netdev - RE: [PATCH] net: phy: Enhance fixed PHY to support 10G and 5G

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Message-ID:
 <BL3PR12MB64503DF203DC50B78494F985C327A@BL3PR12MB6450.namprd12.prod.outlook.com>
Date: Tue, 27 Jun 2023 09:30:18 +0000
From: Revanth Kumar Uppala <ruppala@...dia.com>
To: Russell King <linux@...linux.org.uk>, Andrew Lunn <andrew@...n.ch>,
	Narayan Reddy <narayanr@...dia.com>
CC: "hkallweit1@...il.com" <hkallweit1@...il.com>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "linux-tegra@...r.kernel.org"
	<linux-tegra@...r.kernel.org>
Subject: RE: [PATCH] net: phy: Enhance fixed PHY to support 10G and 5G



> -----Original Message-----
> From: Russell King <linux@...linux.org.uk>
> Sent: Friday, June 23, 2023 7:25 PM
> To: Andrew Lunn <andrew@...n.ch>; Revanth Kumar Uppala
> <ruppala@...dia.com>; Narayan Reddy <narayanr@...dia.com>
> Cc: hkallweit1@...il.com; netdev@...r.kernel.org; linux-
> tegra@...r.kernel.org
> Subject: Re: [PATCH] net: phy: Enhance fixed PHY to support 10G and 5G
> 
> External email: Use caution opening links or attachments
> 
> 
> On Fri, Jun 23, 2023 at 03:29:13PM +0200, Andrew Lunn wrote:
> > On Fri, Jun 23, 2023 at 12:28:49PM +0000, Revanth Kumar Uppala wrote:
> > >
> > >
> > > > -----Original Message-----
> > > > From: Andrew Lunn <andrew@...n.ch>
> > > > Sent: Wednesday, June 21, 2023 11:00 PM
> > > > To: Revanth Kumar Uppala <ruppala@...dia.com>
> > > > Cc: hkallweit1@...il.com; netdev@...r.kernel.org; linux-
> > > > tegra@...r.kernel.org; Narayan Reddy <narayanr@...dia.com>
> > > > Subject: Re: [PATCH] net: phy: Enhance fixed PHY to support 10G
> > > > and 5G
> > > >
> > > > External email: Use caution opening links or attachments
> > > >
> > > >
> > > > On Wed, Jun 21, 2023 at 10:28:53PM +0530, Revanth Kumar Uppala wrote:
> > > > > Add 10G and 5G speed entries for fixed PHY framework.These are
> > > > > needed for the platforms which doesn't have a PHY driver.
> > > > >
> > > > > Signed-off-by: Revanth Kumar Uppala <ruppala@...dia.com>
> > > > > Signed-off-by: Narayan Reddy <narayanr@...dia.com>
> > > >
> > > > This is the second time you have sent me this patch. You have
> > > > failed to answer the questions i asked you the last time.....
> > > Apologies for sending twice.
> > > C45 registers are not defined in the kernel as of now. But, we need to display
> the speed as 5G/10G when the same is configured as fixed link in DT node.
> > > It will be great if you can share any data for handling this.
> > > As of now, with this change we have taken care of providing proper speed
> log in kernel when 5G/10G is added as fixed links in DT node.
> >
> > This is architecturally wrong. As i said, swphy emulates a C22 PHY,
> > and a C22 PHY does not support speeds greater than 1G. To make swphy
> > really support 5G and 10G, you would need to add C45 support, and then
> > extend the default genphy driver to look at the C45 registers as well.
> >
> > However, that is all pointless. As i said, phylink fixed-link is not
> > limited to 1G speeds. Given what i see in Cc: i assume this is for a
> > tegre SoC? And that uses a Synopsys MAC? So you probably want to
> > modify the dwc driver to use phylink.
> 
> Absolutely correct. I seem to recall having had this come up before, and I think it
> was explained at the time, but I don't seem to find anything in my "sent"
> mailboxes for the start of 2022 to present.
> 
> (To nvidia)
> 
> The classical swphy/fixed-phy offers a software emulated clause 22 PHY so that
> phylib can be re-used to make a fixed link work without needing special code
> paths in phylib nor in MAC drivers.
> 
> However, clause 22 PHYs do not support speeds in excess of 1G, so this places a
> hard ceiling on the speed that can be supported with this method. PHYLIB's
> clause 45 support is specific to vendor PHYs, and the "generic" implementation
> only supports 10G speed. Emulating a specific vendor PHY to achieve this old
> way of supporting fixed links when we have a better way is really a waste of
> effort.
> 
> The "better way" is phylink, which makes fixed links work without needing to
> resort to PHY emulation, and thus it can support any speed that a MAC happens
> to support. This is the modern way.
We will look into what you have suggested.
Thanks for the feedback,
Revanth Uppala
> 
> We (the phylib and phylink maintainers) will not entertain extending the old now
> legacy method using swphy/fixed-phy for fixed links to include any faster
> speeds.
> 
> Thanks.
> 
> --
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