powerpc/spufs: Don't spu_acquire_saved unnecessarily in regs read

With most file readers (eg cat, dd), reading a context's regs file will
result in two reads: the first to read the data, and the second to
return EOF. Because each read performs a spu_acquire_saved, we end up
descheduling and re-scheduling the context twice.

This change does a simple check to see if we'd return EOF before
calling spu_acquire_saved(), saving the extra schedule operation.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/arch/powerpc/platforms/cell/spufs/file.c b/arch/powerpc/platforms/cell/spufs/file.c
index f139cd8b594d..b6f7b917b56c 100644
--- a/arch/powerpc/platforms/cell/spufs/file.c
+++ b/arch/powerpc/platforms/cell/spufs/file.c
@@ -548,6 +548,11 @@ spufs_regs_read(struct file *file, char __user *buffer,
         int ret;
         struct spu_context *ctx = file->private_data;
 
+        /* pre-check for file position: if we'd return EOF, there's no point
+         * causing a deschedule */
+        if (*pos >= sizeof(ctx->csa.lscsa->gprs))
+                return 0;
+
         ret = spu_acquire_saved(ctx);
         if (ret)
                 return ret;