linux-kernel - RE: [PATCH 2/4] usbnet: smsc95xx: align tx-buffer to word

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Date:   Tue, 2 Oct 2018 13:19:07 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Ben Dooks' <ben.dooks@...ethink.co.uk>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     "oneukum@...e.com" <oneukum@...e.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kernel@...ts.codethink.co.uk" 
        <linux-kernel@...ts.codethink.co.uk>
Subject: RE: [PATCH 2/4] usbnet: smsc95xx: align tx-buffer to word

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@...ethink.co.uk>
> [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....

> -	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

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