Date: Mon, 04 Dec 2023 11:22:04 +0100
From: Heiko Stübner <heiko@...ech.de>
To: Florian Fainelli <f.fainelli@...il.com>, andrew@...n.ch,
 hkallweit1@...il.com, Quentin Schulz <quentin.schulz@...obroma-systems.com>
Cc: linux@...linux.org.uk, davem@...emloft.net, edumazet@...gle.com,
 kuba@...nel.org, pabeni@...hat.com, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, Heiko Stuebner <heiko.stuebner@...rry.de>
Subject:
 Re: [PATCH] net: mdio: enable optional clock when registering a phy from
 devicetree

Am Montag, 4. Dezember 2023, 11:14:12 CET schrieb Quentin Schulz:
> Hi Florian, Heiko,
> 
> On 12/1/23 23:41, Florian Fainelli wrote:
> > On 12/1/23 06:24, Heiko Stuebner wrote:
> >> From: Heiko Stuebner <heiko.stuebner@...rry.de>
> >>
> >> The ethernet-phy binding (now) specifys that phys can declare a clock
> >> supply. Phy driver itself will handle this when probing the phy-driver.
> >>
> >> But there is a gap when trying to detect phys, because the mdio-bus needs
> >> to talk to the phy to get its phy-id. Using actual phy-ids in the dt like
> >>         compatible = "ethernet-phy-id0022.1640",
> >>                      "ethernet-phy-ieee802.3-c22";
> >> of course circumvents this, but in turn hard-codes the phy.
> > 
> > But it is the established practice for situations like those where you 
> > need specific resources to be available in order to identify the device 
> > you are trying to probe/register.
> > 
> > You can get away here with the clock API because it can operate on 
> > device_node, and you might be able with a bunch of other "resources" 
> > subsystems, but for instance with regulators, that won't work, we need a 
> > "struct device" which won't be created because that is exactly what we 
> > are trying to do.
> > 
> > Also this only works for OF, not for ACPI or other yet to come firmware 
> > interface.
> > 
> > Sorry but NACK.
> > 
> > I am sympathetic to the idea that if you have multiple boards and you 
> > may have multiple PHY vendors this may not really scale, but in 2023 you 
> > have boot loaders aware of the Device Tree which can do all sorts of 
> > live DTB patching to provide the kernel with a "perfect" view of the world.
> 
> There's a strong push towards unifying the device tree across all pieces 
> of SW involved, sometimes going as far as only having one binary passed 
> between SW stages (e.g. U-Boot passes its own DT to TF-A, and then to 
> the Linux kernel without actually loading anything aside from the Linux 
> kernel Image binary) if I remember correctly (haven't really followed 
> tbh). So, this is kinda a step backward for this effort. I don't like 
> relying on bootloader to make the kernel work, this is usually not a 
> great thing. I understand the reasons but am still a bit sad to not see 
> this done in the kernel.
> 
> Heiko, I would personally put the ID of the PHY to be the most likely 
> encountered in the Linux kernel Device Tree so that if we somehow have a 
> broken bootloader, there's a chance some devices still work properly. HW 
> department said ksz9131 so we can go forward with this.

hmm, I was more of the mind of having either all or none work ;-) 
[i.e. keeping the c.22 compatible in the main dt and having firmware
 add the phy-id]

I.e. a bootloader doing the correct detection and fixup would insert the
matching phy-id and a broken bootloader would not do this.

Having some boards work that by chance have the right phy and others break
would possibly create a wild goose chase if the bootloader support for
phy-id-handling breaks somewhere down the line.


Heiko

> In U-Boot DT, we 
> would need a -u-boot.dtsi we change to the auto-detection compatible and 
> we do the magic the Linux kernel doesn't want to do and hope it's fine 
> for U-Boot maintainers. Once properly detected, we fixup the DT before 
> booting the kernel.

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