Re: [PATCH net-next] net: dsa: mt7530: support setting ageing time

Vladimir Oltean
2020-12-08T20:56:42Z

On Tue, Dec 08, 2020 at 03:00:28PM +0800, DENG Qingfang wrote:
> MT7530 has a global address age control register, so use it to set
> ageing time.
>
> The applied timer is (AGE_CNT + 1) * (AGE_UNIT + 1) seconds
>
> Signed-off-by: DENG Qingfang <dqfext@gmail.com>
> ---
> drivers/net/dsa/mt7530.c | 41 ++++++++++++++++++++++++++++++++++++++++
> drivers/net/dsa/mt7530.h | 13 +++++++++++++
> 2 files changed, 54 insertions(+)
>
> diff --git a/drivers/net/dsa/mt7530.c b/drivers/net/dsa/mt7530.c
> index 6408402a44f5..99bf8fed6536 100644
> --- a/drivers/net/dsa/mt7530.c
> +++ b/drivers/net/dsa/mt7530.c
> @@ -870,6 +870,46 @@ mt7530_get_sset_count(struct dsa_switch *ds, int port, int sset)
> return ARRAY_SIZE(mt7530_mib);
> }
>
> +static int
> +mt7530_set_ageing_time(struct dsa_switch *ds, unsigned int msecs)
> +{
> + struct mt7530_priv *priv = ds->priv;
> + unsigned int secs = msecs / 1000;
> + unsigned int tmp_age_count;
> + unsigned int error = -1;
> + unsigned int age_count;
> + unsigned int age_unit;
> +
> + /* Applied timer is (AGE_CNT + 1) * (AGE_UNIT + 1) seconds */
> + if (secs < 1 || secs > (AGE_CNT_MAX + 1) * (AGE_UNIT_MAX + 1))
> + return -ERANGE;
> +
> + /* iterate through all possible age_count to find the closest pair */
> + for (tmp_age_count = 0; tmp_age_count <= AGE_CNT_MAX; ++tmp_age_count) {
> + unsigned int tmp_age_unit = secs / (tmp_age_count + 1) - 1;
> +
> + if (tmp_age_unit <= AGE_UNIT_MAX) {
> + unsigned int tmp_error = secs -
> + (tmp_age_count + 1) * (tmp_age_unit + 1);
> +
> + /* found a closer pair */
> + if (error > tmp_error) {
> + error = tmp_error;
> + age_count = tmp_age_count;
> + age_unit = tmp_age_unit;
> + }

I feel that the error calculation is just snake oil. This would be enough:

if (tmp_age_unit <= AGE_UNIT_MAX)
break;

Explanation:

You are given a number X, and must find A and B such that the error
E = |(A x B) - X| should be minimum, with
1 <= A <= A_max
1 <= B <= B_max

It is logical to try with A=1 first. If X / A <= B_max, then of course,
B = X / 1, and the error E is 0.

If that doesn't work out, and B > B_max, then you go to A=2. That gives
you another B = X / 2, and the error E is 1. You check again if B <=
B_max. If it is, that's your answer. B = X / 2, with an error E of 1.

You get my point. Iterating ascendingly through A, and calculating B as
X / A, already gives you the smallest error as soon as it satisfies the
B <= B_max requirement.

> +
> + /* found the exact match, so break the loop */
> + if (!error)
> + break;
> + }
> + }
> +
> + mt7530_write(priv, MT7530_AAC, AGE_CNT(age_count) | AGE_UNIT(age_unit));
> +
> + return 0;
> +}
> +
> static void mt7530_setup_port5(struct dsa_switch *ds, phy_interface_t interface)
> {
> struct mt7530_priv *priv = ds->priv;
> @@ -2564,6 +2604,7 @@ static const struct dsa_switch_ops mt7530_switch_ops = {
> .phy_write = mt753x_phy_write,
> .get_ethtool_stats = mt7530_get_ethtool_stats,
> .get_sset_count = mt7530_get_sset_count,
> + .set_ageing_time = mt7530_set_ageing_time,
> .port_enable = mt7530_port_enable,
> .port_disable = mt7530_port_disable,
> .port_change_mtu = mt7530_port_change_mtu,
> diff --git a/drivers/net/dsa/mt7530.h b/drivers/net/dsa/mt7530.h
> index ee3523a7537e..32d8969b3ace 100644
> --- a/drivers/net/dsa/mt7530.h
> +++ b/drivers/net/dsa/mt7530.h
> @@ -161,6 +161,19 @@ enum mt7530_vlan_egress_attr {
> MT7530_VLAN_EGRESS_STACK = 3,
> };
>
> +/* Register for address age control */
> +#define MT7530_AAC 0xa0
> +/* Disable ageing */
> +#define AGE_DIS BIT(20)
> +/* Age count */
> +#define AGE_CNT_MASK GENMASK(19, 12)
> +#define AGE_CNT_MAX 0xff
> +#define AGE_CNT(x) (AGE_CNT_MASK & ((x) << 12))
> +/* Age unit */
> +#define AGE_UNIT_MASK GENMASK(11, 0)
> +#define AGE_UNIT_MAX 0xfff
> +#define AGE_UNIT(x) (AGE_UNIT_MASK & (x))
> +
> /* Register for port STP state control */
> #define MT7530_SSP_P(x) (0x2000 + ((x) * 0x100))
> #define FID_PST(x) ((x) & 0x3)
> --
> 2.25.1