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

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Date:	Thu, 5 Jun 2014 09:58:14 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	"'fugang.duan@...escale.com'" <fugang.duan@...escale.com>,
	"davem@...emloft.net" <davem@...emloft.net>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"shawn.guo@...aro.org" <shawn.guo@...aro.org>,
	"Fabio.Estevam@...escale.com" <Fabio.Estevam@...escale.com>,
	"ezequiel.garcia@...e-electrons.com" 
	<ezequiel.garcia@...e-electrons.com>,
	"bhutchings@...arflare.com" <bhutchings@...arflare.com>,
	"stephen@...workplumber.org" <stephen@...workplumber.org>,
	"Frank.Li@...escale.com" <Frank.Li@...escale.com>,
	"eric.dumazet@...il.com" <eric.dumazet@...il.com>
Subject: RE: [PATCH v3 1/6] net: fec: Factorize the .xmit transmit function

From: fugang.duan@...escale.
> > 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@...escale.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.

> Can you give me more training for this ?  Sorry, I don't understand the means.

Single 'if' statements can have measurable performance impact on ethernet
code paths - so it isn't a good idea to add ones that aren't strictly required.

If the function had an extra argument 'tx_ring' (don't add one) which
would be constant at all the call sites and the code read:
	base = tx_ring ? fep->tx_bd_base : fep->rx_bd_base;
when the compiler inlines the function calls it would be able to
optimise away the conditional.

> > 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.
> >
> BDs consumes more than single page, but it use " dma_alloc_coherent()" to allocate continuous memory,
> I don't know why it is cannot do it like this.
> 
> > > +
> > > +	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
> >
> You means:
> if (fep->bufdesc_ex)
> 	bufdesc_size = sizeof(struct bufdesc_ex);
> else
> 	bufdesc_size = sizeof(struct bufdesc);
> index = ((const char *)bdp - (const char *)base)/bufdesc_size;

No, that is still a conditional in the normal path.
Assign fep->bufdesc_size during initialisation.

Then end up with something like:
static
int fec_enet_get_bd_index(struct bufdesc *base, struct bufdesc *bdp,
		struct fec_enet_private *fep)
{
	return ((const char *)bdp - (const char *)base)/fep->bufdesc_size;
}

	David



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