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SubjectRE: [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@codethink.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
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes,
> MK1 1PT, UK
> Registration No: 1397386 (Wales)

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