phy_mode passed from the DTS to the driver from PHY_INTERFACE_MODE_RGMII_ID
> > to PHY_INTERFACE_MODE_RGMII to ensure correct operation and adherence to
> > the definition.

If the delays dependent on the phy-mode are going to be implemented at
the MAC end, then changing the PHY mode indicated to phylink and phylib
to PHY_INTERFACE_MODE_RGMII is the right thing to be doing. However,
as mentioned in the documentation and by Andrew, this is discouraged.

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