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I had > > an open question about whether we wanted to implement the fix slightly > > differently to avoid asserts triggering elsewhere in the code [1] > > Hmm. That patch is odd. > Yes, a little. > It says > > "When VRAM limit (usable_size) is no longer 2M aligned (e.g. because flat > CCS storage size rounds it down to 4K)," [...] > > but that was never true, even before my fix. The usable size was > either 64kB aligned or 128kB aligned - it has *never* been 2M aligned > as far as I can tell. > > Now, obviously, if alignment never did anything at all (because CCS > was already completely aligned), then that 2M alignment could be true. > > So maybe the 2M alignment existed in practice, but it was certainly > never guaranteed by any code, and any 2M assumptions were completely > bogus long before my fix as far as I can tell. Yes, this just happened to be true because of the rounded-up 128K alignment. Nothing actually enforced it, nor was it required. > > But that commit is odd in another way: it changes the code to do > something completely different. > > Before, it didn't use "usable_size" at all, it used thje *actual* > physical size. Which really *was* 2M aligned, becuase I don't think > Intel has ever sold any other configurations than 16/24/64GB of > memory. > > So I don't think [1] actually exposes anything in my fix and I think > the whole 2M limit wrt that code is actually an entirely separate > issue. Also correct. Your change prompted us to audit and clean up our code. That said, with your fix alone and CONFIG_DRM_XE_DEBUG enabled, our old bogus assertion will trigger, but it is meaningless. In any case, no production kernel enables this option. > > But I admittedly *really* don't know this hardware very well at all. > > But yes, this kind of "my fix exposed *another* bug" was why I was > worried about backporting it without further testing. That was my concern too, but our audit showed that the only issue was a bogus assertion that fell out. It isn't a blocker for getting your fix into stable. Matt > > Linus