netdev - Re: [PATCH net-next 1/5 v2] net: gemini: Look up L3 maxlen from table

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Date:   Sat, 7 Jul 2018 18:14:09 +0200
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>,
        Janos Laube <janos.dev@...il.com>,
        Paulius Zaleckas <paulius.zaleckas@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        Hans Ulli Kroll <ulli.kroll@...glemail.com>,
        Florian Fainelli <f.fainelli@...il.com>
Subject: Re: [PATCH net-next 1/5 v2] net: gemini: Look up L3 maxlen from table

On Wed, Jul 04, 2018 at 08:33:20PM +0200, Linus Walleij wrote:
> The code to calculate the hardware register enumerator
> for the maximum L3 length isn't entirely simple to read.
> Use the existing defines and rewrite the function into a
> table look-up.

A matter of habit. ;-)
I think that if you just renamed 'n' to 'best_index' and avoided
reusing 'max_l3_len' (as you already did) the idea could be made
more obvious without expading the code to twice the lines.

The max_len[] array in the original code documents how the HW reacts
to a specific setting.

Acked-by: Michał Mirosław <mirq-linux@...e.qmqm.pl>

Best Regards,
Michał Mirosław

> 
> Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
> ---
> ChangeLog v1->v2:
> - No changes, just resending with the rest.
> ---
>  drivers/net/ethernet/cortina/gemini.c | 61 ++++++++++++++++++++-------
>  1 file changed, 46 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/net/ethernet/cortina/gemini.c b/drivers/net/ethernet/cortina/gemini.c
> index 6d7404f66f84..8fc31723f700 100644
> --- a/drivers/net/ethernet/cortina/gemini.c
> +++ b/drivers/net/ethernet/cortina/gemini.c
> @@ -401,26 +401,57 @@ static int gmac_setup_phy(struct net_device *netdev)
>  	return 0;
>  }
>  
> -static int gmac_pick_rx_max_len(int max_l3_len)
> -{
> -	/* index = CONFIG_MAXLEN_XXX values */
> -	static const int max_len[8] = {
> -		1536, 1518, 1522, 1542,
> -		9212, 10236, 1518, 1518
> -	};
> -	int i, n = 5;
> +/* The maximum frame length is not logically enumerated in the
> + * hardware, so we do a table lookup to find the applicable max
> + * frame length.
> + */
> +struct gmac_max_framelen {
> +	unsigned int max_l3_len;
> +	u8 val;
> +};
>  
> -	max_l3_len += ETH_HLEN + VLAN_HLEN;
> +static const struct gmac_max_framelen gmac_maxlens[] = {
> +	{
> +		.max_l3_len = 1518,
> +		.val = CONFIG0_MAXLEN_1518,
> +	},
> +	{
> +		.max_l3_len = 1522,
> +		.val = CONFIG0_MAXLEN_1522,
> +	},
> +	{
> +		.max_l3_len = 1536,
> +		.val = CONFIG0_MAXLEN_1536,
> +	},
> +	{
> +		.max_l3_len = 1542,
> +		.val = CONFIG0_MAXLEN_1542,
> +	},
> +	{
> +		.max_l3_len = 9212,
> +		.val = CONFIG0_MAXLEN_9k,
> +	},
> +	{
> +		.max_l3_len = 10236,
> +		.val = CONFIG0_MAXLEN_10k,
> +	},
> +};
> +
> +static int gmac_pick_rx_max_len(unsigned int max_l3_len)
> +{
> +	const struct gmac_max_framelen *maxlen;
> +	int maxtot;
> +	int i;
>  
> -	if (max_l3_len > max_len[n])
> -		return -1;
> +	maxtot = max_l3_len + ETH_HLEN + VLAN_HLEN;
>  
> -	for (i = 0; i < 5; i++) {
> -		if (max_len[i] >= max_l3_len && max_len[i] < max_len[n])
> -			n = i;
> +	for (i = 0; i < ARRAY_SIZE(gmac_maxlens); i++) {
> +		maxlen = &gmac_maxlens[i];
> +		if (maxtot <= maxlen->max_l3_len)
> +			return maxlen->val;
>  	}
>  
> -	return n;
> +	return -1;
>  }
>  
>  static int gmac_init(struct net_device *netdev)
> -- 
> 2.17.1
>