Re: [PATCH RFT 1/2] phylib: add device reset GPIO support

[Sergei Shtylyov]

Hello.

On 05/12/2016 09:42 PM, Uwe Kleine-König wrote:

> [we already talked about this patch in #armlinux, I'm now just
> forwarding my comments on the list. Background was that I sent an easier
> and less complete patch with the same idea. See
> http://patchwork.ozlabs.org/patch/621418/]
>
> [added Linus Walleij to Cc, there is a question for you/him below]
>
> On Fri, Apr 29, 2016 at 01:12:54AM +0300, Sergei Shtylyov wrote:
> > --- net-next.orig/Documentation/devicetree/bindings/net/phy.txt
> > +++ net-next/Documentation/devicetree/bindings/net/phy.txt
> > @@ -35,6 +35,8 @@ Optional Properties:
> >  - broken-turn-around: If set, indicates the PHY device does not correctly
> >    release the turn around line low at the end of a MDIO transaction.
> >
> > +- reset-gpios: The GPIO phandle and specifier for the PHY reset signal.
> > +
> >  Example:
> >
> >  ethernet-phy@0 {
>
> This is great.
>
> > Index: net-next/drivers/net/phy/at803x.c
> > ===================================================================
> > --- net-next.orig/drivers/net/phy/at803x.c
> > +++ net-next/drivers/net/phy/at803x.c
> > @@ -65,7 +65,6 @@ MODULE_LICENSE("GPL");
> > [...]
>
> My patch breaks this driver. I wasn't aware of it.

   I tried to be as careful as I could but still it looks that I didn't 
succeed at that too...

[...]
> > Index: net-next/drivers/net/phy/mdio_device.c
> > ===================================================================
> > --- net-next.orig/drivers/net/phy/mdio_device.c
> > +++ net-next/drivers/net/phy/mdio_device.c
[...]
> > @@ -117,9 +126,16 @@ static int mdio_probe(struct device *dev
> >  	struct mdio_driver *mdiodrv = to_mdio_driver(drv);
> >  	int err = 0;
> >
> > -	if (mdiodrv->probe)
> > +	if (mdiodrv->probe) {
> > +		/* Deassert the reset signal */
> > +		mdio_device_reset(mdiodev, 0);
> > +
> >  		err = mdiodrv->probe(mdiodev);
> >
> > +		/* Assert the reset signal */
> > +		mdio_device_reset(mdiodev, 1);
>
> I wonder if it's safe to do this in general. What if ->probe does
> something with the phy that is lost by resetting but that is relied on
> later?

   Well, I thought that config_init() method is designed for that but indeed 
the LXT driver writes to BMCR in its probe() method and hence is broken. Thank 
you for noticing...

[...]
> > Index: net-next/drivers/of/of_mdio.c
> > ===================================================================
> > --- net-next.orig/drivers/of/of_mdio.c
> > +++ net-next/drivers/of/of_mdio.c
> > @@ -44,6 +44,7 @@ static int of_get_phy_id(struct device_n
> >  static void of_mdiobus_register_phy(struct mii_bus *mdio,
> >  				    struct device_node *child, u32 addr)
> >  {
> > +	struct gpio_desc *gpiod;
> >  	struct phy_device *phy;
> >  	bool is_c45;
> >  	int rc;
> > @@ -52,10 +53,17 @@ static void of_mdiobus_register_phy(stru
> >  	is_c45 = of_device_is_compatible(child,
> >  					 "ethernet-phy-ieee802.3-c45");
> >
> > +	gpiod = fwnode_get_named_gpiod(&child->fwnode, "reset-gpios");
> > +	/* Deassert the reset signal */
> > +	if (!IS_ERR(gpiod))
> > +		gpiod_direction_output(gpiod, 0);
>
> This is wrong I think. You must only ignore -ENODEV, all other error

   At least -ENOSYS should also be ignored (it's returned when gpiolib is not 
configured), right? When does -ENODEV gets returned (it's not easy to follow)?

> codes should be passed to the caller.

   The caller doesn't care anyway...

> (I see that's not trivial because
> of_mdiobus_register_phy returns void.)

   I've made this function *void* in net-next.

> In my patch I used devm_gpiod_get_array which has the nice property that
> I can already pass GPIOD_OUT_LOW in flags. Also this binds the lifetime
> of the gpio to the device which is nice and IMHO the right direction for
> the phylib (i.e. better embracing of the device model).
>
> This cannot be used here easily however because there is no struct
> device yet and this is only created after the phy id is determined.

   Your last patch [1] didn't make use of the managed device API (devm) 
either, I didn't quite get to the bottom of that...

> The
> phy id is either read from the device tree or must be read from the phy
> which might fail if reset is not deasserted.

> Principally there is no reason however that the phy_id must be known
> before the struct device is created however.

   It's just that the code is cleaner that way...

[1] http://paste.debian.net/683630/

> Best regards
> Uwe

MBR, Sergei