RE: [PATCH v3 1/6] net: fec: Factorize the .xmit transmit function

[David Laight <David.Laight@ACULAB.COM>]

From: Fugang Duan
> Make the code more readable and easy to support other features like
> SG, TSO, moving the common transmit function to one api.
> 
> And the patch also factorize the getting BD index to it own function.
> 
> Signed-off-by: Fugang Duan <B38611@freescale.com>
> ---
>  drivers/net/ethernet/freescale/fec_main.c |   87 ++++++++++++++++++-----------
>  1 files changed, 54 insertions(+), 33 deletions(-)
> 
> diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
> index 802be17..32c2276 100644
> --- a/drivers/net/ethernet/freescale/fec_main.c
> +++ b/drivers/net/ethernet/freescale/fec_main.c
> @@ -287,6 +287,25 @@ struct bufdesc *fec_enet_get_prevdesc(struct bufdesc *bdp, struct fec_enet_priva
>  		return (new_bd < base) ? (new_bd + ring_size) : new_bd;
>  }
> 
> +static inline
> +int fec_enet_get_bd_index(struct bufdesc *bdp, struct fec_enet_private *fep)
> +{
> +	struct bufdesc *base;
> +	int index;
> +
> +	if (bdp >= fep->tx_bd_base)
> +		base = fep->tx_bd_base;
> +	else
> +		base = fep->rx_bd_base;

You really don't want the above conditional.
It is known from the call site - but the compiler can't determine that
and remove the test.
You may not want to rely on the tx and rx descriptors being allocated
as a single entity - particularly now that they (probably) consume
more than a single page.

> +
> +	if (fep->bufdesc_ex)
> +		index = (struct bufdesc_ex *)bdp - (struct bufdesc_ex *)base;
> +	else
> +		index = bdp - base;

Save the sizeof the descriptor structure as (say) fep->bufdesc_size
(Possibly replacing bufdesc_ex)
and then just calculate:
	index = ((const char *)bdp - (const char *)base)/fep->bufdesc_size;

	David