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:

quoted

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 <redacted>
---
 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