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Re: [PATCH] natsemi: long cable, short cable

To: jgarzik@xxxxxxxxx
Subject: Re: [PATCH] natsemi: long cable, short cable
From: Gary Spiess <Gary.Spiess@xxxxxxxxxxxx>
Date: Mon, 14 Feb 2005 22:35:34 -0600
Cc: netdev@xxxxxxxxxxx
Sender: netdev-bounce@xxxxxxxxxxx
User-agent: Mozilla Thunderbird 1.0 (Windows/20041206)

I just read your message of Feb 10th, 2005 regarding the cable problems of natsemi.c. My investigation started late last week. On Feb 14th, I got serious and nailed it down. I submitted a patch of my own without realizing there was other work in progress on the same issue. Please consider replacing the patch you didn't like with mine.

What was happening with my DP83815D was that the TSTDAT value being read in the do_cable_magic function was near 0x1f, regardless of cable length. The code that fixes short cable problems actually works for short cables, but messes up long ones. The cable magic couldn't figure out the length of the cable and always applied the fix. I went looking for the history of this patch on the Internet and found a DP83815 Linux driver written and maintained by National Semiconductor. I was struck by the difference in the initialization sequences. natsemi.c always applies the sequence documented for the DP83815B. dp83815.c applies different sequences for chip revisions B, C, and D+. I used the D initialization for my D chip, and again watched the values being returned from TSTDAT in do_cable magic. A 1m cable returned something like 0xF8 and a 90m cable return something like 0xB8. Now the register was returning a value like it must have done with previous revisions of the chip. The magic is properly applied in the proper situation.

Since there is no public documentation on these PHY manipulations, I minimized the 'fix' to the setting of the DSPCFG register. If I only turn on the bit that makes the coefficient readable, do_cable_magic now appears to have the right information to decide the cable length automatically.

Ain't life great?


in reply to

 Gary Spiess (Gary.Spiess@xxxxxxxxxxxx)
 MobileLan Wireless Products Group, Intermec Technology Corp
 voice: 319 369-3580  fax: 319 369-3804

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