linux-kernel - Re: [PATCH v4 3/5] dt-bindings: phy: tegra: Add Tegra194 support

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Date:   Thu, 17 Oct 2019 14:01:28 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     JC Kuo <jckuo@...dia.com>, Rob Herring <robh@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jon Hunter <jonathanh@...dia.com>,
        Kishon Vijay Abraham I <kishon@...com>,
        linux-tegra@...r.kernel.org,
        Linux USB List <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        devicetree@...r.kernel.org, Nagarjuna Kristam <nkristam@...dia.com>
Subject: Re: [PATCH v4 3/5] dt-bindings: phy: tegra: Add Tegra194 support

On Thu, Oct 17, 2019 at 03:48:52PM +0800, JC Kuo wrote:
> Hi Thierry, Hi Rob, Hi Kishon,
> Please let me know your thoughts of the below implementation.
> 
> 1. Add a "bool disable_gen2" to "phy->attrs" structure.
> 2. In _of_phy_get() of phy-core.c to add the follow to parse a generic property.
> 
> 	phy->attrs.disable_gen2 = of_property_read_bool(args.np,
> 							"usb-disable-gen2");

Regarding this, I'm not sure how Rob imagined the generic properties to
work. Perhaps he was thinking about something like the max-link-speed
property found in the PCI bindings.

We could have something like this:

  - max-link-speed:
      If present this property specifies the USB generation supported on
      the PHY/port. Must be:
        1: for USB 3.1 Gen 1 (a.k.a. USB 3.0)
        2: for USB 3.1 Gen 2

I'm not sure if we need to consider anything prior to USB 3.0. I suppose
we could do a similar mapping to what I proposed for the PHY ->set_mode
callback:

  - max-link-speed:
      If present this property specifies the USB generation supported on
      the PHY/port. Must be:
        0x0100: for USB 1.0 (Low-Speed)
        0x0101: for USB 1.1 (Full-Speed)
        0x0200: for USB 2.0 (Hi-Speed)
        0x0300: for USB 3.0 (SuperSpeed) (a.k.a. USB 3.1 Gen 1)
        0x0301: for USB 3.1 (SuperSpeed 10 Gbit/s) (a.k.a. USB 3.1 Gen 2)
        0x0302: for USB 3.2 (SuperSpeed 20 Gbit/s) (a.k.a. USB 3.2 Gen 2 x 2)
        ...

Or those could just be sequentially enumerated, like in the above
example.

Rob, any thoughts?

Thierry

> 3. In individual phy driver, to add SOC/PHY specific programming accordingly.
> 
> Thanks,
> JC
> 
> On 10/14/19 9:40 PM, Rob Herring wrote:
> > On Mon, Oct 14, 2019 at 8:17 AM Thierry Reding <thierry.reding@...il.com> wrote:
> >>
> >> On Wed, Oct 09, 2019 at 06:39:00PM -0500, Rob Herring wrote:
> >>> On Wed, Oct 09, 2019 at 10:43:41AM +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. To deal with
> >>>> this, a new device node property "nvidia,disable-gen2" was added to
> >>>> Tegra194 that be used to specifically disable Gen 2 speed for a
> >>>> particular USB 3.0 port so that the port can be limited to Gen 1 speed
> >>>> and avoid the instability.
> >>>
> >>> I suspect this may be a common issue and we should have a common
> >>> property. Typically, this kind of property is in the controller though
> >>> and supports multiple speed limits. See PCI bindings for inspiration.
> >>
> >> Given that support for gen 2 speeds is dependent on signal trace length,
> >> it doesn't really make sense to restrict the whole controller to a given
> >> speed if only the signal trace for a single port exceeds the limit for
> >> which gen 2 would work.
> >>
> >> Also, the USB PHYs are in a different hardware block than the USB
> >> controller, so this really is a property of the PHY block, not the USB
> >> controller.
> > 
> > Okay, but still should be common for USB PHYs IMO.
> > 
> > Rob
> > 

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