netdev - Re: [RFT net-next v4 5/5] net: stmmac: dwmac-meson8b: propagate rate changes to the parent clock

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Date:   Mon, 15 Jan 2018 12:50:31 +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 v4 5/5] net: stmmac: dwmac-meson8b: propagate
 rate changes to the parent clock

On Sun, 2018-01-14 at 22:48 +0100, Martin Blumenstingl wrote:
> On Meson8b the only valid input clock is MPLL2. The bootloader
> configures that to run at 500002394Hz which cannot be divided evenly
> down to 125MHz using the m250_div clock. Currently the common clock
> framework chooses a m250_div of 2 - with the internal fixed
> "divide by 10" this results in a RGMII TX clock of 125001197Hz (120Hz
> above the requested 125MHz).
> 
> Letting the common clock framework propagate the rate changes up to the
> parent of m250_mux allows us to get the best possible clock rate. With
> this patch the common clock framework calculates a rate of
> very-close-to-250MHz (249999701Hz to be exact) for the MPLL2 clock
> (which is the mux input). Dividing that by 2 (which is an internal,
> fixed divider for the RGMII TX clock) gives us an RGMII TX clock of
> 124999850Hz (which is only 150Hz off the requested 125MHz, compared to
> 1197Hz based on the MPLL2 rate set by u-boot and the Amlogic GPL kernel
> sources).
> 
> SoCs from the Meson GX series are not affected by this change because
> the input clock is FCLK_DIV2 whose rate cannot be changed (which is fine
> since it's running at 1GHz, so it's already a multiple of 250MHz and
> 125MHz).
> 
> Fixes: 566e8251625304 ("net: stmmac: add a glue driver for the Amlogic Meson 8b / GXBB DWMAC")
> Suggested-by: Jerome Brunet <jbrunet@...libre.com>
> Signed-off-by: Martin Blumenstingl <martin.blumenstingl@...glemail.com>

Reviewed-by: Jerome Brunet <jbrunet@...libre.com>