linux-kernel - Re: [PATCH RESEND net-next v2] net: stmmac: dwmac: Add a fixup for the Micrel KSZ9131 PHY

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Message-ID: <aWS9fbfvuayJpo3a@eichest-laptop>
Date: Mon, 12 Jan 2026 10:23:09 +0100
From: Stefan Eichenberger <eichest@...il.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Maxime Chevallier <maxime.chevallier@...tlin.com>,
	Andrew Lunn <andrew@...n.ch>, andrew+netdev@...n.ch,
	davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, shawnguo@...nel.org, s.hauer@...gutronix.de,
	kernel@...gutronix.de, festevam@...il.com,
	mcoquelin.stm32@...il.com, alexandre.torgue@...s.st.com,
	linux-stm32@...md-mailman.stormreply.com, netdev@...r.kernel.org,
	imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, francesco.dolcini@...adex.com,
	robh@...nel.org,
	Stefan Eichenberger <stefan.eichenberger@...adex.com>
Subject: Re: [PATCH RESEND net-next v2] net: stmmac: dwmac: Add a fixup for
 the Micrel KSZ9131 PHY

On Sat, Jan 10, 2026 at 03:47:21PM +0000, Russell King (Oracle) wrote:
> On Sat, Jan 10, 2026 at 02:42:13PM +0100, Stefan Eichenberger wrote:
> > Hi Maxime,
> > 
> > Not problem, thanks a lot for the feedback and the discussion. I will
> > then proceed with the current approach and send a new version with an
> > updated commit message.
> 
> We could add a flag to:
> 
> /* Generic phy_device::dev_flags */
> #define PHY_F_NO_IRQ            0x80000000
> #define PHY_F_RXC_ALWAYS_ON     0x40000000
> 
> indicating that the MAC requires the full preamble, which the PHY can
> then test for and configure appropiately.
> 
> The question is, whether the requirement for the full preamble applies
> to many MACs, and whether there are PHYs that default to producing
> short preambles.
> 
> Looking at Marvell 88e151x, the only control it has is to pad odd
> nibbles of preambles on copper (page 2, register 16, bit 6.)
> 
> AR8035 seems to make no mention of preamble for the MII interfaces, so
> I guess it has no control over it.
> 
> I've not looked further than that.

>From what I have seen only the S32 and i.MX8MP MAC require the full
preamble because of the errata. I also checked the i.MX93 and i.MX8DX,
they don't mention the errata, so I assume they are not affected.

Not sure if adding the flag would be a bit overkill. However, assuming
we would do it that way. Would ndo_open be the right place to set the
flag in the mac so that the phy knows about it?

I would think about something like:
- Add a flag STMMAC_FLAG_KEEP_PREAMBLE_BEFORE_SFD to stmmac.h
- Add a flag PHY_F_KEEP_PREAMBLE_BEFORE_SFD to phy.h
- Add STMMAC_FLAG_KEEP_PREAMBLE_BEFORE_SFD to priv->plat->flags in the
  mac driver platform probe (e.g. dwmac-imx.c).
- Set PHY_F_KEEP_PREAMBLE_BEFORE_SFD in stmmac_init_phy (during
  ndo_open) if STMMAC_FLAG_KEEP_PREAMBLE_BEFORE_SFD is set under
  priv->plat->flags.
- If PHY_F_KEEP_PREAMBLE_BEFORE_SFD is set in the phy driver keep the
  full preamble if the phy supports it during config_init.

I could send the next version doing it that way, to see if that's the
better approach.

Regards,
Stefan