linux-kernel - Re: [PATCH 2/3] dt-bindings: phy: Add lane<n>-mode property to WIZ (SERDES wrapper)

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Date:   Mon, 30 Dec 2019 11:21:55 -0700
From:   Rob Herring <robh@...nel.org>
To:     Jyri Sarha <jsarha@...com>
Cc:     Kishon Vijay Abraham I <kishon@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        devicetree@...r.kernel.org, Tomi Valkeinen <tomi.valkeinen@...com>,
        Praneeth Bajjuri <praneeth@...com>,
        Yuti Amonkar <yamonkar@...ence.com>,
        Swapnil Kashinath Jakhade <sjakhade@...ence.com>,
        Roger Quadros <rogerq@...com>
Subject: Re: [PATCH 2/3] dt-bindings: phy: Add lane<n>-mode property to WIZ
 (SERDES wrapper)

On Mon, Dec 30, 2019 at 2:37 AM Jyri Sarha <jsarha@...com> wrote:
>
> On 24/12/2019 23:31, Rob Herring wrote:
> > On Fri, Dec 20, 2019 at 5:52 AM Jyri Sarha <jsarha@...com> wrote:
> >>
> >> On 19/12/2019 21:08, Rob Herring wrote:
> >>> On Mon, Dec 09, 2019 at 06:22:11PM +0200, Jyri Sarha wrote:
> >>>> Add property to indicate the usage of SERDES lane controlled by the
> >>>> WIZ wrapper. The wrapper configuration has some variation depending on
> >>>> how each lane is going to be used.
> >>>>
> >>>> Signed-off-by: Jyri Sarha <jsarha@...com>
> >>>> ---
> >>>>  .../devicetree/bindings/phy/ti,phy-j721e-wiz.yaml    | 12 ++++++++++++
> >>>>  1 file changed, 12 insertions(+)
> >>>>
> >>>> diff --git a/Documentation/devicetree/bindings/phy/ti,phy-j721e-wiz.yaml b/Documentation/devicetree/bindings/phy/ti,phy-j721e-wiz.yaml
> >>>> index 94e3b4b5ed8e..399725f65278 100644
> >>>> --- a/Documentation/devicetree/bindings/phy/ti,phy-j721e-wiz.yaml
> >>>> +++ b/Documentation/devicetree/bindings/phy/ti,phy-j721e-wiz.yaml
> >>>> @@ -97,6 +97,18 @@ patternProperties:
> >>>>        Torrent SERDES should follow the bindings specified in
> >>>>        Documentation/devicetree/bindings/phy/phy-cadence-dp.txt
> >>>>
> >>>> +  "^lane[1-4]-mode$":
> >>>> +    allOf:
> >>>> +      - $ref: /schemas/types.yaml#/definitions/uint32
> >>>> +      - enum: [0, 1, 2, 3, 4, 5, 6]
> >>>> +    description: |
> >>>> +     Integer describing static lane usage for the lane indicated in
> >>>> +     the property name. For Sierra there may be properties lane0 and
> >>>> +     lane1, for Torrent all lane[1-4]-mode properties may be
> >>>> +     there. The constants to indicate the lane usage are defined in
> >>>> +     "include/dt-bindings/phy/phy.h". The lane is assumed to be unused
> >>>> +     if its lane<n>-use property does not exist.
> >>>
> >>> The defines were intended to be in 'phys' cells. Does putting both lane
> >>> and mode in the client 'phys' properties not work?
> >>>
> >>
> >> Let me first check if I understood you. So you are suggesting something
> >> like this:
> >>
> >> dp-phy {
> >>         #phy-cells = <5>; /* 1 for phy-type and 4 for lanes = 5 */
> >>         ...
> >> };
> >>
> >> dp-bridge {
> >>         ...
> >>         phys = <&dp-phy PHY_TYPE_DP 1 1 0 0>; /* lanes 0 and 1 for DP */
> >
> > Yes, but I think the lanes can be a single cell mask. And I'd probably
> > make that the first cell which is generally "which PHY" and make
> > type/mode the 2nd cell. I'd look for other users of PHY_TYPE_ defines
> > and match what they've done if possible.
> >
>
> I see. This will cause some head ache on the driver implementation side,
> as there is no way for the phy driver to peek the lane use or type from
> the phy client's device tree node.

Yes, there is a way. Not really fast, but use
for_each_node_with_property(node, "phys") and filter on ones matching
your phy's node.

> It also looks to me that the phy
> API[1] has to be extended quite a bit before the phy client can pass the
> lane usage information to the phy driver. It will cause some pain to
> implement the extension without breaking the phy API and causing a nasty
> cross dependency over all the phy client domains.

Not really a concern from a binding standpoint. Bindings shouldn't be
designed around some OS's current design or limitations.

There's already several cases using PHY_TYPE_* in phy cells, so I'm
not sure what the issue is.

> Also, there is not much point in putting the PHY_TYPE constant to the
> phy client's node, as normally the phy client driver will know quite
> well what PHY_TYPE to use. E.g. a SATA driver will always select
> PHY_TYPE_SATA and a PCIE driver will select PHY_TYPE_PCIE, etc.

Good point. That could work as well.

> Kishon, if we have to take this road it also starts to sound like we
> will have to move the phy client's phandle to point to the phy wrapper
> node, if we want to keep the actual phy driver wrapper agnostic. Then we
> can make the wrapper to act like a proxy that forwards the phy_ops calls
> to the actual phy driver. Luckily the per lane phy-type selection is not
> a blocker for our j721e DisplayPort functionality.
>
> Best regards,
> Jyri
>
> [1] include/linux/phy/phy.h
>
>
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