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

Tue Apr 19 18:13:21 UTC 2016

On Tue, Apr 19, 2016 at 5:59 PM, Marek Olšák <maraeo at gmail.com> wrote:
> 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

If you mean to say that they are interchangeable, than no.
It's true that for array formats we *never* need to do swaps, but for
non-array formats there are cases where we need to do it and cases
where we don't need to do it.

For example, when writing to depth buffer through st_DrawPixels
(glDrawPixels) with GL_FLOAT, than the format used is
PIPE_FORMAT_Z32_FLOAT.
In the make_texture() part, we need to configure endian swap for the
staging buffer, but after you return from it and go into
st_create_texture_sampler_view(), you want the destination texture to
be configured without endian swapping. Both those buffers are
configured with PIPE_FORMAT_Z32_FLOAT, so that's why is_array can't be
used instead of a dedicated flag.

And that's just the example I had now. There are more examples.

Oded