[Mesa-dev] [PATCH 2/2] util/u_helpers: return correct number of bound buffers

[Patrick Rudolph]

Ok, first of all bind some buffers:

pipe->set_vertex_buffers(pipe, 0, 1, &vtxbuf);
pipe->set_vertex_buffers(pipe, 1, 1, &vtxbuf);
pipe->set_vertex_buffers(pipe, 2, 1, &vtxbuf);

num_vtxbufs is now 3 as it should be.

Now you are unbinding buffers, one after another starting at slot 0:
pipe->set_vertex_buffers(pipe, 0, 1, NULL);

enabled_buffers = (1ull << num_vtxbufs) - 1 = 0x7;
util_set_vertex_buffers_mask(...);
enabled_buffers = 0x6;
num_vtxbufs = util_last_bit(enabled_buffers) = 3


pipe->set_vertex_buffers(pipe, 1, 1, NULL);

enabled_buffers = (1ull << num_vtxbufs) - 1 = 0x7;
util_set_vertex_buffers_mask(...);
enabled_buffers = 0x5;
num_vtxbufs = util_last_bit(enabled_buffers) = 3


pipe->set_vertex_buffers(pipe, 2, 1, NULL);

enabled_buffers = (1ull << num_vtxbufs) - 1 = 0x7;
util_set_vertex_buffers_mask(...);
enabled_buffers = 0x3;
num_vtxbufs = util_last_bit(enabled_buffers) = 2

There are no buffers bound any more, but num_vtxbufs is now 2 instead of
0.

There would be no problem if you start at slot 2 going down to 0.
There would be no problem if you unbind all buffers at once.

I hope this clarifies the problem.

Kind Regards,
Patrick

On 2015-12-09 07:10 PM, Ilia Mirkin wrote:
> I'm probably just being dense... can you provide an exact sequence of
> calls that would cause this logic to fail? Seems like it should work
> as-is...
> 
> On Sun, Dec 6, 2015 at 4:12 AM, Patrick Rudolph <siro at das-labor.org> wrote:
>> In case a state tracker unbinds every slot by a seperate
>> pipe->set_vertex_buffers() call, starting from slot zero, the number
>> of bound buffers would not reach zero at all. Unbinding all buffers
>> at once or starting at the top-most slot results in correct behaviour.
>>
>> Calculating the correct number of bound buffers fixes a NULL pointer
>> dereference in nvc0_validate_vertex_buffers_shared().
>>
>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93004
>>
>> Signed-off-by: Patrick Rudolph <siro at das-labor.org>
>> ---
>>  src/gallium/auxiliary/util/u_helpers.c | 8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/gallium/auxiliary/util/u_helpers.c b/src/gallium/auxiliary/util/u_helpers.c
>> index 09619c1..09020b0 100644
>> --- a/src/gallium/auxiliary/util/u_helpers.c
>> +++ b/src/gallium/auxiliary/util/u_helpers.c
>> @@ -81,7 +81,13 @@ void util_set_vertex_buffers_count(struct pipe_vertex_buffer *dst,
>>                                     const struct pipe_vertex_buffer *src,
>>                                     unsigned start_slot, unsigned count)
>>  {
>> -   uint32_t enabled_buffers = (1ull << *dst_count) - 1;
>> +   unsigned i;
>> +   uint32_t enabled_buffers = 0;
>> +
>> +   for (i = 0; i < *dst_count; i++) {
>> +      if (dst[i].buffer || dst[i].user_buffer)
>> +         enabled_buffers |= (1ull << i);
>> +   }
>>
>>     util_set_vertex_buffers_mask(dst, &enabled_buffers, src, start_slot,
>>                                  count);
>> --
>> 2.4.3
>>
>> _______________________________________________
>> mesa-dev mailing list
>> mesa-dev at lists.freedesktop.org
>> http://lists.freedesktop.org/mailman/listinfo/mesa-dev