Can I run cable across attic decking or is that a violation?

The plywood doesn't change anything, unfortunatley. The rule is about accessible attic spaces where the cable runs across framing rather than along it. If the cable runs perpendicular to the joists, it has to be protected, and decking on top doesn't count beacuse the cable sits above it right where feet go. I ran into this exact thing in my own attic a couple years back and ended up pulling the decking section up rather than trying to build around it.

When it runs along the side of a joist, stapling is fine on its own, no extra protection needed. The rule only kicks in when the cable crosses framing instead of following it. So the cleanest fix is to lift a section of that plywood, staple the 12/2 to the side of the joists underneath, following the framing the whole way, and then put the decking back over it. A little extra hassle now but nobody can ever step on it and the inspector will have nothing to say about it. If you relly don't want to pull the decking up, the other approach people use is building guard strips, basially running a board on each side of the cable so it sits protected in a little channel. But you'd be building a little tunnel on top of the decking, which eats headroom and looks janky, and you'd need to make sure it's built right where the cable crosses. Pulling up the decking is honestly easier if the sections are screwed down and not too many. One thing to check before you start: whether that attic area is considered accessible, meaning reachable from the hatch or scuttle opening. If the crossing section is way out at the far end where nobody can get near it, the protection rules are more relaxed. But if your shortest path is anywhere near the access point, just plan on running it along the joist sides. Take a couple photos before you cover everything back up too, makes the inspector visit go faster when they can see what's under the decking.

Agree with running it along the joist sides, but another option is just drilling the joists and going through them. That's what I did on my garage build two summers ago, maybe 14 holes in 2x8s. Keep each hole at least 1-1/4 inch from the edge of the lumber so a stray nail can't reach it. Bit slower than stapling but the cable ends up fully out of the way and you don't touch the plywood at all. Also check your staple spacing, no more than 4.5 feet between supports and within about a foot of each box.

First time posting here, mostly been lurking. My situation was a bit diffrent last year, finished shed with no exposed framing at all, so I just ran the cable in plastic conduit the whole way. Made things dead simple since it's protected everywhere and I could clip it wherever it needed to go. Costs a bit more but for one short run it was nothing. Wondering if anyone's done that up in an attic, might dodge the whole decking headache without unscrewing anything. Only worry would be fishing it over the joists without kinking the pipe. Also heads up from my own inspection, they wanted the cable secured within about a foot of every box, so leave yourself slack for that when measuring.

Did basically this run in my own garage attic last fall and the impact driver with a long bit was the thing that saved me. The decking up there was screwed down and pulling screws went way faster than I expected, maybe half an hour for the sections I needed up. Ran the 12/2 along the joist sides with regular staples and covered it back up, no issues at inspection. Only thing I'd add is leave yourself a little slack at each end before you staple anything, I cut mine short on the first outlet and had to splice in a junction box I didn't plan on. Check where your access hatch is relative to the crossing run too, like was said above.

Following this since I've got basically the same setup going on over my garage, except my walkway boards run the other direction from yours. Mine are just laying loose on top though, no screws, so I figure I can shift one out of the way while I'm up there and slide it back after. What I can't figure out is where the access zone ends. My ladder comes up near the middle of the attic and the far corners are a real crawl to get to. Is there like a set distance from the hatch, or is it more of a judgment call?

Update: went the drilled-holes route instead of messing with the decking, worked out fine so far. One thing I'm still not sure on though. Once the cable is running through the joists, does it still need stapling along the run somewhere, or is it supported enough just by passing through each hole? Never done it this way before.