Subject: Re: [net-next PATCH v3 3/3] net: phy: Add Airoha AN8855 Internal Switch Gigabit PHY

From: Christian Marangi <[email-masked]>
Date: Wed, 6 Nov 2024 19:11:18 +0100

On Wed, Nov 06, 2024 at 05:19:03PM +0100, Andrew Lunn wrote:
> > +static const u8 dsa_r50ohm_table[] = {
> > +	127, 127, 127, 127, 127, 127, 127, 127, 127, 127,
> > +	127, 127, 127, 127, 127, 127, 127, 126, 122, 117,
> > +	112, 109, 104, 101,  97,  94,  90,  88,  84,  80,
> > +	78,  74,  72,  68,  66,  64,  61,  58,  56,  53,
> > +	51,  48,  47,  44,  42,  40,  38,  36,  34,  32,
> > +	31,  28,  27,  24,  24,  22,  20,  18,  16,  16,
> > +	14,  12,  11,   9
> > +};
> > +
> > +static int en8855_get_r50ohm_val(struct device *dev, const char *calib_name,
> > +				 u8 *dest)
> > +{
> > +	u32 shift_sel, val;
> > +	int ret;
> > +	int i;
> > +
> > +	ret = nvmem_cell_read_u32(dev, calib_name, &val);
> > +	if (ret)
> > +		return ret;
> > +
> > +	shift_sel = FIELD_GET(AN8855_SWITCH_EFUSE_R50O, val);
> > +	for (i = 0; i < ARRAY_SIZE(dsa_r50ohm_table); i++)
> > +		if (dsa_r50ohm_table[i] == shift_sel)
> > +			break;
> 
> Is an exact match expected? Should this be >= so the nearest match is
> found?
>

As strange as this is, yes this is what the original code does.

> > +
> > +	if (i < 8 || i >= ARRAY_SIZE(dsa_r50ohm_table))
> > +		*dest = dsa_r50ohm_table[25];
> > +	else
> > +		*dest = dsa_r50ohm_table[i - 8];
> > +
> > +	return 0;
> > +}
> > +
> > +static int an8855_probe(struct phy_device *phydev)
> > +{
> > +	struct device *dev = &phydev->mdio.dev;
> > +	struct device_node *node = dev->of_node;
> > +	struct air_an8855_priv *priv;
> > +	int ret;
> > +
> > +	/* If we don't have a node, skip get calib */
> > +	if (!node)
> > +		return 0;
> 
> phydev->priv will be a NULL pointer, causing problems in
> an8855_config_init()
> 

Quite unlikely scenario since for the switch, defining the internal PHY
in an MDIO node is mandatory but yes it's a fragility.

2 solution:
- I check priv in config_init and skip that section
- I always set phydev->priv 

Solution 1 is safer (handle case where for some reason
en8855_get_r50ohm_val fails (it's really almost impossible)) but error
prone if the PHY gets extended with other parts and priv starts to gets
used for other thing.

Solution 2 require an extra bool to signal full calibrarion read and is
waste more resource (in case calib is not needed...)

Anyway thanks for the review!

-- 
	Ansuel