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

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6D8AE76@saturn3.aculab.com>
Date:	Thu, 6 Oct 2011 15:10:28 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Eli Cohen" <eli@....mellanox.co.il>,
	"Thadeu Lima de Souza Cascardo" <cascardo@...ux.vnet.ibm.com>
Cc:	"Eli Cohen" <eli@...lanox.co.il>, <netdev@...r.kernel.org>,
	<linuxppc-dev@...ts.ozlabs.org>,
	"Yevgeny Petrilin" <yevgenyp@...lanox.co.il>
Subject: RE: [PATCH] mlx4_en: fix transmit of packages when blue frame isenabled

>  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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