[Piglit] [PATCH] gl-1.0/swapbuffers-behavior: Try avoid reading from real front

[Thomas Hellstrom]

Hi Brian,

On 06/21/2017 04:20 PM, Brian Paul wrote:
> On 06/21/2017 06:24 AM, Thomas Hellstrom wrote:
>> Window systems (read dri3) that allocate a fake frontbuffer on demand 
>> will
>> effectively read out from the real front before the first swapbuffers 
>> after
>> fake front creation, and the real front buffer content is subject to 
>> errors
>> caused by, among other things, window reparenting. So increase the 
>> likelyhood
>> of not reading out from the real front by advertizing early that we will
>> access the front buffer. After all, real front consistency is not the 
>> objective
>> of this test.
>>
>> Fixes sporadic failures due to window reparenting on dri3/vmwgfx.
>>
>> Cc: Brian Paul <brianp at vmware.com>
>> Signed-off-by: Thomas Hellstrom <thellstrom at vmware.com>
>> ---
>>   tests/spec/gl-1.0/swapbuffers-behavior.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/tests/spec/gl-1.0/swapbuffers-behavior.c 
>> b/tests/spec/gl-1.0/swapbuffers-behavior.c
>> index d914020..15a07ac 100644
>> --- a/tests/spec/gl-1.0/swapbuffers-behavior.c
>> +++ b/tests/spec/gl-1.0/swapbuffers-behavior.c
>> @@ -62,6 +62,7 @@ piglit_display(void)
>>
>>       /* Clear back buffer to green */
>>       glDrawBuffer(GL_BACK);
>> +    glReadBuffer(GL_FRONT);
>
> The comment no longer agrees with the code.
I'll fix that.

>
>>       glClearColor(green[0], green[1], green[2], green[3]);
>>       glClear(GL_COLOR_BUFFER_BIT);
>>
>> @@ -69,7 +70,6 @@ piglit_display(void)
>>       piglit_swap_buffers();
>>
>>           /* Front buffer sanity-check */
>> -    glReadBuffer(GL_FRONT);
>
> Same here.

Actually that comment still makes sense since it's followed by a 
frontbuffer piglit_probe
>
>
>>       if (!piglit_probe_rect_rgb_silent(0, 0, piglit_width, 
>> piglit_height,
>>                         green)) {
>>           printf("SwapBuffers apparently failed!\n");
>>
>
> These changes seem to defeat the purpose of the test.
>
> Would a different approach work?  How about after the initial 
> glViewport call we simply clear the front buffer?  That should trigger 
> creation of the fake front buffer, right?

Yes, as long as we access the front buffer before SwapBuffers we should 
be safe. But I don't quite understand how creating the fake front buffer 
using glReadBuffer() would differ form creating it by a write access 
with respect to the test functionality? All this patch is doing is 
calling glReadBuffer() a little earlier than before?

/Thomas

>
> -Brian
>