[xdp-hints] Re: [RFC bpf-next 01/23] ice: make RX hash reading code more reusable

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Larysa Zaremba <larysa.zaremba@intel.com>
Date: Thu, 24 Aug 2023 21:26:40 +0200

> Previously, we only needed RX hash in skb path,
> hence all related code was written with skb in mind.
> But with the addition of XDP hints via kfuncs to the ice driver,
> the same logic will be needed in .xmo_() callbacks.
> 
> Separate generic process of reading RX hash from a descriptor
> into a separate function.
> 
> Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>

I like the patch, except three minors above,

> ---
>  drivers/net/ethernet/intel/ice/ice_txrx_lib.c | 37 +++++++++++++------
>  1 file changed, 26 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/ice/ice_txrx_lib.c b/drivers/net/ethernet/intel/ice/ice_txrx_lib.c
> index c8322fb6f2b3..8f7f6d78f7bf 100644
> --- a/drivers/net/ethernet/intel/ice/ice_txrx_lib.c
> +++ b/drivers/net/ethernet/intel/ice/ice_txrx_lib.c
> @@ -63,28 +63,43 @@ static enum pkt_hash_types ice_ptype_to_htype(u16 ptype)
>  }
>  
>  /**
> - * ice_rx_hash - set the hash value in the skb
> + * ice_get_rx_hash - get RX hash value from descriptor
> + * @rx_desc: specific descriptor
> + *
> + * Returns hash, if present, 0 otherwise.
> + */
> +static u32
> +ice_get_rx_hash(const union ice_32b_rx_flex_desc *rx_desc)

The whole declaration could easily fit into one line :>

> +{
> +	const struct ice_32b_rx_flex_desc_nic *nic_mdid;
> +
> +	if (rx_desc->wb.rxdid != ICE_RXDID_FLEX_NIC)

Not really related: have you tried to measure branch hit/miss here?
Can't it be a candidate for unlikely()?

> +		return 0;
> +
> +	nic_mdid = (struct ice_32b_rx_flex_desc_nic *)rx_desc;
> +	return le32_to_cpu(nic_mdid->rss_hash);

I think the common convention in the kernel is to separate the last
return from the main body with a newline.
To not leave the cast above alone, you can embed it into the declaration.

	const struct ice_32b_rx_flex_desc_nic *mdid = (typeof(mdid))rx_desc;

This is a compile-time cast w/o any maths anyway, so doing it before
checking for the descriptor type doesn't hurt in any way.

	if (!= FLEX)
		return 0;

	return le32_ ...

(or via a ternary)

> +}

[...]

Thanks,
Olek