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To: Maxime Coquelin <maxime.coquelin@redhat.com>, dev@dpdk.org
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From: Olivier Matz <olivier.matz@6wind.com>
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Date: Mon, 3 Oct 2016 18:11:53 +0200
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Subject: Re: [dpdk-dev] [PATCH 1/2] mbuf: add rte_pktmbuff_reset_headroom
	function
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Hi Maxime,

On 09/29/2016 02:20 PM, Maxime Coquelin wrote:
> Some application use rte_mbuf_raw_alloc() function to improve
> performance by not resetting mbuf's fields to their default state.
> 
> This can be however problematic for mbuf consumers that need some
> headroom, meaning that data_off field gets decremented after
> allocation. When the mbuf is re-used afterwards, there might not
> be enough room for the consumer to prepend anything, if the data_off
> field is not reset to its default value.
> 
> This patch adds a new rte_pktmbuf_reset_headroom() function that
> applications can call to reset the data_off field.
> This patch also replaces current data_off affectations in the mbuf
> lib with a call to this function.
> 
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>

Sounds like a good idea. Just one small comment below.

>  
>  /**
> + * Reset the data_off field of a packet mbuf to its default value.
> + *
> + * The given mbuf must have only one segment.
> + *
> + * @param m
> + *   The packet mbuf's data_off field has to be reset.
> + */
> +static inline void rte_pktmbuf_reset_headroom(struct rte_mbuf *m)
> +{
> +	m->data_off = RTE_MIN(RTE_PKTMBUF_HEADROOM, (uint16_t)m->buf_len);
> +}

Maybe we should also highlight in the API comment that the segment
should be empty.


Thanks,
Olivier