netdev - Re: [RFT net-next v3 3/5] net: stmmac: dwmac-meson8b: fix internal RGMII clock configuration

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Date:   Fri, 29 Dec 2017 18:57:14 +0100
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        netdev@...r.kernel.org, ingrassia@...genesys.com
Cc:     linus.luessing@...3.blue, khilman@...libre.com,
        linux-amlogic@...ts.infradead.org, narmstrong@...libre.com,
        peppe.cavallaro@...com, alexandre.torgue@...com
Subject: Re: [RFT net-next v3 3/5] net: stmmac: dwmac-meson8b: fix internal
 RGMII clock configuration

On Thu, 2017-12-28 at 23:21 +0100, Martin Blumenstingl wrote:
> While testing the dwmac-meson8b with an RGMII PHY on Meson8b we
> discovered that the m25_div is not actually a divider but rather a gate.
> This matches with the datasheet which describes bit 10 as "Generate
> 25MHz clock for PHY". Back when the driver was written it was assumed
> that this was a divider (which could divide by 5 or 10) because other
> clock bits in the datasheet were documented incorrectly.

Maybe this bit 10 is indeed a 5/10 divider, as amlogic claims it is. Maybe, as
Emiliano suggested, the output rate of div250 actually needs to be 250Mhz in
RGMII, before being divided by 10 to produce the 25MHz of div25

IOW, maybe we need this intermediate rate.
It would not be surprising, 1GBps usually requires a 125MHz clock somewhere. 

This is still doable:
* Keep the divider
* drop CLK_SET_RATE_PARENT on div25
* call set_rate on div250 with 250MHz then on div25 with 25Mhz


> 
> Tests have shown that without bit 10 set the RTL8211F RGMII PHY on
> Odroid-C1 (using a Meson8b SoC) does not work.
> On GXBB and newer SoCs (where the driver was initially tested with RGMII
> PHYs) this is not a problem because the input clock is running at 1GHz.
> The m250_div clock's biggest possible divider is 7 (3-bit divider, with
> value 0 being reserved). Thus we end up with a m250_div of 4 and a
> "m25_div" of 10 (= register value 1).
> 
> Instead it turns out that the Ethernet IP block seems to have a fixed
> "divide by 10" clock internally. This means that bit 10 is a gate clock
> which enables the RGMII clock output.
> 
> This replaces the "m25_div" clock with a clock gate called "m25_en"
> which ensures that we can set this bit to 1 whenever we enable RGMII
> mode. This however means that we are now missing a "divide by 10" after
> the m250_div (and before our new m25_en), otherwise the common clock
> framework thinks that the rate of the m25_en clock is 10-times higher
> than it is in the actual hardware. That is solved by adding a
> fixed-factor clock which divides the m250_div output by 10.
> 
> Fixes: 566e8251625304 ("net: stmmac: add a glue driver for the Amlogic Meson 8b / GXBB DWMAC")
> Reported-by: Emiliano Ingrassia <ingrassia@...genesys.com>
> Signed-off-by: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
> ---
>  .../net/ethernet/stmicro/stmmac/dwmac-meson8b.c    | 66 +++++++++++++---------
>  1 file changed, 38 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac-meson8b.c b/drivers/net/ethernet/stmicro/stmmac/dwmac-meson8b.c
> index 1c14210df465..7199e8c08536 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/dwmac-meson8b.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/dwmac-meson8b.c
> @@ -40,9 +40,7 @@
>  #define PRG_ETH0_CLK_M250_DIV_SHIFT    7
>  #define PRG_ETH0_CLK_M250_DIV_WIDTH    3
>  
> -/* divides the result of m25_sel by either 5 (bit unset) or 10 (bit set) */
> -#define PRG_ETH0_CLK_M25_DIV_SHIFT     10
> -#define PRG_ETH0_CLK_M25_DIV_WIDTH     1
> +#define PRG_ETH0_GENERATE_25M_PHY_CLOCK        10
>  
>  #define PRG_ETH0_INVERTED_RMII_CLK     BIT(11)
>  #define PRG_ETH0_TX_AND_PHY_REF_CLK    BIT(12)
> @@ -63,8 +61,11 @@ struct meson8b_dwmac {
>         struct clk_divider      m250_div;
>         struct clk              *m250_div_clk;
>  
> -       struct clk_divider      m25_div;
> -       struct clk              *m25_div_clk;
> +       struct clk_fixed_factor fixed_div10;
> +       struct clk              *fixed_div10_clk;
> +
> +       struct clk_gate         m25_en;
> +       struct clk              *m25_en_clk;

Maybe it could be the topic of another series but we don't need to keep all
those structures around, thanks to devm

all clk_divider, fixed_factor, gate and mux can go away
You only need to keep  the'struct clk*' you are going to use later on

at the moment it would be m25_en_clk only.

>  
>         u32                     tx_delay_ns;