linux-kernel - Re: [PATCH v2 2/3] net: phy: Add helper for fixing RGMII PHY mode based on internal mac delay

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Message-ID: <8da7450f-ef1a-4d8f-9081-a31585e2da19@lunn.ch>
Date: Mon, 20 Oct 2025 20:51:44 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Inochi Amaoto <inochiama@...il.com>
Cc: Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Chen Wang <unicorn_wang@...look.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Heiner Kallweit <hkallweit1@...il.com>,
	Russell King <linux@...linux.org.uk>,
	"Russell King (Oracle)" <rmk+kernel@...linux.org.uk>,
	Han Gao <rabenda.cn@...il.com>, Icenowy Zheng <uwu@...nowy.me>,
	Vivian Wang <wangruikang@...as.ac.cn>, Yao Zi <ziyao@...root.org>,
	netdev@...r.kernel.org, devicetree@...r.kernel.org,
	sophgo@...ts.linux.dev, linux-kernel@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org, Yixun Lan <dlan@...too.org>,
	Longbin Li <looong.bin@...il.com>
Subject: Re: [PATCH v2 2/3] net: phy: Add helper for fixing RGMII PHY mode
 based on internal mac delay

On Mon, Oct 20, 2025 at 05:54:58PM +0800, Inochi Amaoto wrote:
> The "phy-mode" property of devicetree indicates whether the PCB has
> delay now, which means the mac needs to modify the PHY mode based
> on whether there is an internal delay in the mac.
> 
> This modification is similar for many ethernet drivers. To simplify
> code, define the helper phy_fix_phy_mode_for_mac_delays(speed, mac_txid,
> mac_rxid) to fix PHY mode based on whether mac adds internal delay.
> 
> Signed-off-by: Inochi Amaoto <inochiama@...il.com>
> ---
>  drivers/net/phy/phy-core.c | 43 ++++++++++++++++++++++++++++++++++++++
>  include/linux/phy.h        |  3 +++
>  2 files changed, 46 insertions(+)
> 
> diff --git a/drivers/net/phy/phy-core.c b/drivers/net/phy/phy-core.c
> index 605ca20ae192..4f258fb409da 100644
> --- a/drivers/net/phy/phy-core.c
> +++ b/drivers/net/phy/phy-core.c
> @@ -101,6 +101,49 @@ const char *phy_rate_matching_to_str(int rate_matching)
>  }
>  EXPORT_SYMBOL_GPL(phy_rate_matching_to_str);
> 
> +/**
> + * phy_fix_phy_mode_for_mac_delays - Convenience function for fixing PHY
> + * mode based on whether mac adds internal delay
> + *
> + * @interface: The current interface mode of the port
> + * @mac_txid: True if the mac adds internal tx delay
> + * @mac_rxid: True if the mac adds internal rx delay
> + *
> + * Return fixed PHY mode, or PHY_INTERFACE_MODE_NA if the interface can
> + * not apply the internal delay
> + */

I think a helper like this is a good idea. But there are a couple of
things i don't like about this implementation.

I don't like returning PHY_INTERFACE_MODE_NA on error. I would prefer
-EINVAL.  of_get_phy_mode() passed a phy_interface_t *, and returns an
errno. The same would be good here.

I find:

phy_fix_phy_mode_for_mac_delays(interface, true, false)

hard to read. You cannot see what these true/false mean. Which is Rx
and which is Tx?

Rather than true false, how about passing an
PHY_INTERFACE_MODE_RGMII_* values?

PHY_INTERFACE_MODE_RGMII_ID would indicate the MAC is doing both
delays.  PHY_INTERFACE_MODE_RGMII_RXID the MAC is implementing the RX
delay? I'm open to other ideas here.

	Andrew