Reenabled linux-xen and xen-image build

[people/pmueller/ipfire-2.x.git] / src / patches / suse-2.6.27.25 / patches.arch / ppc-spufs-07-Don-t-spu_acquire_saved-unnecessarily.patch

Subject: Don't spu_acquire_saved unnecessarily in regs read
From: Jeremy Kerr <jk@ozlabs.org>
References: 447133 - LTC50070

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>
Signed-off-by: Olaf Hering <olh@suse.de>
---
 arch/powerpc/platforms/cell/spufs/file.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/arch/powerpc/platforms/cell/spufs/file.c
+++ b/arch/powerpc/platforms/cell/spufs/file.c
@@ -551,6 +551,11 @@ spufs_regs_read(struct file *file, char 
 	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;