Re: [PATCH V3 05/13] drm: bridge: icn6211: Add DSI lane count DT property parsing

[Marek Vasut <marex@denx.de>]

On 3/11/22 21:38, Jagan Teki wrote:
> Hi Marek,
> 
> Small correction in the previous comment.
> 
> On Sat, Mar 12, 2022 at 2:05 AM Jagan Teki <jagan@amarulasolutions.com> wrote:
>>
>> Hi Marek,
>>
>> On Sat, Mar 12, 2022 at 1:32 AM Marek Vasut <marex@denx.de> wrote:
>>>
>>> On 3/11/22 17:29, Maxime Ripard wrote:
>>>> On Fri, Mar 11, 2022 at 11:36:58AM +0100, Marek Vasut wrote:
>>>>> On 3/10/22 15:18, Maxime Ripard wrote:
>>>>>> On Thu, Mar 10, 2022 at 01:47:13PM +0100, Marek Vasut wrote:
>>>>>>> On 3/10/22 11:53, Maxime Ripard wrote:
>>>>>>>> On Tue, Mar 08, 2022 at 10:41:05PM +0100, Marek Vasut wrote:
>>>>>>>>> On 3/8/22 17:21, Maxime Ripard wrote:
>>>>>>>>>> On Tue, Mar 08, 2022 at 03:47:22PM +0100, Marek Vasut wrote:
>>>>>>>>>>> On 3/8/22 14:49, Maxime Ripard wrote:
>>>>>>>>>>>> On Tue, Mar 08, 2022 at 02:27:40PM +0100, Marek Vasut wrote:
>>>>>>>>>>>>> On 3/8/22 13:51, Maxime Ripard wrote:
>>>>>>>>>>>>>> On Tue, Mar 08, 2022 at 11:29:59AM +0100, Marek Vasut wrote:
>>>>>>>>>>>>>>> On 3/8/22 11:07, Jagan Teki wrote:
>>>>>>>>>>>>>>>> On Tue, Mar 8, 2022 at 3:19 PM Marek Vasut <marex@denx.de> wrote:
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> On 3/8/22 09:03, Jagan Teki wrote:
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Hi,
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> [...]
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> @@ -314,7 +321,9 @@ static const struct drm_bridge_funcs chipone_bridge_funcs = {
>>>>>>>>>>>>>>>>>>>           static int chipone_parse_dt(struct chipone *icn)
>>>>>>>>>>>>>>>>>>>           {
>>>>>>>>>>>>>>>>>>>                  struct device *dev = icn->dev;
>>>>>>>>>>>>>>>>>>> +       struct device_node *endpoint;
>>>>>>>>>>>>>>>>>>>                  struct drm_panel *panel;
>>>>>>>>>>>>>>>>>>> +       int dsi_lanes;
>>>>>>>>>>>>>>>>>>>                  int ret;
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>                  icn->vdd1 = devm_regulator_get_optional(dev, "vdd1");
>>>>>>>>>>>>>>>>>>> @@ -350,15 +359,42 @@ static int chipone_parse_dt(struct chipone *icn)
>>>>>>>>>>>>>>>>>>>                          return PTR_ERR(icn->enable_gpio);
>>>>>>>>>>>>>>>>>>>                  }
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> +       endpoint = of_graph_get_endpoint_by_regs(dev->of_node, 0, 0);
>>>>>>>>>>>>>>>>>>> +       dsi_lanes = of_property_count_u32_elems(endpoint, "data-lanes");
>>>>>>>>>>>>>>>>>>> +       icn->host_node = of_graph_get_remote_port_parent(endpoint);
>>>>>>>>>>>>>>>>>>> +       of_node_put(endpoint);
>>>>>>>>>>>>>>>>>>> +
>>>>>>>>>>>>>>>>>>> +       if (!icn->host_node)
>>>>>>>>>>>>>>>>>>> +               return -ENODEV;
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> The non-ports-based OF graph returns a -19 example on the Allwinner
>>>>>>>>>>>>>>>>>> Display pipeline in R16 [1].
