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Most PPC chips have been bi-endian for a long time but, as far as I know, everyone treated it as a big-endian processor. For example, it's big-endian all the wa

Most PPC chips have been bi-endian for a long time but, as far as I know, everyone treated it as a big-endian processor. For example, it's big-endian all the way in the automotive industry, where PPC is incredibly popular. Where has the demand come from to start supporting little-endian mode suddenly? I assume the existence of a Debian/Ubuntu port is evidence of such demand.

rwmj 13y ago
IBM have been leading the work and contributing a lot of patches for ppc64le in Fedora.
pedrocr 13y ago
>Where has the demand come from to start supporting little-endian mode suddenly? I found a Debian discussion on this and the only reasons presented were GPU computing and porting apps that assume little-endian. I assume the GPU computing is the big reason. As we move to unified memory architectures between CPU and GPU they need to both use the same endianness and I guess GPUs are usually little-endian to match x86.
- dman 13y ago
  Can you post the link about GPU computing. I am a bit perplexed about what powerpc has to do with GPUs.
  - _delirium 13y ago
    > I am a bit perplexed about what powerpc has to do with GPUs. As others note that IBM is funding most of the work, my guess is the main use-case here is IBM's POWER-based HPC clusters that have PowerPC CPUs augmented by a bunch of GPU coprocessors for Cuda/OpenCL offloading. IBM is trying to position POWER clusters as competing with x86-based clusters for certain kinds of work (especially scientific computing), and to match x86-based clusters decked out with GPUs they probably need to have GPU options for their POWER-based clusters as well. The idea, as I read it (have not had occasion to encounter it myself) is that things are a lot easier if your CPU and GPU have the same endianness, or else you have to byte-swap whenever transferring data to/from the GPU (and there are even more complications if you have unified memory spaces). Since most (all?) commercially available GPUs are little-endian, if you want PowerPC CPUs along with GPUs for auxiliary processing, and want the endianness matched, the little-endian mode of PowerPC becomes important.
    - dman 13y ago
      Interesting!
gsnedders 13y ago
POWER8 (not yet shipping) has far better little-endian support than prior PPC chips (many converted to/from big-endian when in little-endian mode!), as many customers of IBM apparently want better little-endian support (either to interface with existing little-endian hardware, to be more similar to other hardware, etc.), and IBM is paying for almost all the engineering work going on for the ppc64le ports. IBM need the software to work to be able to sell the hardware, so they're investing a lot in this.

rwmj 13y ago

IBM have been leading the work and contributing a lot of patches for ppc64le in Fedora.

pedrocr 13y ago

>Where has the demand come from to start supporting little-endian mode suddenly? I found a Debian discussion on this and the only reasons presented were GPU computing and porting apps that assume little-endian. I assume the GPU computing is the big reason. As we move to unified memory architectures between CPU and GPU they need to both use the same endianness and I guess GPUs are usually little-endian to match x86.

dman 13y ago
Can you post the link about GPU computing. I am a bit perplexed about what powerpc has to do with GPUs.
- _delirium 13y ago
  > I am a bit perplexed about what powerpc has to do with GPUs. As others note that IBM is funding most of the work, my guess is the main use-case here is IBM's POWER-based HPC clusters that have PowerPC CPUs augmented by a bunch of GPU coprocessors for Cuda/OpenCL offloading. IBM is trying to position POWER clusters as competing with x86-based clusters for certain kinds of work (especially scientific computing), and to match x86-based clusters decked out with GPUs they probably need to have GPU options for their POWER-based clusters as well. The idea, as I read it (have not had occasion to encounter it myself) is that things are a lot easier if your CPU and GPU have the same endianness, or else you have to byte-swap whenever transferring data to/from the GPU (and there are even more complications if you have unified memory spaces). Since most (all?) commercially available GPUs are little-endian, if you want PowerPC CPUs along with GPUs for auxiliary processing, and want the endianness matched, the little-endian mode of PowerPC becomes important.
  - dman 13y ago
    Interesting!

gsnedders 13y ago

POWER8 (not yet shipping) has far better little-endian support than prior PPC chips (many converted to/from big-endian when in little-endian mode!), as many customers of IBM apparently want better little-endian support (either to interface with existing little-endian hardware, to be more similar to other hardware, etc.), and IBM is paying for almost all the engineering work going on for the ppc64le ports. IBM need the software to work to be able to sell the hardware, so they're investing a lot in this.