[Mesa-dev] [PATCH v2 04/11] gallium: add endian_format field to struct pipe_resource

Tue Apr 19 14:59:26 UTC 2016

On Tue, Apr 19, 2016 at 3:11 PM, Oded Gabbay <oded.gabbay at gmail.com> wrote:
> On Mon, Apr 18, 2016 at 6:03 PM,
> Ilia Mirkin <imirkin at alum.mit.edu> wrote:
>> On Mon, Apr 18, 2016 at 10:47 AM, Oded Gabbay <oded.gabbay at gmail.com> wrote:
>>> On Thu, Apr 14, 2016 at 6:44 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
>>>> On Thu, Apr 14, 2016 at 11:08 AM, Oded Gabbay <oded.gabbay at gmail.com> wrote:
>>>>>> Wouldn't it make more sense to handle such issues in transfer_map?
>>>>>> (i.e. create a staging memory area, and decode into it)? This assumes
>>>>>> that the transfer_map() call has enough information to "do the right
>>>>>> thing". I don't think it does today, but perhaps it could be taught?
>>>>> It doesn't have all the info today, that's for sure. I imagine though
>>>>> we can add parameters to it.
>>>>>
>>>>>> That way everything that's in a pipe_resource is in some
>>>>>> tightly-controlled format, and we specify the LE <-> BE parameters
>>>>>> when converting between CPU-read/written and GPU-read/written data. I
>>>>>> believe this is a better match for what's really happening, too. What
>>>>>> do you think?
>>>>>>
>>>>>>   -ilia
>>>>>
>>>>> Unless I'm missing something, I think, at the end of the day, it will
>>>>> be the same issues as in my solution - per code path per format is a
>>>>> different case. That's because you will still need to "teach"
>>>>> transfer_map, per each transfer per format what to do. So one will
>>>>> need to go and debug every single code path there is in mesa for
>>>>> drawing/copying/reading/textures/etc., like what I did in the last 1.5
>>>>> months. It's a great learning experience but it won't give anything
>>>>> generic.
>>>>>
>>>>> Again, for example, in st_ReadPixels, I imagine you will need to give
>>>>> "different orders" to transfer_map for the two different scenarios -
>>>>> H/W blit and fallback. So what's the gain here ?
>>>>>
>>>>> If I'm missing something, please tell me.
>>>>
>>>> One of us is... let's figure out which one :)
>>>>
>>>> Here's my proposal:
>>>>
>>>> All data stored inside of resources is stored in a driver-happy
>>>> format. The driver ensures that it's stored in proper endianness, etc.
>>>> (Much like it does today wrt proper stride.)
>>>>
>>>> Blitting(/copying) between resources doesn't require any additional
>>>> information, since you have the format(s) of the respective resources,
>>>> and it's all inside the driver, so the driver does whatever it needs
>>>> to do to make it all "work".
>>>>
>>>> *Accessing and modifying* resources (directly) from the CPU is what
>>>> becomes tricky. The state tracker may have incorrect expectations of
>>>> the actual backing data. There are a few different ways to resolve
>>>> this. The one I'm proposing is that you only ever return a pointer to
>>>> the directly underlying data if it matches the CPU's expectations
>>>> (which will only be the case for byte-oriented array formats like
>>>> PIPE_FORMAT_R8G8B8A8_* & co). Everything else, like e.g.
>>>> PIPE_FORMAT_R5G6B5_UNORM and countless others, will have to go through
>>>> a bounce buffer.
>>>>
>>>> At transfer map time, you convert the data from GPU-style to
>>>> CPU-style, and copy back the relevant bits at unmap/flush time.
>>>>
>>>> This presents a nice clean boundary for this stuff. Instead of the
>>>> state tracker trying to guess what the driver will do and feeding it
>>>> endiannesses that it can't possibly guess properly, the tracking logic
>>>> is relegated to the driver, and we extend the interfaces to allow the
>>>> state tracker to access the data in a proper way.
>>>>
>>>> I believe the advantage of this scheme is that beyond adding format
>>>> parameters to pipe_transfer_map() calls, there will not need to be any
>>>> adjustments to the state trackers.
>>>>
>>>> One yet-to-be-resolved issue is what to do about glMapBuffer* - it
