[PATCH 1/2] drm/nouveau: replace ffsll with __ffs64

[Ben Skeggs]

On Mon, Feb 10, 2014 at 6:51 AM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> The ffsll function is a lot slower than the __ffs64 built-in which
> compiles to a single instruction on 64-bit. It's also nice to avoid
> custom versions of standard functions. Note that __ffs == ffs - 1.
>
> Signed-off-by: Ilia Mirkin <imirkin at alum.mit.edu>
Merged both patches in the series.

Thanks.
> ---
>
> I wrote a user-space program to test these out and make sure that the
> functions behaved as expected. The logic in abi16 had to be flipped around a
> bit since __ffs doesn't distinguish between 0 and 1. There's a minor
> difference in that init->channel is going to get returned as 0 for ENOSPC vs
>  -1, but I can't imagine that'd matter.
>
>  drivers/gpu/drm/nouveau/core/core/parent.c |  2 +-
>  drivers/gpu/drm/nouveau/core/os.h          | 11 -----------
>  drivers/gpu/drm/nouveau/nouveau_abi16.c    |  4 ++--
>  3 files changed, 3 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/gpu/drm/nouveau/core/core/parent.c b/drivers/gpu/drm/nouveau/core/core/parent.c
> index 313380c..dee5d12 100644
> --- a/drivers/gpu/drm/nouveau/core/core/parent.c
> +++ b/drivers/gpu/drm/nouveau/core/core/parent.c
> @@ -49,7 +49,7 @@ nouveau_parent_sclass(struct nouveau_object *parent, u16 handle,
>
>         mask = nv_parent(parent)->engine;
>         while (mask) {
> -               int i = ffsll(mask) - 1;
> +               int i = __ffs64(mask);
>
>                 if (nv_iclass(parent, NV_CLIENT_CLASS))
>                         engine = nv_engine(nv_client(parent)->device);
> diff --git a/drivers/gpu/drm/nouveau/core/os.h b/drivers/gpu/drm/nouveau/core/os.h
> index 191e739..3cd6120 100644
> --- a/drivers/gpu/drm/nouveau/core/os.h
> +++ b/drivers/gpu/drm/nouveau/core/os.h
> @@ -23,17 +23,6 @@
>
>  #include <asm/unaligned.h>
>
> -static inline int
> -ffsll(u64 mask)
> -{
> -       int i;
> -       for (i = 0; i < 64; i++) {
> -               if (mask & (1ULL << i))
> -                       return i + 1;
> -       }
> -       return 0;
> -}
> -
>  #ifndef ioread32_native
>  #ifdef __BIG_ENDIAN
>  #define ioread16_native ioread16be
> diff --git a/drivers/gpu/drm/nouveau/nouveau_abi16.c b/drivers/gpu/drm/nouveau/nouveau_abi16.c
> index 900fae0..b701117 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_abi16.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_abi16.c
> @@ -270,8 +270,8 @@ nouveau_abi16_ioctl_channel_alloc(ABI16_IOCTL_ARGS)
>                 return nouveau_abi16_put(abi16, -EINVAL);
>
>         /* allocate "abi16 channel" data and make up a handle for it */
> -       init->channel = ffsll(~abi16->handles);
> -       if (!init->channel--)
> +       init->channel = __ffs64(~abi16->handles);
> +       if (~abi16->handles == 0)
>                 return nouveau_abi16_put(abi16, -ENOSPC);
>
>         chan = kzalloc(sizeof(*chan), GFP_KERNEL);
> --
> 1.8.3.2
>
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