2012/10/12 Daniel Vetter <daniel.vetter at ffwll.ch>: > On Fri, Oct 12, 2012 at 4:47 AM, Paulo Zanoni <przanoni at gmail.com> wrote: >> 2012/10/11 Daniel Vetter <daniel.vetter at ffwll.ch>: >>> ... since they don't apply to pre-pch platforms and could actually be >>> harmful. >>> >>> Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch> >> >> Ok, so I checked the specs and yes, these bit definitions don't exist. >> The problem here is that instead of "must-be-zero", the spec says >> "Reserved. Software must preserve the contents of these bits" for bits >> 29:16 (and also some others). So maybe by setting everything to 0 >> instead of enabling bits 17, 18, 20-23 we could actually be breaking >> things? Either way, both the old and new code don't follow the >> specification. > > Indeed, I've overlooked the "must be preserved" wording, and it goes > back to gen2 when the ADPA reg was added. So I've fired up all my > gen2/gen3 machines, and they have all zeros in these registers. Ok, so please do a final test: try to write something to those "must-be-preserved" bits and check if the values stay or not. If after writing 1 to bits 17-18, 20-23 you read 0, then you have my Reviewed-by: Paulo Zanoni <paulo.r.zanoni at intel.com>. I still do plan to test the 6 patches of the series on hsw later btw. > And we > never write anything in there. I suspect this is simply a hint from > the Bspec authors to driver writes that they might eventually use > these reserved bits in future hw platforms. And if the driver > preserves the bit settings, it will automatically work. I think MBZ is > mostly used for bit ranges that have been used once, but are no longer > implemented (e.g. bits 12:13 do something in gen2/3 but not on later > hw gens). > >> Maybe on non-pch-split we could try to read ADPA and erase all the >> bits except the "must-be-preserved" ones? > > See above, I think we can clear them - at least all my old hw seems to > work perfectly well with all these bits being zero. > -Daniel > -- > Daniel Vetter > Software Engineer, Intel Corporation > +41 (0) 79 365 57 48 - http://blog.ffwll.ch -- Paulo Zanoni