>>>> maps a buffer, it's formatless (at map time), and yet the GPU will be
>>>> required to interpret it correctly. We could decree that PIPE_BUFFER
>>>> is just *always* an array of R8_UNORM and thus never needs any type of
>>>> swapping. The driver needs to adjust accordingly to deal with accesses
>>>> that don't fit that pattern (and where parameters can't be fed to the
>>>> GPU to interpret it properly).
>>>>
>>>> I think something like the above will work. And I think it presents a
>>>> cleaner barrier than your proposal, because none of the "this GPU can
>>>> kinda-sorta understand BE, but not everywhere" details are ever
>>>> exposed to the state tracker.
>>>>
>>>> Thoughts?
>>>>
>>>>   -ilia
>>>
>>> Ilia,
>>>
>>> To make the GPU do a conversion during blitting, I need to configure
>>> registers. This is done in a couple of functions in the r600g driver
>>> (r600_translate_texformat, r600_colorformat_endian_swap,
>>> r600_translate_colorformat and r600_translate_colorswap).
>>>
>>> The problem is that transfer_map/unmap don't call directly to those
>>> functions. They call other functions which eventually call those 4
>>> functions. Among those "other" functions, there are several function
>>> calls which are *not* in the r600g driver. i.e. we go back to generic
>>> util functions. For example:
>>>
>>> #0  r600_translate_colorformat
>>> #1  evergreen_init_color_surface
>>> #2  evergreen_set_framebuffer_state
>>> #3  util_blitter_custom_depth_stencil
>>> #4  r600_blit_decompress_depth
>>> #5  r600_texture_transfer_map
>>>
>>> Am I allowed to now pass information from transfer_map/unmap all the
>>> way down to the 4 functions I mentioned through all these layers as
>>> additional parameters ? I preferred to put it in pipe_resource as that
>>> information goes all the way down to those functions, but if I can't
>>> use that, then what's an acceptable alternative ?
>>>
>>> This time, I would like to get an agreement *before* I implement it.
>>
>> Probably a good idea. And as issues are investigated, people's
>> opinions on the "correct" way might shift. Let's think about this...
>>
>> So clearly *a* correct way to handle this would be to stop all the
>> lying. What's the lie? The lie is the PIPE_FORMAT. It talks about e.g.
>> R5G6B5 but makes no mention of the byte layout in memory for those 16
>> bits. Really what we have right now is a format and an *implicit*
>> endian ordering, which is the CPU's. But what happens when the CPU and
>> GPU don't agree?
>>
>> There's a path we could take which would be to add an endianness
>> alongside each format (be it by doubling formats, or an explicit
>> second field). This would be a very far-reaching change though, and I
>> doubt you'll want to do it. What we're left with is having a format
>> and an *implicit* endianness. Which means that the consumers of the
>> format need to be able to work out the implicit endianness involved.
>> And the endianness will be GPU endian for regular resources, and CPU
>> endian for "staging" resources. So it's definitely tempting to stick
>> the endian thing into a private field of the resource, like Rob is
>> suggesting - when creating a staging texture in
>> transfer_map/unmap/flush, set the endianness the cpu endian. Otherwise
>> set it to gpu endian. And I think this is somewhat similar to your
>> former approach.
>>
>> What do you think?
>>
>>   -ilia
>
> I don't think I have any other choice but to stick it as a private
> field, because the endian parameter simple can't go through all the
> function calls as an additional parameter. The reason is that
> set_framebuffer_state() function types are called from
> st_invalidate_state, where I don't have any idea about the "correct"
> endianess, so I can't add the endian parameter to that function type.
>
> The only thing that is propagated through all layers is r600_texture.
> I'll try to use that.
>
> Marek, Michel,
> Do you think it is OK to add the endian mark to that private structure ?

Yes, but doesn't util_format_description::is_array provide that info already?

Marek