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: 566e8251 ("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>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Tested-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>