netdev - RE: [PATCH net] net/mlx4_en: Doorbell is byteswapped in Little Endian archs

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Date:	Mon, 5 Jan 2015 09:46:35 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Amir Vadai' <amirv@...lanox.com>,
	"David S. Miller" <davem@...emloft.net>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Yevgeny Petrilin <yevgenyp@...lanox.com>,
	Wei Yang <weiyang@...ux.vnet.ibm.com>
Subject: RE: [PATCH net] net/mlx4_en: Doorbell is byteswapped in Little
 Endian archs

> iowrite32() will byteswap it's argument on big endian archs.
> iowrite32be() will byteswap on little endian archs.
> Since we don't want to do this unnecessary byteswap on the fast path,
> doorbell is stored in the NIC's native endianness. Using the right
> iowrite() according to the arch endianness.

Someone seems to have forgotten that the byteswap within iowrite32()
on ppc (and more recent sparc) is almost certainly 'free'.
So the value ought to be stored in the byte order required by iowrite32().

	David

> Fixes: 6a4e812 ("net/mlx4_en: Avoid calling bswap in tx fast path")
> Signed-off-by: Amir Vadai <amirv@...lanox.com>
> ---
>  drivers/net/ethernet/mellanox/mlx4/en_tx.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx4/en_tx.c b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> index a308d41..6477cc7 100644
> --- a/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> +++ b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> @@ -962,7 +962,16 @@ netdev_tx_t mlx4_en_xmit(struct sk_buff *skb, struct net_device *dev)
>  		tx_desc->ctrl.owner_opcode = op_own;
>  		if (send_doorbell) {
>  			wmb();
> -			iowrite32(ring->doorbell_qpn,
> +		/* Since there is no iowrite*_native() that writes the value
> +		 * as is, without byteswapping - using the one the doesn't do
> +		 * byteswapping in the relevant arch endianness.
> +		 */
> +#if defined(__LITTLE_ENDIAN)
> +			iowrite32(
> +#else
> +			iowrite32be(
> +#endif
> +				  ring->doorbell_qpn,
>  				  ring->bf.uar->map + MLX4_SEND_DOORBELL);
>  		} else {
>  			ring->xmit_more++;
> --
> 1.9.3

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