Date: Thu, 27 Jun 2024 17:44:54 +0300
From: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To: Aradhya Bhatia <a-bhatia1@...com>, Rob Herring <robh@...nel.org>,
 Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Jyri Sarha <jyri.sarha@....fi>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 Neil Armstrong <neil.armstrong@...aro.org>, David Airlie
 <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, DRI Development List
 <dri-devel@...ts.freedesktop.org>,
 Devicetree List <devicetree@...r.kernel.org>,
 Linux Kernel List <linux-kernel@...r.kernel.org>, Nishanth Menon
 <nm@...com>, Vignesh Raghavendra <vigneshr@...com>,
 Praneeth Bajjuri <praneeth@...com>, Udit Kumar <u-kumar1@...com>,
 Francesco Dolcini <francesco@...cini.it>,
 Alexander Sverdlin <alexander.sverdlin@...mens.com>,
 Randolph Sapp <rs@...com>, Devarsh Thakkar <devarsht@...com>,
 Jayesh Choudhary <j-choudhary@...com>, Jai Luthra <j-luthra@...com>
Subject: Re: [PATCH 2/4] dt-bindings: display: ti: Add schema for AM625 OLDI
 Transmitter

On 14/05/2024 08:08, Aradhya Bhatia wrote:
> Hi Rob,
> 
> Thank you for reviewing the patches!
> 
> On 14/05/24 01:00, Rob Herring wrote:
>> On Mon, May 13, 2024 at 02:07:44PM +0530, Aradhya Bhatia wrote:
>>> Hi Laurent,
>>>
>>> Thank you for reviewing the patches!
>>>
>>> On 13-May-24 01:04, Laurent Pinchart wrote:
>>>> Hi Aradhya,
>>>>
>>>> Thank you for the patch.
>>>>
>>>> On Sun, May 12, 2024 at 01:00:53AM +0530, Aradhya Bhatia wrote:
>>>>> Add devicetree binding schema for AM625 OLDI Transmitters.
>>>>>
>>>>> Signed-off-by: Aradhya Bhatia <a-bhatia1@...com>
>>>>> ---
>>>>>   .../bindings/display/ti/ti,am625-oldi.yaml    | 153 ++++++++++++++++++
>>>>>   MAINTAINERS                                   |   1 +
>>>>>   2 files changed, 154 insertions(+)
>>>>>   create mode 100644 Documentation/devicetree/bindings/display/ti/ti,am625-oldi.yaml
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/display/ti/ti,am625-oldi.yaml b/Documentation/devicetree/bindings/display/ti/ti,am625-oldi.yaml
>>>>> new file mode 100644
>>>>> index 000000000000..0a96e600bc0b
>>>>> --- /dev/null
>>>>> +++ b/Documentation/devicetree/bindings/display/ti/ti,am625-oldi.yaml
>>>>> @@ -0,0 +1,153 @@
>>>>> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
>>>>> +%YAML 1.2
>>>>> +---
>>>>> +$id: http://devicetree.org/schemas/display/ti/ti,am625-oldi.yaml#
>>>>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>>>>> +
>>>>> +title: Texas Instruments AM625 OLDI Transmitter
>>>>> +
>>>>> +maintainers:
>>>>> +  - Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
>>>>> +  - Aradhya Bhatia <a-bhatia1@...com>
>>>>> +
>>>>> +description: |
>>>>> +  The AM625 TI Keystone OpenLDI transmitter (OLDI TX) supports serialized RGB
>>>>> +  pixel data transmission between host and flat panel display over LVDS (Low
>>>>> +  Voltage Differential Sampling) interface. The OLDI TX consists of 7-to-1 data
>>>>> +  serializers, and 4-data and 1-clock LVDS outputs. It supports the LVDS output
>>>>> +  formats "jeida-18", "jeida-24" and "vesa-18", and can accept 24-bit RGB or
>>>>> +  padded and un-padded 18-bit RGB bus formats as input.
>>>>> +
>>>>> +properties:
>>>>> +  reg:
>>>>> +    maxItems: 1
>>>>> +
>>>>> +  clocks:
>>>>> +    maxItems: 1
>>>>> +    description: serial clock input for the OLDI transmitters
>>>>> +
>>>>> +  clock-names:
>>>>> +    const: s_clk
>>>>> +
>>>>> +  ti,companion-oldi:
>>>>> +    $ref: /schemas/types.yaml#/definitions/phandle
>>>>> +    description:
>>>>> +      phandle to companion OLDI transmitter. This property is mandatory for the
>>>>> +      primarty OLDI TX if the OLDI TXes are expected to work either in dual-lvds
>>>>> +      mode or in clone mode. This property should point to the secondary OLDI
>>>>> +      TX.
>>>>> +
>>>>> +  ti,secondary-oldi:
>>>>> +    type: boolean
>>>>> +    description: Boolean property to mark an OLDI TX as secondary node.
>>>>> +
>>>>> +  ti,oldi-io-ctrl:
>>>>> +    $ref: /schemas/types.yaml#/definitions/phandle
>>>>> +    description:
>>>>> +      phandle to syscon device node mapping OLDI IO_CTRL registers found in the
>>>>> +      control MMR region. This property is needed for OLDI interface to work.
