netdev - RE: [PATCH] usbnet: smsc95xx: simplify tx

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Date:   Wed, 03 Oct 2018 17:25:48 +0100
From:   Ben Dooks <ben.dooks@...ethink.co.uk>
To:     David Laight <David.Laight@...lab.com>
Cc:     netdev@...r.kernel.org, oneukum@...e.com, davem@...emloft.net,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kernel@...ts.codethink.co.uk
Subject: RE: [PATCH] usbnet: smsc95xx: simplify tx_fixup code

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@...ethink.co.uk>
>> ---
>>  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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