RE: [PATCH] usbnet: smsc95xx: simplify tx_fixup code

[Ben Dooks]

On 2018-10-03 14:36, David Laight wrote:
> From: Ben Dooks
> > Sent: 02 October 2018 17:56
> >
> > The smsc95xx_tx_fixup is doing multiple calls to skb_push() to
> > put an 8-byte command header onto the packet. It would be easier
> > to do one skb_push() and then copy the data in once the push is
> > done.
> >
> > Signed-off-by: Ben Dooks <ben.dooks@xxxxxxxxxxxxxxx>
> > ---
> >  drivers/net/usb/smsc95xx.c | 25 +++++++++++++------------
> >  1 file changed, 13 insertions(+), 12 deletions(-)
> >
> > diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
> > index cb19aea139d3..813ab93ee2c3 100644
> > --- a/drivers/net/usb/smsc95xx.c
> > +++ b/drivers/net/usb/smsc95xx.c
> > @@ -2006,6 +2006,7 @@ static struct sk_buff *smsc95xx_tx_fixup(struct 
> > usbnet *dev,
> >  	bool csum = skb->ip_summed == CHECKSUM_PARTIAL;
> >  	int overhead = csum ? SMSC95XX_TX_OVERHEAD_CSUM : 
> > SMSC95XX_TX_OVERHEAD;
> >  	u32 tx_cmd_a, tx_cmd_b;
> > +	void *ptr;
>
> It might be useful to define a structure for the header.
> You might need to find the 'store unaligned 32bit word' macro though.
> (Actually that will probably be better than the memcpy() which might
> end up doing memory-memory copies rather than storing the register.)
> Although if/when you add the tx alignment that won't be needed because 
> the
> header will be aligned.

Ok, might be worth doing.

I did try to do a "u32 tx_cmd[2]" but the code generated ended up 
storing
stuff onto the stack before copying into the packet. I agree that 
possibly
going to the "put_unaligned" function might be nicer too.

If we did enable tx-align all the time then we'd not have to care about 
the
alignment, but I didn't want to do that if possible as that would end up
sending up to 3 bytes extra per packet.

I am trying not too do too many changes at one time to allow roll back.

> >  	/* We do not advertise SG, so skbs should be already linearized */
> >  	BUG_ON(skb_shinfo(skb)->nr_frags);
> > @@ -2019,6 +2020,9 @@ static struct sk_buff *smsc95xx_tx_fixup(struct 
> > usbnet *dev,
> >  		return NULL;
> >  	}
> >
> > +	tx_cmd_b = (u32)skb->len;
> > +	tx_cmd_a = tx_cmd_b | TX_CMD_A_FIRST_SEG_ | TX_CMD_A_LAST_SEG_;
> > +
> >  	if (csum) {
> >  		if (skb->len <= 45) {
> >  			/* workaround - hardware tx checksum does not work
> > @@ -2035,21 +2039,18 @@ static struct sk_buff 
> > *smsc95xx_tx_fixup(struct usbnet *dev,
> >  			skb_push(skb, 4);
> >  			cpu_to_le32s(&csum_preamble);
>
> Not related, but csum_preamble = cpu_to_le32(csum_preamble) is likely 
> to
> generate better code (at least for some architectures).
>
> 	David
>
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