Re: [PATCH 2/4] usbnet: smsc95xx: align tx-buffer to word

[Ben Dooks]

On 02/10/18 14:19, David Laight wrote:
> From: Ben Dooks
> > Sent: 02 October 2018 10:27
> >
> > The tegra driver requires alignment of the buffer, so try and
> > make this better by pushing the buffer start back to an word
> > aligned address. At the worst this makes memcpy() easier as
> > it is word aligned, at best it makes sure the usb can directly
> > map the buffer.
> >
> > Signed-off-by: Ben Dooks <ben.dooks@xxxxxxxxxxxxxxx>
> > [todo - make this configurable]
> > ---
> >   drivers/net/usb/Kconfig    | 12 ++++++++++++
> >   drivers/net/usb/smsc95xx.c | 22 ++++++++++++++++++++--
> >   2 files changed, 32 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/usb/Kconfig b/drivers/net/usb/Kconfig
> ...
> > +static bool align_tx = IS_ENABLED(CONFIG_USB_NET_SMSC95XX_TXALIGN);
> > +module_param(align_tx, bool, 0644);
> > +MODULE_PARM_DESC(align_tx, "Align TX buffers to word boundaries");
>
> DM doesn't like module parameters.
>
> >   static bool turbo_mode = IS_ENABLED(CONFIG_USB_NET_SMSC95XX_TURBO);
> >   module_param(turbo_mode, bool, 0644);
> >   MODULE_PARM_DESC(turbo_mode, "Enable multiple frames per Rx transaction");
> > @@ -2005,10 +2009,18 @@ 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;
> > +	u32 data_len;
> > +	uintptr_t align = 0;
> >
> >   	/* We do not advertise SG, so skbs should be already linearized */
> >   	BUG_ON(skb_shinfo(skb)->nr_frags);
> >
> > +	if (IS_ENABLED(CONFIG_USB_NET_SMSC95XX_TXALIGN) && align_tx) {
> > +		align = (uintptr_t)skb->data & 3;
> > +		if (align)
> > +			overhead += 4 - align;
>
> Better to calculate the pad size once:
> 		align = (-(long)skb->data) & 3;
> should do it - and you can unconditionally add it in.
>
> > +	}
> > +
> >   	/* Make writable and expand header space by overhead if required */
> >   	if (skb_cow_head(skb, overhead)) {
> >   		/* Must deallocate here as returning NULL to indicate error
> > @@ -2037,16 +2049,22 @@ static struct sk_buff *smsc95xx_tx_fixup(struct usbnet *dev,
> >   		}
> >   	}
> >
> > +	data_len = skb->len;
> > +	if (align)
> > +		skb_push(skb, 4 - align);
> > +
> >   	skb_push(skb, 4);
>
> You don't want to call skb_push() twice.
> IIRC really horrid things happen if the data has to be copied.
> (Actually what happens to the alignment in that case??)
> And there is another skb_push() below....

The driver does it /multiple/ times depending on the path used.
Is it wise to try and make a separate patch to skb_push() once
and also move the tx_cmd_a and tx_cmd_b bit to a single point?

> > -	tx_cmd_b = (u32)(skb->len - 4);
> > +	tx_cmd_b = (u32)(data_len);
>
> You don't need the cast here at all (if it was ever needed).
> Actually you don't need the new 'data_len' variable.
> Just set tx_cmd_b earlier.
>
> ...
>
> 	David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)


--
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

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