[Mesa-dev] Are gallium unpack_rgba_8unorm/pack_rgba_8unorm safe for in-place conversion ?

[Jose Fonseca]

On 09/02/16 19:17, Axel Davy wrote:
> Hi,
>
> We'd need to do some formats conversion in gallium nine, and if possible
> we would like to do them in-place.
>
> unpack_rgba_8unorm/pack_rgba_8unorm doesn't seem to explicitly allow
> in-place conversion,
> but the generated code seems to be fine with that.
>
> Can we rely on these functions to be safe for in-place conversion ?
> Could we add that somewhere as requirement ?
>
> CC-ing vmware guys, as they probably know.
>
> Yours,
>
> Axel Davy

It might work in practice now, but I don't think this is same it should 
be dependent upon.

Take BGRA <-> RGBA: currently we read the dword, swizzle, and write 
dword back, but it's an implementation detail.  One could imagine at 
some point the implementation be changed to read/write bytes at a time, 
which would break if done in place.


My recommendation is, if you want to do this things in place: malloc a 
temp with the size of a row of pixels, convert into temp row then memcpy 
into src.

You can even add util_convert_format_inplace() helper if you want.

And if there's any hot-path (e.g., BGRA<->RGBA) just add a special code 
path to this new util_convert_format_inplace that avoids the temp copy.

Jose