linux-kernel - Re: [PATCH NET v3 1/2] net: phy: Add phy loopback support in net phy framework

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Date:   Sat, 24 Jun 2017 14:15:27 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Andrew Lunn <andrew@...n.ch>
CC:     <f.fainelli@...il.com>, <liguozhu@...ilicon.com>,
        <linuxarm@...wei.com>, <linux-kernel@...r.kernel.org>,
        <netdev@...r.kernel.org>, <davem@...emloft.net>,
        <tremyfr@...il.com>
Subject: Re: [PATCH NET v3 1/2] net: phy: Add phy loopback support in net phy
 framework



On 2017/6/24 11:40, Yunsheng Lin wrote:
> Hi, Andrew
> 
> On 2017/6/24 11:12, Andrew Lunn wrote:
>>> +int phy_loopback(struct phy_device *phydev, bool enable)
>>> +{
>>> +	struct phy_driver *phydrv = to_phy_driver(phydev->mdio.dev.driver);
>>> +	int ret = 0;
>>> +
>>> +	if (enable && phydev->loopback_enabled)
>>> +		return -EBUSY;
>>> +
>>> +	if (!enable && !phydev->loopback_enabled)
>>> +		return -EINVAL;
>>> +
>>> +	if (phydev->drv && phydrv->set_loopback)
>>> +		ret = phydrv->set_loopback(phydev, enable);
>>
>>    	else
>> 		ret = -EOPNOTSUPP;
>>
>>> +
>>> +	if (ret)
>>> +		return ret;
>>> +
>>> +	phydev->loopback_enabled = enable;
>>> +
>>> +	return 0;
>>> +}
>>> +EXPORT_SYMBOL(phy_loopback);
>>
>> One of the comments we made of the PHY code in the hns driver is that
>> its locking is completely broken. You have made the same error
>> here. The core needs to hold the mutex while calling into the PHY
>> driver.
> Do you mean hns_nic_config_phy_loopback need to hold the mutex while
> calling phy_loopback? and other place that calling phy_* function?
I took some time looking into how to take mutex in phy core, here is what
I find:
phy_resume call phydrv->resume without take mutex.
if phy driver implement resume function, for example marvell_resume, then

static int marvell_resume(struct phy_device *phydev)
{
	int err;

	/* Resume the fiber mode first */
	if (!(phydev->supported & SUPPORTED_FIBRE)) {
		err = phy_write(phydev, MII_MARVELL_PHY_PAGE, MII_M1111_FIBER);
		if (err < 0)
			goto error;

		/* With the page set, use the generic resume */
		err = genphy_resume(phydev);
		if (err < 0)
			goto error;

		/* Then, the copper link */
		err = phy_write(phydev, MII_MARVELL_PHY_PAGE, MII_M1111_COPPER);
		if (err < 0)
			goto error;
	}

the code above executes without holding a mutex expect genphy_resume

	/* With the page set, use the generic resume */
	return genphy_resume(phydev);

error:
	phy_write(phydev, MII_MARVELL_PHY_PAGE, MII_M1111_COPPER);
	return err;
}

So I think the correct way to hold a lock is in phy_* function, not in genphy_*,
and current genphy_resume and phy_resume is broken in this way.
Please let me know if I misunderstand the mutex taking in phy core.

Best Regards
Yunsheng Lin