Strange PMAC IDE performance problem

[Paul Mackerras]

I found a register at 0xf3020200 on my powermac G3 which affects the
speed of disk transfers.  This is at offset 0x200 from the base
address of the IDE interface and it is a 32-bit little-endian
register.  After booting Linux, it has 0x2f8526 in it, but under MacOS
it has 0x211025.  Putting 0x211025 in (with xmon) increased the speed
reported by hdparm -tT /dev/hda from 1.89MB/s to 9.7MB/s !  The disk
is a WDC 24300ACL (as reported by hdparm -i /dev/hda).  The other
hdparm settings were -d1 -u1 -m16.

Paul.

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