Re: [RFT net-next v4 3/5] net: stmmac: dwmac-meson8b: fix internal RGMII clock configuration

[Jerome Brunet <jbrunet@baylibre.com>]

On Sun, 2018-01-14 at 22:48 +0100, Martin Blumenstingl wrote:
> Tests (using an oscilloscope and an Odroid-C1 board with a RTL8211F
> RGMII PHY) have shown that the PRG_ETH0 register behaves as follows:
> - bit 4 is a mux to choose between two parent clocks. according to the
>   public S805 datasheet the only supported parent clock is MPLL2 (this
>   was not verified using the oscilloscope).
>   The public S805/S905 datasheet claims that this bit is reserved.
> - bits 9:7 control a one-based divider (register value 1 means "divide
>   by 1", etc.) for the input clock. we call this clock the "m250_div"
>   clock because it's value is always supposed to be (close to) 250MHz
>   (see below for an explanation).
>   The description in the public S805/S905 datasheet is a bit cryptic,
>   but it comes down to "input clock = 250MHz * value" (which could also
>   be expressed as "250MHz = input clock / value")
> - there seems to be an internal fixed divide-by-2 clock which takes the
>   output from the m250_div and divides it by 2. This is not unusual on
>   Amlogic SoCs, since the SDIO (MMC) driver also uses an internal fixed
>   divide-by-2 clock.
>   This is not documented in the public S805/S905 datasheet
> - bit 10 controls a gate clock which enables or disables the RGMII TX
>   clock (which is an output on the MAC/SoC and an input in the PHY). we
>   call this the "rgmii_tx_en" clock. if this bit is set to "0" the RGMII
>   TX clock output is close to 0
>   The description for this bit in the public S805/S905 datasheet is
>   "Generate 25MHz clock for PHY". Based on these tests it's believed
>   that this is wrong, and should probably read "Generate the 125MHz
>   RGMII TX clock for the PHY"
> - the RGMII TX clock has to be set to 125MHz - the IP block adjusts the
>   output (automatically) depending on the line speed (RGMII specifies
>   that Gbit connections use a 125MHz clock, 100Mbit/s connections use a
>   25MHz clock and 10Mbit/s connections use a 2.5MHz clock. only Gbit and
>   100Mbit/s were tested with an oscilloscope). Due to the requirement
>   that this clock always has to be set to 125MHz and due to the fixed
>   divide-by-2 parent clock this means that m250_div will always end up
>   with a rate of (close to) 250MHz.
> - bits 6:5 are the TX delay, which is also named "clock phase" in some
>   of Amlogic's older GPL kernel sources.
> 
> The PHY also has an XTAL_IN pin where a 25MHz clock has to be provided.
> Tests with the oscilloscope have shown that this is routed to a crystal
> right next to the RTL8211F PHY. The same seems to be true on the Khadas
> VIM2 (which uses a GXM SoC) board - however the 25MHz crystal is on the
> other side of the PCB there.
> 
> This updates the clocks in the dwmac-meson8b driver by replacing the
> "m25_div" with the "rgmii_tx_en" clock and additionally introducing a
> fixed divide-by-2 clock between "m250_div" and "rgmii_tx_en".
> Now we also need to set a frequency of 125MHz on the RGMII clock
> (opposed to the 25MHz we set before, with that non-existing
> divide-by-5-or-10 divider).
> 
> Special thanks go to Linus Lüssing for testing the various bits and
> checking the results with an oscilloscope on his Odroid-C1!
> 
> Fixes: 566e8251625304 ("net: stmmac: add a glue driver for the Amlogic Meson 8b / GXBB DWMAC")
> Reported-by: Emiliano Ingrassia <ingrassia@epigenesys.com>
> Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>

Looks Good
Acked-by: Jerome Brunet <jbrunet@baylibre.com>

Not related to this particular change but we still need to re-factor the clock
registration of this driver. We carry a lot of useless pointers in our private
data. I guess this is something for another day.