For the benefit of anyone else who's interested in this thread, with the
previous changes you may see "unexpected interrupt" on the IDE device.
Investigation showed this only happens when the Linux IDE driver is being run
in MEMORY mode for sector writes to the Compact Flash (CF), and only at the
very start of a multi-sector write transaction. i.e. the CF ~BUSY/READY line
transitions from low to high when it is configured for MEMORY mode.
The reason for the event can be explained by text found in the Sandisk product
spec that describes the WRITE SECTOR command. The text states ...
"When this command is accepted, the CompactFlash Memory Card sets
BSY, then sets DRQ and clears BSY, then waits for the host to fill
the sector buffer with the data to be written. No interrupt is
generated to start the first buffer fill operation."
The Linux IDE code (intended for IO mode) does not expect to receive an
interrupt when the CF is ready to accept the first buffer fill of a WRITE
SECTOR command. Instead, the IDE code polls the device and waits for DRQ to be
asserted.
The 'unexpected interrupt' message is therefore harmless, as the DRQ is being
serviced from polling anyway.
The 'quick hack' solution is to add code in the IDE driver that sets an
'ignore_interrupt' flag just prior to issuing the WRITE SECTOR command to the
drive when the CF is configured for MEMORY mode, and code in the IDE interrupt
handler that senses this flag and does not log an 'unexpected interrupt'
message when the flag is asserted. I'm sure there's a cleaner way of doing it,
but this works.
For reference, with corresponding PCI changes for our arch to support DMA
access to the CF memory buffer for PIO transfers, we get approx 4x the
performance that we did with IO based simple PIO.
i.e. it's not as fast as true UDMA would be, but for a CF controller such as
PCI1510 which doesn't support DMA natively, it's a big improvement and a lower
CPU overhead.
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