Re: [PATCH v5 3/5] dt-bindings: phy: tegra: Add Tegra194 support

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

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On Thu, Jan 09, 2020 at 09:31:27AM +0800, JC Kuo wrote:
> Hi Rob,
> For now we have two options.
> 1. reusing the existing "maximum-speed" which is documented in usb/generic.txt
> Pro: no need to add new property
> Con: only "super-speed" and "super-speed-plus" are valid for Tegra XUSB PADCTL,
> therefore extra code/document change is required.
> 
> +	if (device_property_present(&port->dev, "maximum-speed")) {
> +		maximum_speed =  usb_get_maximum_speed(&port->dev);
> +		if (maximum_speed == USB_SPEED_SUPER)
> +			usb3->disable_gen2 = true;
> +		else if (maximum_speed == USB_SPEED_SUPER_PLUS)
> +			usb3->disable_gen2 = false;
> +		else
> +			return -EINVAL;
> +	}
> 
> 2. introducing a new proprietary "nvidia,disable-gen2" property
> Pro: its logic perfectly matches the need, and code change is minimum
> 
> +        usb3->disable_gen2 = of_property_read_bool(np, "nvidia,disable-gen2");
> 
> Con: it's a new and proprietary property.
> 
> Please let me know which one do you prefer or there is something else
> works better.

I think the first version is much clearer. maximum-speed = "super-speed"
is very clear and explicit. nvidia,disable-gen2 is less so. While it may
be true in this case what "disable-gen2" does, using the generic
bindings has the advantage that it'll be more familiar to people already
familiar with other USB device tree bindings.

Also, the nvidia,disable-gen2 property only perfectly matches the need
because it reflects the name of the variable. If you rewrote the code to
do something like this:

	if (port->maximum_speed <= USB_SPEED_SUPER) {
		/* disable gen2 */
		...
	}

Then all of a sudden the "maximum-speed" property is a perfect match. In
general, bindings should be natural to the domain of the bus or device
that they describe rather than a reflection of how the driver programs
the device's registers.

On a side-note: we should also update the usb/generic.txt binding to
describe the "super-speed-plus" value for the maximum-speed property.

