Gregory's Serverlooks like there's a rare problem when reading sectors, around 1 in 100,000 reads. ugh these can be hard to track down
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13 comments
 looks like some individual data bits are getting flipped. what's really odd is that it is repeatable and it seems to be the same bits at the same addresses. also -- there seem to be two overlapping bugs. one causes a few bits to be corrupted for single sector reads, the other causes an entire sector's data to be shifted by several bytes.  huh, once I connect the logic analyzer, the problem takes much longer to reproduce; the probability of data corruption drops dramatically. interesting but it makes things much harder to debug. @tubetime hey @azonenberg weren’t you looking for examples of probes changing behavior of the DUT? looks like it is skipping a transfer here somehow. it transfers B66D, then it skips 6DDB and goes to DBB6. the preempt line seems to deassert right before arb/gnt goes high. that doesn't seem right.  @tubetime A coincidence that DBB6 == 6DDB << 1 ? Could it be slipping a bit? @jond no that is just the pattern in the disk image. presumably a disk area that was never written to after being formatted and still contains this bit pattern. the smoking gun: an invalid bus transfer. both rd and wr are asserted at the same time. before that we see the missing data on the bus: 6DDB. so the card thinks it is in a DMA cycle even though it is not, and it's putting data on the bus when it shouldn't be! i deleted 3 characters from my verilog and it works now. 😅 (i still need to test writes, and i need to test it without the logic analyzer card) @tubetime As the joke goes, its all about knowing where to tap...  @tubetime Ugh, that's NEVER good. I see lots of oscilloscope probing in your future. |
i thought it might be a timing issue caused by disabling USB debugging, but it still happens (with roughly the same statistics) with USB debugging turned on.