From: Ben Dooks <ben.dooks@codethink.co.uk>
To: David Laight <David.Laight@ACULAB.COM>,
"netdev@vger.kernel.org" <netdev@vger.kernel.org>
Cc: "oneukum@suse.com" <oneukum@suse.com>,
"davem@davemloft.net" <davem@davemloft.net>,
"linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-kernel@lists.codethink.co.uk"
<linux-kernel@lists.codethink.co.uk>
Subject: Re: [PATCH 2/4] usbnet: smsc95xx: align tx-buffer to word
Date: Tue, 2 Oct 2018 14:35:59 +0100 [thread overview]
Message-ID: <ee3ad7ff-3110-67ff-a4a4-3e647f311dbd@codethink.co.uk> (raw)
In-Reply-To: <59988ed22559410881addfecf58335eb@AcuMS.aculab.com>
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@codethink.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....
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
https://www.codethink.co.uk/privacy.html
next prev parent reply other threads:[~2018-10-02 13:43 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-02 9:26 SMSC95XX driver updates Ben Dooks
2018-10-02 9:26 ` [PATCH 1/4] usbnet: smsc95xx: add kconfig for turbo mode Ben Dooks
2018-10-02 12:46 ` Andrew Lunn
2018-10-02 9:26 ` [PATCH 2/4] usbnet: smsc95xx: align tx-buffer to word Ben Dooks
2018-10-02 13:19 ` David Laight
2018-10-02 13:35 ` Ben Dooks [this message]
2018-10-02 16:56 ` [PATCH] usbnet: smsc95xx: simplify tx_fixup code Ben Dooks
2018-10-03 13:36 ` David Laight
2018-10-03 16:25 ` Ben Dooks
2018-10-05 21:24 ` David Miller
2018-10-06 11:27 ` Ben Dooks
2018-10-06 17:28 ` David Miller
2018-10-08 8:41 ` David Laight
2018-10-04 16:53 ` [Linux-kernel] [PATCH 2/4] usbnet: smsc95xx: align tx-buffer to word Ben Hutchings
2018-10-02 9:26 ` [PATCH 3/4] usbnet: smsc95xx: check for csum being in last four bytes Ben Dooks
2018-10-02 9:45 ` Sergei Shtylyov
2018-10-04 16:55 ` [Linux-kernel] " Ben Hutchings
2018-10-02 9:26 ` [PATCH 4/4] usbnet: smsc95xx: fix rx packet alignment Ben Dooks
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