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> We need to have a helper to return host_node for non-ports as I have
>>>>>>>>>>>>>>>>>> done it for drm_of_find_bridge.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> [1] https://patchwork.amarulasolutions.com/patch/1805/
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> The link points to a patch marked "DO NOT MERGE", maybe that patch is
>>>>>>>>>>>>>>>>> missing the DSI host port@0 OF graph link ? Both port@0 and port@1 are
>>>>>>>>>>>>>>>>> required, see:
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/display/bridge/chipone,icn6211.yaml#n53
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> What is "non-ports-based OF graph" ?
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> I don't see drm_of_find_bridge() in linux-next , what is that ?
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> port@0 is optional as some of the DSI host OF-graph represent the
>>>>>>>>>>>>>>>> bridge or panel as child nodes instead of ports. (i think dt-binding
>>>>>>>>>>>>>>>> has to fix it to make port@0 optional)
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> The current upstream DT binding document says:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>            required:
>>>>>>>>>>>>>>>              - port@0
>>>>>>>>>>>>>>>              - port@1
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> So port@0 is mandatory.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> In the binding, sure, but fundamentally the DT excerpt Jagan provided is
>>>>>>>>>>>>>> correct. If the bridge supports DCS, there's no reason to use the OF
>>>>>>>>>>>>>> graph in the first place: the bridge node will be a child node of the
>>>>>>>>>>>>>> MIPI-DSI controller (and there's no obligation to use the OF-graph for a
>>>>>>>>>>>>>> MIPI-DSI controller).
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> I believe port@0 should be made optional (or downright removed if
>>>>>>>>>>>>>> MIPI-DCS in the only control bus).
>>>>>>>>>>>>>
>>>>>>>>>>>>> That's out of scope of this series anyway, so Jagan can implement patches
>>>>>>>>>>>>> for that mode if needed.
>>>>>>>>>>>>
>>>>>>>>>>>> Not really? You can't count on the port@0 being there generally
>>>>>>>>>>>> speaking, so you can't count on data-lanes being there either, which
>>>>>>>>>>>> exactly what you're doing in this patch.
>>>>>>>>>>>
>>>>>>>>>>> I can because the upstream DT bindings currently say port@0 must be present,
>>>>>>>>>>> see above. If that requirement should be relaxed, sure, but that's a
>>>>>>>>>>> separate series.
>>>>>>>>>>
>>>>>>>>>> And another upstream DT bindings say that you don't need them at all.
>>>>>>>>>
>>>>>>>>> Which "another upstream DT bindings" do you refer to ?
>>>>>>>>
>>>>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/display/mipi-dsi-bus.txt
>>>>>>>>
>>>>>>>>>> Yes, there's a conflict. Yes, it's unfortunate. But the generic DSI
>>>>>>>>>> binding is far more relevant than a single bridge driver.
>>>>>>>>>
>>>>>>>>> That seems to be the wrong way around, how can generic subsystem-wide
>>>>>>>>> binding take precedence over specific driver binding ?
>>>>>>>>
>>>>>>>> This is the binding of the bus. You're part of that bus. You're a child
>>>>>>>> node of that bus, but nothing ever mandates that your parent node uses
>>>>>>>> the same convention. And some don't. And since your bridge can be
>>>>>>>> connected to pretty much any DSI controller, you have to use the lowest
>>>>>>>> common denominator, not make up some new constraints that not all
>>>>>>>> controller will be able to comply with.
>>>>>>>
>>>>>>> It seems to me the ICN6211 DT bindings currently further constraint the
>>>>>>> generic DSI bus bindings, and that seems OK to me.
