Re: [PATCH 09/11 v2] RDMA/cxgb4: Support on-chip SQs.

[Roland Dreier <rdreier-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org>]

 > We would remove it I guess.   I see that mlx4 assumes
 > pgprot_writecombine() exists and works correctly across all platforms.
 > See mlx4_ib_mmap().

It might be more accurate to say that mlx4 assumes pgprot_writecombine()
is defined on all platforms.  Any platform that hasn't defined it (or
where the concept of write combining doesn't make sense) just falls back
to defining it as pgprot_noncached().

>From a quick grep it seems that /arch/powerpc has no definition of
pgprot_writecombine() but your code seems to say that one could be
given.

 > If all platforms support pgprot_writecombine() correctly, then we
 > don't need ocqp_supported().

As I said, it's a bit tricky.  All platforms get some definition of
pgprot_writecombine(), but some platforms might define it as just giving
a non-cached mapping, which might lead to dire performance (perhaps
worse than just putting the SQ in host memory).  I'm not sure what the
best way to handle it is -- I suppose if you wanted to get fancy you
could do a quick benchmark on startup (if such a thing is possible).

 - R.
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