I think I figured this one out... posting here on the off-chance someone runs into this again and does a search
Had pretty much the same issue, MCF5480 @ 160MHz on a custom board, kernel 2.6.10, flaky NFS root fs no matter what i did to the settings.
In the end I traced it down to large (1514 byte) outgoing packets just disappearing somewhere in or below the FEC driver. Lowering the MTU to below 1k made everything work fine.
It seems the FEC buffer underruns on outbound packets that are larger than its FIFO, which in turn points to the DMA not transferring the remaining bytes fast enough at sub-demoboard clock speeds.
Raising the transmit watermark and alarm levels to 512k fixed the issue:
#define FEC_FECTFAR_ALARM (0x200)
#define FEC_FECTFWR_XWMRK (0x00000007)
Of course, this is kindof dependent on the clock speed.
Message Edited by klaus on
2008-03-07 07:12 PM