On Tue, 23 Oct 2012 22:03:21 +0200 Daniel Vetter <daniel at ffwll.ch> wrote: > On Tue, Oct 23, 2012 at 9:57 PM, Ben Widawsky <ben at bwidawsk.net> wrote: > >> On Tue, Oct 23, 2012 at 4:57 PM, Ben Widawsky <ben at bwidawsk.net> wrote: > >> > Actually, after we introduce the FLSH_CNTL patch from Jesse/me later in the > >> > series, I think we just want a POSTING_READ on that register. It is > >> > technically "required" by our desire to some day WC the registers, and > >> > should synchronize everything else for us. > >> > > >> > After a quick read of memory_barriers.txt (again), I think mmiowb is > >> > actually what we might want in addition to the POSTING_READ I'd add. > >> > >> Imo we have special rules for the igd, since clearly not all registers > >> in our 4mb mmio window are equal. So I'd prefer the keep the readback > >> of the last pte write (to ensure those are flushed out) and maybe also > >> add a readback of the gfx_flsh_cntl reg (like I've seen in some > >> internal vlv tree). Just to be paranoid. > >> -Daniel > > > > What's your definition of flush? I think we just need one read to > > satisfy the device I/O flush, and I think the write to the regsiter > > should satisy the TLB flush. That leads me to the conclusion that just > > the POSTING_READ of the FLSH_CNTL register is sufficient. > > > > If you want me to do both, I have no problem with that either, and I'll > > just update the comment to say that we believe it is unnecessary. > > > > I don't really care either way. > > I think we need both, otherwise one of the pte writes might not have > arrived at the endpoint when the chip is purging the tlb already, > which might give is a tiny window where we load an invalid entry into > the tlb. Tiny window, but I'm paranoid ;-) And yes, I know that's more > strict than what docs say, but our igd also seems to be less coherent > than what docs claim. > -Daniel Hmm, that's a good point. If we write the last pte, and then flsh_cntl, and then posting_read on flsh_cntl; I think it is also fine. (Unless writes can be reordered) -- Ben Widawsky, Intel Open Source Technology Center
