<div dir="ltr"> <div class="gmail_extra"> <div class="gmail_quote">On Wed, Jun 25, 2014 at 10:59 AM, Neil Roberts <<a href="mailto:neil@linux.intel.com" target="_blank">neil@linux.intel.com</a>> wrote: <blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">Hi, I've been looking into this bug a bit more. I don't think disabling the blorp path on Sandybridge is the right thing to do. The only notable difference between the blorp code path and the meta code path seems to be that the blorp code forces the use of non-normalized coordinates whereas the meta code path doesn't. It looks like the linear filtering on Sandybridge is not very precise and switching to non-normalized coordinates is enough to tip it over the balance and make the test fail. You can also make the meta code path fail if you change the test to use a rectangle texture instead of a 2D texture. That will make it also use non-normalized coordinates. Therefore I think the meta path isn't really any better than the blorp path and the problem is with the sampler. I've made a patch for piglit to demonstrate using rectangle textures here: <a href="https://github.com/bpeel/piglit/commit/69d431af43825ea184e224c9dec7fca6" target="_blank">https://github.com/bpeel/piglit/commit/69d431af43825ea184e224c9dec7fca6</a> I made a little test program to demonstrate the results of the linear filtering here: <a href="https://github.com/bpeel/glthing/blob/wip/simple-longblit/src/main.c" target="_blank">https://github.com/bpeel/glthing/blob/wip/simple-longblit/src/main.c</a> This creates a 2x1 rectangle texture with a black pixel on the left and a red pixel on the right. It then renders this using a triangle strip into a 256x1 texture where the texture coordinates are set up such that the center of the first pixel in the destination samples the center of the left pixel in the source and the center of the rightmost pixel samples the center of the right pixel. Linear interpolation is used so that if the rendering is working preciesly the red channel of the result should have exactly the values from 0 to 255. On Haswell, this gives the values you would expect from 0 to 255. However on Sandybridge I get the following result: </blockquote><div>Good analysis Neil. I get perfect output on IVB too using RECT and</div><div>2D textures.</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"> 00 00* 00* 00* 04 04* 04* 04* 08 08* 08* 08* 0c 0c* 0c* 0c* 10 10* 10* 10* 14 14* 14* 14* 18 18* 18* 18* 1c 1c* 1c* 1c* 20 20* 20* 20* 24 24* 24* 24* 28 28* 28* 28* 2c 2c* 2c* 2c* 30 30* 30* 30* 34 34* 34* 34* 38 38* 38* 38* 3c 3c* 3c* 3c* 40 40* 40* 40* 44 44* 44* 44* 48 48* 48* 48* 4c 4c* 4c* 4c* 50 50* 50* 50* 54 54* 54* 54* 58 58* 58* 58* 5c 5c* 5c* 5c* 60 60* 60* 60* 64 64* 64* 64* 68 68* 68* 68* 6c 6c* 6c* 6c* 70 70* 70* 70* 74 74* 74* 74* 78 78* 78* 78* 7c 7c* 7c* 7c* 80 80* 80* 80* 83* 83* 83* 83* 87* 87* 87* 87* 8b* 8b* 8b* 8b* 8f* 8f* 8f* 8f* 93* 93* 93* 93* 97* 97* 97* 97* 9b* 9b* 9b* 9b* 9f* 9f* 9f* 9f* a3* a3* a3* a3* a7* a7* a7* a7* ab* ab* ab* ab* af* af* af* af* b3* b3* b3* b3* b7* b7* b7* b7* bb* bb* bb* bb* bf* bf* bf* bf* c3* c3* c3* c3* c7* c7* c7* c7* cb* cb* cb* cb* cf* cf* cf* cf* d3* d3* d3* d3* d7* d7* d7* d7* db* db* db* db* df* df* df* df* e3* e3* e3* e3* e7* e7* e7* e7* eb* eb* eb* eb* ef* ef* ef* ef* f3* f3* f3* f3* f7* f7* f7* f7* fb* fb* fb* fb* Maximum diff = 4 (1.568627%) (The asterisk marks when the value is not correct) With a 2D texture this gives: 00 00* 04* 04* 04 04* 08* 08* 08 08* 0c* 0c* 0c 0c* 10* 10* 10 10* 14* 14* 14 14* 18* 18* 18 18* 1c* 1c* 1c 1c* 20* 20* 20 20* 24* 24* 24 24* 28* 28* 28 28* 2c* 2c* 2c 2c* 30* 30* 30 30* 34* 34* 34 34* 38* 38* 38 38* 3c* 3c* 3c 3c* 40* 40* 40 40* 44* 44* 44 44* 48* 48* 48 48* 4c* 4c* 4c 4c* 50* 50* 50 50* 54* 54* 54 54* 58* 58* 58 58* 5c* 5c* 5c 5c* 60* 60* 60 60* 64* 64* 64 64* 68* 68* 68 68* 6c* 6c* 6c 6c* 70* 70* 70 70* 74* 74* 74 74* 78* 78* 78 78* 7c* 7c* 7c 7c* 80* 80* 80 80* 83* 83 83* 83* 87* 87 87* 87* 8b* 8b 8b* 8b* 8f* 8f 8f* 8f* 93* 93 93* 93* 97* 97 97* 97* 9b* 9b 9b* 9b* 9f* 9f 9f* 9f* a3* a3 a3* a3* a7* a7 a7* a7* ab* ab ab* ab* af* af af* af* b3* b3 b3* b3* b7* b7 b7* b7* bb* bb bb* bb* bf* bf bf* bf* c3* c3 c3* c3* c7* c7 c7* c7* cb* cb cb* cb* cf* cf cf* cf* d3* d3 d3* d3* d7* d7 d7* d7* db* db db* db* df* df df* df* e3* e3 e3* e3* e7* e7 e7* e7* eb* eb eb* eb* ef* ef ef* ef* f3* f3 f3* f3* f7* f7 f7* f7* fb* fb fb* fb* ff* ff Maximum diff = 2 (0.784314%) Piglit effectively has a default tolerance of 1% so the 2D texture version would pass. However the results are still clumped together in fours and they are just offset slightly. It doesn't look like it's actually any better. Is this amount of imprecision something that we should expect on Sandybridge? If so perhaps we should increase the tolerance level in the test. </blockquote><div> Increasing the tolerance will hide any imprecision issues in other platforms. </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"> - Neil </blockquote></div> </div></div>