netdev - Re: [PATCH 3/4] net/fec: set phy_speed to the optimal frequency 2.5 MHz

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Date:	Tue, 20 Sep 2011 09:50:17 +0200
From:	Lothar Waßmann <LW@...O-electronics.de>
To:	Shawn Guo <shawn.guo@...aro.org>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	linux-arm-kernel@...ts.infradead.org, patches@...aro.org
Subject: Re: [PATCH 3/4] net/fec: set phy_speed to the optimal frequency 2.5 MHz

Hi,

Shawn Guo writes:
> With the unnecessary 1 bit left-shift on fep->phy_speed during the
> calculation, the phy_speed always runs at the half frequency of the
> optimal one 2.5 MHz.
> 
> The patch removes that 1 bit left-shift to get the optimal phy_speed.
> 
> Signed-off-by: Shawn Guo <shawn.guo@...aro.org>
> ---
>  drivers/net/fec.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/net/fec.c b/drivers/net/fec.c
> index 5ef0e34..04206e4 100644
> --- a/drivers/net/fec.c
> +++ b/drivers/net/fec.c
> @@ -1007,7 +1007,7 @@ static int fec_enet_mii_init(struct platform_device *pdev)
>  	/*
>  	 * Set MII speed to 2.5 MHz (= clk_get_rate() / 2 * phy_speed)
>  	 */
> -	fep->phy_speed = DIV_ROUND_UP(clk_get_rate(fep->clk), 5000000) << 1;
> +	fep->phy_speed = DIV_ROUND_UP(clk_get_rate(fep->clk), 5000000);
>  	writel(fep->phy_speed, fep->hwp + FEC_MII_SPEED);
>  
>  	fep->mii_bus = mdiobus_alloc();
>
The left shift accounts for the fact, that the MII_SPEED bitfield
starts at pos 1 in the register. Thus the divider value has to be
shifted left to occupy the correct bit positions in the register.

According to my measurements on the TX28 the original code works
correctly!
Did you measure the actual frequency on the MDC pin after you change?


Lothar Waßmann
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