netdev - Re: [PATCH net-next 11/13] net: ethernet: aquantia: Refactoring buffers copying.

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Date:   Wed, 15 Feb 2017 22:31:18 +0100
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     Pavel Belous <Pavel.Belous@...antia.com>,
        "David S . Miller" <davem@...emloft.net>
Cc:     netdev@...r.kernel.org,
        Simon Edelhaus <Simon.Edelhaus@...antia.com>,
        Alexey Andriyanov <Alexey.Andriyanov@...antia.com>
Subject: Re: [PATCH net-next 11/13] net: ethernet: aquantia: Refactoring
 buffers copying.

On 15.02.2017 21:01, Pavel Belous wrote:
> From: Pavel Belous <pavel.belous@...antia.com>
>
> This fix simplified copying data to the ring buffer.
> Also, there was an error in the code when the second memcpy
> is called with zero length. It didn't break the driver, but it's bad.
>
> Signed-off-by: Pavel Belous <pavel.belous@...antia.com>
> ---
>  drivers/net/ethernet/aquantia/atlantic/aq_ring.c | 25 ++++++++++++------------
>  1 file changed, 13 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/net/ethernet/aquantia/atlantic/aq_ring.c b/drivers/net/ethernet/aquantia/atlantic/aq_ring.c
> index 4c40644..8ebed0d 100644
> --- a/drivers/net/ethernet/aquantia/atlantic/aq_ring.c
> +++ b/drivers/net/ethernet/aquantia/atlantic/aq_ring.c
> @@ -108,18 +108,19 @@ void aq_ring_tx_append_buffs(struct aq_ring_s *self,
>  			     struct aq_ring_buff_s *buffer,
>  			     unsigned int buffers)
>  {
> -	if (likely(self->sw_tail + buffers < self->size)) {
> -		memcpy(&self->buff_ring[self->sw_tail], buffer,
> -		       sizeof(buffer[0]) * buffers);
> -	} else {
> -		unsigned int first_part = self->size - self->sw_tail;
> -		unsigned int second_part = buffers - first_part;
> -
> -		memcpy(&self->buff_ring[self->sw_tail], buffer,
> -		       sizeof(buffer[0]) * first_part);
> -
> -		memcpy(&self->buff_ring[0], &buffer[first_part],
> -		       sizeof(buffer[0]) * second_part);
> +	int buff_len = min(self->size - self->sw_tail, buffers);
> +
> +	memcpy(&self->buff_ring[self->sw_tail],
> +	       buffer,
> +	       sizeof(struct aq_ring_buff_s) * buff_len);
> +
> +	/* We are in the end of the ring.
> +	 *  Copy remains data to beginning of the ring
> +	 */
> +	if (buffers > buff_len) {
> +		memcpy(self->buff_ring,
> +		       &buffer[buff_len],
> +		       sizeof(struct aq_ring_buff_s) * (buffers - buff_len));
>  	}
>  }

Well, you should really try to avoid copying the tx buffers _at all_.
E.g. by passing self->buff_ring to aq_ring_tx_append_buffs() instead of
the temporary array.

Regards,
Lino
buffer array.