>>>>> +
>>>>> +  ports:
>>>>> +    $ref: /schemas/graph.yaml#/properties/ports
>>>>> +
>>>>> +    properties:
>>>>> +      port@0:
>>>>> +        $ref: /schemas/graph.yaml#/properties/port
>>>>> +        description: Parallel RGB input port
>>>>> +
>>>>> +      port@1:
>>>>> +        $ref: /schemas/graph.yaml#/properties/port
>>>>> +        description: LVDS output port
>>>>> +
>>>>> +    required:
>>>>> +      - port@0
>>>>> +      - port@1
>>>>> +
>>>>> +allOf:
>>>>> +  - if:
>>>>> +      properties:
>>>>> +        ti,secondary-oldi: true
>>>>> +    then:
>>>>> +      properties:
>>>>> +        ti,companion-oldi: false
>>>>> +        ti,oldi-io-ctrl: false
>>>>> +        clocks: false
>>>>> +        clock-names: false
>>>>> +
>>>>> +    else:
>>>>> +      required:
>>>>> +        - ti,oldi-io-ctrl
>>>>> +        - clocks
>>>>> +        - clock-names
>>>>> +
>>>>> +required:
>>>>> +  - reg
>>>>> +  - ports
>>>>> +
>>>>> +additionalProperties: false
>>>>> +
>>>>> +examples:
>>>>> +  - |
>>>>> +    #include <dt-bindings/soc/ti,sci_pm_domain.h>
>>>>> +
>>>>> +    oldi_txes {
>>>>> +        #address-cells = <1>;
>>>>> +        #size-cells = <0>;
>>>>> +        oldi: oldi@0 {
>>>>> +            reg = <0>;
>>>>> +            clocks = <&k3_clks 186 0>;
>>>>> +            clock-names = "s_clk";
>>>>> +            ti,oldi-io-ctrl = <&dss_oldi_io_ctrl>;
>>>>
>>>> What bus does this device live on ? Couldn't the I/O register space be
>>>> referenced by the reg property ?.
>>>>
>>>
>>> These registers are a part of the system-controller register space
>>> (ctrl_mmr0). The whole register set is owned by the main_conf[0]
>>> devicetree node, with sub-nodes pointing to specific regions. That's why
>>> I cannot reference these registers directly.
>>
>> Then what does 'reg' represent? Looks like you just made up an index. If
>> so, then this should probably be a child of &dss_oldi_io_ctrl instead.
>> Or it should just be merged into that node.
>>
> 
> I did make up an index when I used the 'reg' property. Similar to how
> ports can be indexed. The AM65 has 1 OLDI TX. AM62 and AM62P have 2 OLDI
> TXes each. The index is to help the driver parse through each of them.
> 
> If I push these OLDI TX nodes as child nodes under &dss_oldi_io_ctrl,
> then that would be an inaccurate representation of the hardware.
> 
> The OLDI TXes are very well a part of the DSS hardware. As such, the
> (three) registers that control the functionality have been made a part
> of the DSS video-port (VP) register space, leaving OLDI TXes with no
> direct access to the primary bus (cbass_main) where the DSS sits.
> 
> The IO control registers, on the other hand, do not affect OLDI
> functionality in any way. These are just helper registers that merely
> control the power characteristics of the OLDI data and clock lanes.

Just summarizing my understanding from today's code and ref manual (TRM) 
reading, which mostly repeats what Aradhya said above:

There may be 0, 1 or 2 OLDI instances, depending on the SoC. With two 
instances, they may be (or are currently always?) connected to the same 
display controller videoport, and thus can be used for cloning or 
dual-link lvds.

In the TRM the OLDI instances are depicted to be inside DSS (display 
subsystem), but outside DISPC (display controller). However, the 
registers to configure and control OLDI are in the DISPC register block. 
More specifically, in the DISPC VP1 block, which is the videoport to 
which the OLDIs are connected to, and additionally one bit in the 
DSS_SYSSTATUS register.

The oldi-io-ctrl points to a system-control module, which contains a 
register to control OLDI power up/down. But why is it called 
"oldi-io-ctrl"? The register in question is OLDI_PD_CTRL.

So... How to represent this in the DT?

I think we do want have separate nodes for the OLDIs, so that we can use 
the of_graph to represent how the data flows. OLDI doesn't have its own 
register block, and is controlled via DSS. If the OLDI nodes are not 
children of the DSS node, where could they be?

I think the DT design in these patches matches my understanding of the 
hardware.

  Tomi

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