[Piglit] [PATCH] winsys-framework Default to showing window

James Jones jajones at nvidia.com
Mon Aug 17 10:48:52 PDT 2015

For those who don't want to click-through, I'll just paste the very 
brief portion of the spec on Pixel Ownership that Alex linked to here:

[1] "The first test is to determine if the pixel at location in the 
framebuffer is currently owned by the GL (more precisely, by this GL 
context). If it is not, the window system decides the fate the incoming 
fragment. Possible results are that the fragment is discarded or that 
some subset of the subsequent per-fragment operations are applied to the 
fragment. This test allows the window system to control the GL's 
behavior, for instance, when a GL window is obscured."

The important part is the last sentence: The GL does not guarantee 
rendering to an obscured pixel will do anything.  Unmapped windows are 
definitely obscured pixels.  I've never heard anyone argue against that 
interpretation, though I've heard some grumbling from people that wish 
this spec language didn't exist.  As long as it exists though, we'll 
take advantage of it in our driver, so I don't think it makes any sense 
for tests to ignore it, at least by default.  Any other driver is free 
to implement the same optimization, but I don't know if any other X11 GL 
drivers do at the moment.



(Also present in the OpenGL 4.5 core spec as section 14.9.1, page 461)

On 08/14/2015 04:33 PM, Ilia Mirkin wrote:
> Most tests do indeed use an FBO, either directly or via the fbo
> winsys, and all will be well. However some don't (glean, probably
> others) because they were written in the stone ages. Those are the
> ones that have problems.
> Perhaps we should just suck up the slower-ness and use that as an
> incentive to finally convert those tests over to The New Way (tm).
> Unfortunately I'm ill-equipped to either agree or disagree with your
> claim that "using the default framebuffer without mapping it is
> undefined behavior". Hopefully someone "in the know" can comment?
>    -ilia
> On Fri, Aug 14, 2015 at 7:27 PM, Alexander Goins <agoins at nvidia.com> wrote:
>>  From NVIDIA's side, I don't think it would be that big of a problem to just keep running our tests with PIGLIT_FORCE_WINDOW=1 or -fbo. However, my reasoning is that Piglit should default to the 'correct' behavior according to the OpenGL spec. While the NVIDIA driver is the only one I know of that actually enforces pixel ownership, any driver theoretically could do so while still adhering to the spec.
>> I agree that having a window pop up for every test is annoying, but using the default framebuffer without mapping it is undefined behavior. The 'correct' way to do offscreen rendering would be using FBOs which Piglit supports with the -fbo argument. Is there a reason why this can't/shouldn't be used for running Piglit offscreen? I originally removed the ability to render offscreen with the default framebuffer altogether, but I figured some users might still desire this behavior so instead decided to leave it in as a non-default option.
>> Thanks,
>> Alex
>> -----Original Message-----
>> From: ibmirkin at gmail.com [mailto:ibmirkin at gmail.com] On Behalf Of Ilia Mirkin
>> Sent: Friday, August 14, 2015 8:07 AM
>> To: Alexander Goins
>> Cc: piglit at lists.freedesktop.org
>> Subject: Re: [Piglit] [PATCH] winsys-framework Default to showing window
>> Interesting. PIGLIT_FORCE_WINDOW was introduced in this change:
>> commit b797406fb850c5e3c9fdaaebae36127b5e5f8954
>> Author: Paul Berry <stereotype441 at gmail.com>
>> Date:   Tue Jun 11 16:34:32 2013 -0700
>>      Add PIGLIT_FORCE_WINDOW environment variable.
>>      Setting PIGLIT_FORCE_WINDOW=1 causes all piglit windowsystem tests
>>      (those that weren't invoked with the "-fbo" flag) to show the window
>>      prior to calling piglit_display().  Setting PIGLIT_FORCE_WINDOW=0 (the
>>      default) preserves piglit's existing behaviour, which is to only show
>>      the window when the test is run in "manual" mode (with no "-auto"
>>      argument supplied).
>>      Setting PIGLIT_FORCE_WINDOW=1 slows down the piglit run, and produces
>>      more annoying windows on the screen, but it has been observed to
