[Cbe-oss-dev] [PATCH] powerpc/spufs: don't spu_acquire_saved unnecessarily in regs read

[Jeremy Kerr]

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 at ozlabs.org>

---
 arch/powerpc/platforms/cell/spufs/file.c |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/powerpc/platforms/cell/spufs/file.c b/arch/powerpc/platforms/cell/spufs/file.c
index f139cd8..b6f7b91 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;