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 16:14:39 +0800
From:	Shawn Guo <shawn.guo@...escale.com>
To:	Lothar Waßmann <LW@...O-electronics.de>
CC:	Shawn Guo <shawn.guo@...aro.org>, <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

On Tue, Sep 20, 2011 at 09:50:17AM +0200, Lothar Waßmann wrote:
> 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.
> 
Oops, I missed that.

> 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?
> 
I should have done that before sending this patch.  I'm working home
these days and have not got the chance get into the lab.  Yes, I
should have sent this patch as an RFC at least.  Sorry about this,
and thank you for pointing this out.

Will drop this patch from the v2 of the series.

-- 
Regards,
Shawn

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