[Mesa-dev] [PATCH 4/7] mesa/main: add USAGE_PERSISTENT_WRITE_MAP flag to buffer UsageHistory

[Nicolai Hähnle]

On 08.01.2016 18:09, Ian Romanick wrote:
> On 01/07/2016 04:57 PM, Nicolai Hähnle wrote:
>> From: Nicolai Hähnle <nicolai.haehnle at amd.com>
>>
>> We will want to disable minmax index caching for buffers that are used in this
>> way.
>
> I think this is too heavy handed.  It seems like what you want is to
> disable caching of draws in the mapped range while it is mapped.  If an
> app has a 16mb buffer, maps the last 8mb, unmaps it, then draws, it
> would be a shame to lose the cache forever.  To avoid having to do all
> the range checking in the draw and cache-invalidate code, you could just
> clear the USAGE_PERSISTENT_WRITE_MAP bit in the unmap.  Right?

I like that idea. An alternative would be to just check the access flags 
in the cache itself, i.e. an additional check like:

if (bufferObj->Mappings[MAP_USER].AccessFlags &
     (GL_MAP_PERSISTENT_BIT | GL_MAP_WRITE_BIT) ==
     (GL_MAP_PERSISTENT_BIT | GL_MAP_WRITE_BIT))
    // don't use the cache

It's a few additional instructions in vbo_use_minmax_cache and a few 
instructions less in _mesa_map_buffer_range. That's probably a good 
trade-off, right?

Otherwise, perhaps the variable UsageHistory should be renamed if it 
contains non-sticky flags...

Cheers,
Nicolai

>
>> ---
>>   src/mesa/main/bufferobj.c | 6 +++++-
>>   src/mesa/main/mtypes.h    | 1 +
>>   2 files changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/mesa/main/bufferobj.c b/src/mesa/main/bufferobj.c
>> index 8fd9a06..ef2b495 100644
>> --- a/src/mesa/main/bufferobj.c
>> +++ b/src/mesa/main/bufferobj.c
>> @@ -2474,9 +2474,13 @@ _mesa_map_buffer_range(struct gl_context *ctx,
>>         assert(bufObj->Mappings[MAP_USER].AccessFlags == access);
>>      }
>>
>> -   if (access & GL_MAP_WRITE_BIT)
>> +   if (access & GL_MAP_WRITE_BIT) {
>>         bufObj->Written = GL_TRUE;
>>
>> +      if (access & GL_MAP_PERSISTENT_BIT)
>> +         bufObj->UsageHistory |= USAGE_PERSISTENT_WRITE_MAP;
>> +   }
>> +
>>   #ifdef VBO_DEBUG
>>      if (strstr(func, "Range") == NULL) { /* If not MapRange */
>>         printf("glMapBuffer(%u, sz %ld, access 0x%x)\n",
>> diff --git a/src/mesa/main/mtypes.h b/src/mesa/main/mtypes.h
>> index 9e4b2c8..4d625da 100644
>> --- a/src/mesa/main/mtypes.h
>> +++ b/src/mesa/main/mtypes.h
>> @@ -1255,6 +1255,7 @@ typedef enum {
>>      USAGE_SHADER_STORAGE_BUFFER = 0x8,
>>      USAGE_TRANSFORM_FEEDBACK_BUFFER = 0x10,
>>      USAGE_PIXEL_PACK_BUFFER = 0x20,
>> +   USAGE_PERSISTENT_WRITE_MAP = 0x40,
>>   } gl_buffer_usage;
>>
>>
>>
>