>>      produce more repeatable results when testing with the nVidia
>>      proprietary driver for Linux.
>>      Reviewed-by: Chad Versace <chad.versace at linux.intel.com>
>> Which explicitly mentions the nvidia blob driver thing. Having more windows popping up during piglit runs is going to be annoying... and slowing things down will also be unwelcome. Is there anything else that can be done here?
>>    -ilia
>> On Thu, Aug 13, 2015 at 1:49 PM, agoins <agoins at nvidia.com> wrote:
>>> winsys-framework incorrectly assumed that it will always have
>>> ownership of the pixels in its own buffers. If using the default
>>> framebuffer (i.e., when Piglit is not running in FBO mode,) ownership
>>> will not be acquired unless the window is mapped.
>>> While this is not typically a problem because buffers are separate,
>>> NVIDIA's Unified Back Buffer feature makes all windows share a back
>>> buffer while still conforming to the OpenGL spec. If Piglit does not
>>> establish pixel ownership, its output will be clobbered by other windows.
>>> While this problem could be easily fixed by specifying
>>> PIGLIT_FORCE_WINDOW=1 or -fbo, the current default takes advantage of
>>> undefined behavior in the OpenGL spec. A better solution would be to
>>> replace PIGLIT_FORCE_WINDOW with PIGLIT_NO_WINDOW, a flag that allows
>>> tests to be run without mapping a window or using an FBO. The default
>>> behavior, the, would be to map a window. Really, though, if users want
>>> to test with offscreen rendering, they should use FBOs with flag -fbo.
>>> More information on pixel ownership here:
>>> https://www.opengl.org/documentation/specs/version1.1/glspec1.1/node94
>>> .html
>>> ---
>>>   .../piglit-framework-gl/piglit_winsys_framework.c    | 20 ++++++++++----------
>>>   1 file changed, 10 insertions(+), 10 deletions(-)
>>> diff --git a/tests/util/piglit-framework-gl/piglit_winsys_framework.c
>>> b/tests/util/piglit-framework-gl/piglit_winsys_framework.c
>>> index c80e972..fcba91e 100644
>>> --- a/tests/util/piglit-framework-gl/piglit_winsys_framework.c
>>> +++ b/tests/util/piglit-framework-gl/piglit_winsys_framework.c
>>> @@ -53,18 +53,18 @@ run_test(struct piglit_gl_framework *gl_fw,
>>>            int argc, char *argv[])
>>>   {
>>>          struct piglit_winsys_framework *winsys_fw = piglit_winsys_framework(gl_fw);
>>> -       bool force_window = false;
>>> -       const char *env_force_window = getenv("PIGLIT_FORCE_WINDOW");
>>> +       bool no_window = false;
>>> +       const char *env_no_window = getenv("PIGLIT_NO_WINDOW");
>>> -       if (env_force_window != NULL) {
>>> -               if (strcmp(env_force_window, "0") == 0) {
>>> -                       force_window = false;
>>> -               } else if (strcmp(env_force_window, "1") == 0) {
>>> -                       force_window = true;
>>> +       if (env_no_window != NULL) {
>>> +               if (strcmp(env_no_window, "0") == 0) {
>>> +                       no_window = true;
>>> +               } else if (strcmp(env_no_window, "1") == 0) {
>>> +                       no_window = true;
>>>                  } else {
>>> -                       fprintf(stderr, "PIGLIT_FORCE_WINDOW has invalid"
>>> -                               " value: %s\n", env_force_window);
>>> +                       fprintf(stderr, "PIGLIT_NO_WINDOW has invalid"
>>> +                               " value: %s\n", env_no_window);
>>>                          abort();
>>>                  }
>>>          }
>>> @@ -73,7 +73,7 @@ run_test(struct piglit_gl_framework *gl_fw,
>>>                  gl_fw->test_config->init(argc, argv);
>>>          if (!gl_fw->test_config->requires_displayed_window &&
>>> -           piglit_automatic && !force_window) {
>>> +           piglit_automatic && no_window) {
>>>                  enum piglit_result result = PIGLIT_PASS;
>>>                  if (gl_fw->test_config->display)
>>>                          result = gl_fw->test_config->display();
>>> --
>>> 1.9.1
>>> New to the project - please commit for me.
>>> _______________________________________________
>>> Piglit mailing list
>>> Piglit at lists.freedesktop.org
>>> http://lists.freedesktop.org/mailman/listinfo/piglit
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