netdev - Re: BeagleBone Black Ethernet PHY issues

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Message-ID: <CAMuHMdVN_xNLTvy9u6FvK=agSAUzSHxEuyBS37sOA7jpGLwddw@mail.gmail.com>
Date: Wed, 30 Oct 2024 16:08:37 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Roger Quadros <rogerq@...nel.org>
Cc: ext Tony Lindgren <tony@...mide.com>, Siddharth Vadapalli <s-vadapalli@...com>, 
	"open list:TI ETHERNET SWITCH DRIVER (CPSW)" <linux-omap@...r.kernel.org>, netdev <netdev@...r.kernel.org>, 
	Matti Vaittinen <mazziesaccount@...il.com>, Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: BeagleBone Black Ethernet PHY issues

Hi Roger,

On Wed, Oct 30, 2024 at 1:58 PM Roger Quadros <rogerq@...nel.org> wrote:
> On 29/10/2024 19:18, Geert Uytterhoeven wrote:
> > During the last few months, booting kernels on BeagleBone Black
> > sometimes fails with:
> >
> >     +SMSC LAN8710/LAN8720 4a101000.mdio:00: probe with driver SMSC
> > LAN8710/LAN8720 failed with error -5

[...]

> Just wondering if the Reset is happening correctly and it has settled
> before PHY access.
>
> From arch/arm/boot/dts/ti/omap/am335x-bone-common.dtsi
>
> &davinci_mdio_sw {
>         pinctrl-names = "default", "sleep";
>         pinctrl-0 = <&davinci_mdio_default>;
>         pinctrl-1 = <&davinci_mdio_sleep>;
>
>         ethphy0: ethernet-phy@0 {
>                 reg = <0>;
>                 /* Support GPIO reset on revision C3 boards */
>                 reset-gpios = <&gpio1 8 GPIO_ACTIVE_LOW>;
>                 reset-assert-us = <300>;
>                 reset-deassert-us = <13000>;
>         };
> };
>
> Do we need to increase reset-deassert-us for some reason?

Thanks for the hint!

This is indeed on Rev. C3 (my other boards are Rev. A5C or C, but
I don't test boot recent kernels on them, as they are in active use).

Multiplying reset-deassert-us by 10 gives me a booting board.
More experiments reveal that I need a delay of 14 ms to boot
successfully, and 15 ms to avoid the early __mdiobus_read()
failure, too.

> I couldn't find MII ready time after reset de-assert information form the
> PHY datasheet. except the following line [1].
> "For the first 16us after coming out of reset, the MII/RMII interface will run at 2.5 MHz. After this time, it will
> switch to 25 MHz if auto-negotiation is enabled"
>
> [1] 3.8.5 RESETS
> https://ww1.microchip.com/downloads/aemDocuments/documents/UNG/ProductDocuments/DataSheets/LAN8710A-LAN8710Ai-Data-Sheet-DS00002164.pdf

3.8.5.1 Hardware Reset
"A Hardware reset is asserted by driving the nRST input pin low. When
 driven, nRST should be held low for the minimum time detailed in
 Section 5.6.3, "Power-On nRST & Configuration Strap Timing," on page
 60 to ensure a proper transceiver reset."

5.6.3 POWER-ON NRST & CONFIGURATION STRAP TIMING
"For proper operation, nRST must be asserted for no less than trstia."

TABLE 5-8: POWER-ON NRST & CONFIGURATION STRAP TIMING VALUES
"trstia nRST input assertion time min. 100 µS"

On Rev. C3, ETH_RESETn is controlled by an open-drain AND gate.
It is pulled high by a 10K resistor, and has a 4.7µF capacitor to
ground.  That gives an RC constant of 47ms.  As you need 0.7RC to charge
the capacitor above the threshold voltage of a CMOS input (VDD/2),
reset-deassert-us should be at least 33ms. Considering the typical
tolerance of 20% on capacitors, 40ms would be safer. Or perhaps
even 50ms?

If you agree, I can send a patch.
Thanks!

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds