acd(4) and maximum read block size

[Ulrich Spoerlein]

Søren, group,

I'm working on some improvements for recoverdisk[1] to handle optical media
better. I found it an invaluable tool for recovering old CD-Rs. It also
helps with newer DVDs, when your drive sucks wrt. error recovery.

recoverdisk tries to read in 1MB, 64kB and SECTORSIZE chunks. This has
the obvious drawback, that it can't really handle audio CDs or mode 2
CDs with a sectorsize of 2352.

While this is easily fixed, the acd(4) driver seems to allow no reads
greater than 128kB *for audio CDs*. While this is also not terrible, it
results in roughly 700 error messages per CD, as all the 1MB (1046640
bytes, more precisely) chunks will fail.

A side note: Accessing audio CDs via atapicam and dd(1) is completely
impossible. Is the SCSI subsystem limited to multiple-of-512-blocksizes?

[1] /usr/src/tools/tools/recoverdisk

Ulrich Spoerlein
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