Re: [PATCH v3 2/2] PCI: tegra: Support per-lane PHYs

[Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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On Wed, Mar 16, 2016 at 11:01:19AM -0600, Stephen Warren wrote:
> On 03/08/2016 08:48 AM, Thierry Reding wrote:
> > From: Thierry Reding <treding-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> > 
> > The current XUSB pad controller bindings are insufficient to describe
> > PHY devices attached to USB controllers. New bindings have been created
> > to overcome these restrictions. As a side-effect each root port now is
> > assigned a set of PHY devices, one for each lane associated with the
> > root port. This has the benefit of allowing fine-grained control of the
> > power management for each lane.
> 
> > diff --git a/drivers/pci/host/pci-tegra.c b/drivers/pci/host/pci-tegra.c
> 
> > +static int tegra_pcie_port_phy_power_on(struct tegra_pcie_port *port)
> > +{
> > +	struct device *dev = port->pcie->dev;
> > +	unsigned int i;
> > +	int err;
> > +
> > +	for (i = 0; i < port->lanes; i++) {
> > +		err = phy_power_on(port->phys[i]);
> 
> This assume the number of array entries is precisely the number of lanes.
> That seems to contradict the binding update which said the number might not
> match. Perhaps there's an expectation that phy_power_on() is a no-op for
> some "invalid" values like NULL or an error-pointer value? But...
> 
> > +static struct phy *devm_of_phy_optional_get_index(struct device *dev,
> > +						  struct device_node *np,
> > +						  const char *consumer,
> > +						  unsigned int index)
> > +{
> > +	struct phy *phy;
> > +	char *name;
> > +
> > +	name = kasprintf(GFP_KERNEL, "%s-%u", consumer, index);
> > +	if (!name)
> > +		return ERR_PTR(-ENOMEM);
> > +
> > +	phy = devm_of_phy_get(dev, np, name);
> > +	kfree(name);
> > +
> > +	if (IS_ERR(phy) && PTR_ERR(phy) == -ENODEV)
> > +		phy = NULL;
> > +
> > +	return phy;
> > +}
> 
> The error-handling there looks wrong. The function generally returns either
> a valid PHY or an error pointer. However, in the case of -ENODEV, NULL is
> returned. Subsystems are supposed to encode their handles as, and functions
> are supposed to return, either NULL or an error pointer for error cases, not
> both/either. Is the PHY API broken in this regard? If so, then this code is
> fine, but if not it might need a fix.

This function mimics phy_optional_get() which similarily returns NULL
for -ENODEV. The remainder of the PHY API treats NULL pointers as
"dummy" PHYs and returns early. I think that's a sensible approach to
handling optional resources.

It might have been more obvious had I implemented this function within
phy-core.c, but I didn't think it universally useful because it uses a
rather uncommon lookup pattern. I did keep a generic name in case it's
ever deemed useful outside of this driver, at which point it could
simply be moved into phy-core.c without requiring this driver to
change.

Thierry

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