Thread (36 messages) 36 messages, 6 authors, 2011-10-10

RE: [PATCH] mlx4_en: fix transmit of packages when blue frame isenabled

From: David Laight <hidden>
Date: 2011-10-06 14:11:35
Also in: linuxppc-dev

 
 static void mlx4_bf_copy(unsigned long *dst, unsigned long *src,
unsigned bytecnt) {
+	int i;
+	__le32 *psrc = (__le32 *)src;
+
+	/*
+	 * the buffer is already in big endian. For little endian
machines that's
+	 * fine. For big endain machines we must swap since the chipset
swaps again
+	 */
+	for (i = 0; i < bytecnt / 4; ++i)
+		psrc[i] = le32_to_cpu(psrc[i]);
+
 	__iowrite64_copy(dst, src, bytecnt / 8);
 }
That code looks horrid...
1) I'm not sure the caller expects the buffer to be corrupted.
2) It contains a lot of memory cycles.
3) It looked from the calls that this code is copying descriptors,
   so the transfer length is probably 1 or 2 words - so the loop
   is inefficient.
4) ppc doesn't have a fast byteswap instruction (very new gcc might
   use the byteswapping memery access for the le32_to_cpu() though),
   so it would be better getting the byteswap done inside
   __iowrite64_copy() - since that is probably requesting a byteswap
   anyway.
OTOH I'm not at all clear about the 64bit xfers....
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