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Date: Fri, 28 Feb 2020 22:15:04 +0100 From: Antoine Tenart <antoine.tenart@...tlin.com> To: Andrew Lunn <andrew@...n.ch> Cc: Antoine Tenart <antoine.tenart@...tlin.com>, davem@...emloft.net, f.fainelli@...il.com, hkallweit1@...il.com, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, foss@...il.net Subject: Re: [PATCH net-next v2 3/3] net: phy: mscc: RGMII skew delay configuration On Fri, Feb 28, 2020 at 06:26:16PM +0100, Andrew Lunn wrote: > > > What is not clearly defined, is how you combine > > > PHY_INTERFACE_MODE_RGMII* with DT properties. I guess i would enforce > > > that phydev->interface is PHY_INTERFACE_MODE_RGMII and then the delays > > > in DT are absolute values. > > > > So, if there's a value in the device tree, and the mode corresponds > > (RXID for Rx skew), we do use the dt value. That should look like what's > > in the patch (except for the default value used when no configuration is > > provided in the dt). > > No. I would not do that. PHY_INTERFACE_MODE_RGMII_RXID means 2ns delay > for RX. So how do you then interpret the DT property. Is it 2ns + the > DT delay? That would then mean you need negative values in DT if you > want short delays than 2ns. > > Which is why i suggest PHY_INTERFACE_MODE_RGMII. It is then clear you > have a base delay of 0ns, and the DT property then has the absolute > value of the delay. OK I see, to avoid confusion we could either define RGMII_ID or RGMII and fixed delays in the dt if the 2ns one isn't what we need. We may need an RGMII dedicated documentation then, to avoid future confusion :) > > > There is also some discussion about what should go in DT. #defines > > > like you have, or time in pS, and the driver converts to register > > > values. I generally push towards pS. > > > > That would allow a more generic dt binding, and could be used by other > > PHY drivers. The difficulty would be to map the pS to allowed values in > > the h/w. Should we round them to the upper or lower bound? > > I would document the accepted values in DT, and return -EINVAL if DT > does not exactly match one of the listed values. Plus a phydev_err() > message to help debug. OK. > > I also saw the micrel-ksz90x1 dt documentation describes many skews, not > > only one for Rx and one for Tx. How would the generic dt binding would > > look like then? > > It is a balancing act. Do you actually need dt properties for your > hardware? Are the standard 2ns delays sufficient. For many designs it > is. Just because the hardware supports all sorts of configurations, > are they actually needed? It seems like adding delays are needed for a > few boards. But do all the properties exposed for the Micrel PHY every > get used, or is it a case of, the hardware has it, lets make it > available, even if nobody ever uses it? Right, this kind of settings shouldn't be needed for lots of boards, so we can add a per-PHY binding, only when it's needed. The board I'm currently working on do use smaller delays than 2ns and I was told to use even lower ones if the connexion wasn't stable. But then do we really need such a configuration upstream (apart from supporting the 2ns delays)? That's a good question :) Thanks! Antoine -- Antoine Ténart, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
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