Thierry

> Thanks,
> JC
> 
> 
> On 1/6/20 3:10 PM, JC Kuo wrote:
> > On 1/4/20 6:39 AM, Rob Herring wrote:
> >> On Fri, Jan 03, 2020 at 04:18:12PM +0800, JC Kuo wrote:
> >>> Extend the bindings to cover the set of features found in Tegra194.
> >>> Note that, technically, there are four more supplies connected to the
> >>> XUSB pad controller (DVDD_PEX, DVDD_PEX_PLL, HVDD_PEX and HVDD_PEX_PLL)
> >>> , but the power sequencing requirements of Tegra194 require these to be
> >>> under the control of the PMIC.
> >>>
> >>> Tegra194 XUSB PADCTL supports up to USB 3.1 Gen 2 speed, however, it
> >>> is possible for some platforms have long signal trace that could not
> >>> provide sufficient electrical environment for Gen 2 speed. This patch
> >>> adds a "maximum-speed" property to usb3 ports which can be used to
> >>> specify the maximum supported speed for any particular USB 3.1 port.
> >>> For a port that is not capable of SuperSpeedPlus, "maximum-speed"
> >>> property should carry "super-speed".
> >>>
> >>> Signed-off-by: JC Kuo <jckuo-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> >>> ---
> >>> Changes in v5:
> >>> - re-use "maximum-speed" instead of adding "nvidia,disable-gen2"
> >>> Changes in v4: none
> >>> Changes in v3: none
> >>> Changes in v2:
> >>> - fix a typo
> >>>
> >>>  .../phy/nvidia,tegra124-xusb-padctl.txt        | 18 ++++++++++++++++++
> >>>  1 file changed, 18 insertions(+)
> >>>
> >>> diff --git a/Documentation/devicetree/bindings/phy/nvidia,tegra124-xusb-padctl.txt b/Documentation/devicetree/bindings/phy/nvidia,tegra124-xusb-padctl.txt
> >>> index 9fb682e47c29..7d0089006e67 100644
> >>> --- a/Documentation/devicetree/bindings/phy/nvidia,tegra124-xusb-padctl.txt
> >>> +++ b/Documentation/devicetree/bindings/phy/nvidia,tegra124-xusb-padctl.txt
> >>> @@ -37,6 +37,7 @@ Required properties:
> >>>    - Tegra132: "nvidia,tegra132-xusb-padctl", "nvidia,tegra124-xusb-padctl"
> >>>    - Tegra210: "nvidia,tegra210-xusb-padctl"
> >>>    - Tegra186: "nvidia,tegra186-xusb-padctl"
> >>> +  - Tegra194: "nvidia,tegra194-xusb-padctl"
> >>>  - reg: Physical base address and length of the controller's registers.
> >>>  - resets: Must contain an entry for each entry in reset-names.
> >>>  - reset-names: Must include the following entries:
> >>> @@ -62,6 +63,10 @@ For Tegra186:
> >>>  - vclamp-usb-supply: Bias rail for USB pad. Must supply 1.8 V.
> >>>  - vddio-hsic-supply: HSIC PHY power supply. Must supply 1.2 V.
> >>>  
> >>> +For Tegra194:
> >>> +- avdd-usb-supply: USB I/Os, VBUS, ID, REXT, D+/D- power supply. Must supply
> >>> +  3.3 V.
> >>> +- vclamp-usb-supply: Bias rail for USB pad. Must supply 1.8 V.
> >>>  
> >>>  Pad nodes:
> >>>  ==========
> >>> @@ -154,6 +159,11 @@ For Tegra210, the list of valid PHY nodes is given below:
> >>>  - sata: sata-0
> >>>    - functions: "usb3-ss", "sata"
> >>>  
> >>> +For Tegra194, the list of valid PHY nodes is given below:
> >>> +- usb2: usb2-0, usb2-1, usb2-2, usb2-3
> >>> +  - functions: "xusb"
> >>> +- usb3: usb3-0, usb3-1, usb3-2, usb3-3
> >>> +  - functions: "xusb"
> >>>  
> >>>  Port nodes:
> >>>  ===========
> >>> @@ -221,6 +231,11 @@ Optional properties:
> >>>    is internal. In the absence of this property the port is considered to be
> >>>    external.
> >>>  
> >>> +- maximum-speed: Only for Tegra194. A string property that specifies maximum
> >>> +  supported speed of a usb3 port. Valid values are:
> >>> +  - "super-speed-plus": default, the usb3 port supports USB 3.1 Gen 2 speed.
> >>
> >> Not defined as a valid value in usb/generic.txt. '-gen2' instead of 
> >> '-plus' would be clearer IMO. However, is there any need to define the 
> >> maximum speed possible? The purpose of this property is to limit the 
> >> speed below the max.
> >>
> > usb_get_maximum_speed(), which parses "maximum-speed" property, indeed handles
> > string "super-speed-plus". Should "super-speed-plus" be documented in
> > usb/generic.txt"?
> > 
> > static const char *const speed_names[] = {
> > 	[USB_SPEED_UNKNOWN] = "UNKNOWN",
> > 	[USB_SPEED_LOW] = "low-speed",
> > 	[USB_SPEED_FULL] = "full-speed",
> > 	[USB_SPEED_HIGH] = "high-speed",
> > 	[USB_SPEED_WIRELESS] = "wireless",
> > 	[USB_SPEED_SUPER] = "super-speed",
> > 	[USB_SPEED_SUPER_PLUS] = "super-speed-plus",
> > };
> > 
> > A proprietary "nvidia,disable-gen2" property was proposed in earlier revision to
> > "limit the speed below the max". I like it because it fit our needs better and
> > requires only one line of code change.
> > 
> >    usb3->disable_gen2 = of_property_read_bool(np, "nvidia,disable-gen2");
> > 
> > Should I fallback to that approach?
> > 
> > Thanks,
> > JC
> > 
> >>> +  - "super-speed": the usb3 port supports USB 3.1 Gen 1 speed only.
> >>> +
> >>>  For Tegra124 and Tegra132, the XUSB pad controller exposes the following
> >>>  ports:
> >>>  - 3x USB2: usb2-0, usb2-1, usb2-2
> >>> @@ -233,6 +248,9 @@ For Tegra210, the XUSB pad controller exposes the following ports:
> >>>  - 2x HSIC: hsic-0, hsic-1
> >>>  - 4x super-speed USB: usb3-0, usb3-1, usb3-2, usb3-3
> >>>  
> >>> +For Tegra194, the XUSB pad controller exposes the following ports:
> >>> +- 4x USB2: usb2-0, usb2-1, usb2-2, usb2-3
> >>> +- 4x super-speed USB: usb3-0, usb3-1, usb3-2, usb3-3
> >>>  
> >>>  Examples:
> >>>  =========
> >>> -- 
> >>> 2.17.1
> >>>

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