Re: port-amd64/53890: st(4) driver for tapes not working o variable block size

[mlelstv%serpens.de@localhost (Michael van Elst)]

The following reply was made to PR port-amd64/53890; it has been noted by GNATS.

From: mlelstv%serpens.de@localhost (Michael van Elst)
To: gnats-bugs%netbsd.org@localhost
Cc: 
Subject: Re: port-amd64/53890: st(4) driver for tapes not working o variable block size
Date: Sat, 19 Jan 2019 16:27:38 -0000 (UTC)

 mlelstv%serpens.de@localhost (Michael van Elst) writes:
 
 > Looks like this is not the tape driver itself. The kernel physio() routine
 > does a sanity check on the I/O byte offset to be a multiple of DEV_BSIZE
 > (== 512 bytes) and returns EINVAL if that's false. That's why the
 > second read or write fails (the first starts at offset 0).
 
 The sanity check is done because physio() may issue concurrent
 partial I/O requests and keeps track of the individual parts
 by their block number. The block number addresses DEV_BSIZE
 blocks, so the algorithm only works for multiples of 512 bytes.
 
 A simple solution would be to not track absolute but relative
 offsets. This would still fail when physio has to handle I/O of
 multiple memory blocks (the second might be unaligned again),
 but the tape driver only starts I/O for a single block.
 
 -- 
 -- 
                                 Michael van Elst
 Internet: mlelstv%serpens.de@localhost
                                 "A potential Snark may lurk in every tree."