>>>>>>>
>>>>>>> Let me reiterate this again -- if someone wants to relax the requirements
>>>>>>> currently imposed by the ICN6211 DT bindings, fine, but that can be done in
>>>>>>> a separate patchset AND that needs DT bindings update. Furthermore, there
>>>>>>> are no users of this ICN6211 bridge in upstream DTs, so there is currently
>>>>>>> no bridge which would operate without OF graph using this driver.
>>>>>>
>>>>>> And let me reiterate this again: something that used to work for a user
>>>>>> doesn't anymore when your series is applied. This is a textbook
>>>>>> regression. I suggested a way forward, that you don't like for some
>>>>>> reason, fine. But pushing through a regression is just not acceptable.
>>>>>
>>>>> How can this be a regression if this mode of operation could not have ever
>>>>> been supported with valid upstream DT bindings in the first place ?
>>>>>
>>>>> Should we now require that kernel drivers somehow magically support all
>>>>> kinds of random broken DT bindings in addition to ones which pass YAML DT
>>>>> validation ?
>>>>
>>>> The thing is, as I told you multiple times already, it was broken from
>>>> the bridge standpoint, but not from the controller's. If it had been
>>>> correct for the bridge, it wouldn't have been for the controller. So,
>>>> same story.
>>>>
>>>> The only difference is that it wouldn't affect you, but I don't see how
>>>> it's relevant.
>>>
>>> I'm sorry, I do not understand this answer.
>>>
>>>>>>>>>> So figuring it out is very much a prerequisite to that series,
>>>>>>>>>> especially since those patches effectively make the OF-graph mandatory
>>>>>>>>>> in some situations, while it was purely aesthetics before.
>>>>>>>>>
>>>>>>>>> The OF-graph is mandatory per the DT bindings of this driver. If you
>>>>>>>>> implement invalid DT which does not contain port@0, it will fail DT
>>>>>>>>> validation.
>>>>>>>>>
>>>>>>>>> If this requirement should be relaxed, sure, it can and I don't think it
>>>>>>>>> would be hard to do, but I don't see why that should be part of this series,
>>>>>>>>> which follows the upstream DT binding document for this driver.
>>>>>>>>>
>>>>>>>>> If I cannot trust the driver DT bindings to indicate what is and is not
>>>>>>>>> mandatory, what other document can I trust then ...
>>>>>>>>
>>>>>>>> Oh, come on. Doing that, you also require OF-Graph support for the DSI
>>>>>>>> controller you attach to, and you can't require that. This is very
>>>>>>>> different from just requiring a property that doesn't have any impact on
>>>>>>>> any other device, and you know that very well.
>>>>>>>
>>>>>>> Currently the ICN6211 DT bindings DO require that kind of bridge.
>>>>>>
>>>>>> And while this wasn't enforced before, you make it a hard requirement
>>>>>> with this series. This is what changed, and what caused this whole
>>>>>> discussion.
>>>>>
>>>>> The current DT bindings already make it a hard requirement, so no, nothing
>>>>> changed here.
>>>>>
>>>>> Unless what you are trying to ask for is support for broken DT bindings
>>>>> which do not pass YAML DT validation by this driver, but that is very
>>>>> dangerous, because then the question is, how far should we support such
>>>>> broken bindings. What kind of broken is still OK and what kind of broken is
>>>>> no longer OK ?
>>>>
>>>> If it ever worked in a mainline release, it must always work. See:
>>>> https://www.kernel.org/doc/html/latest/devicetree/bindings/ABI.html
>>>
>>>> As far as I'm concerned, it's the sole criteria. So to answer your
>>>> question, if it was broken but worked at some point, yes, we need to
>>>> keep supporting it. If it never worked, no, we don't.
>>>
>>> There are no users of this driver in any mainline release.
>>>
>>> DT is ABI, and ICN6211 DT bindings says port@0 is mandatory. If this
>>> driver worked with some broken downstream DT without port@0, then that
>>> downstream depended on undefined behavior which I cannot fathom how it
>>> can be considered part of kernel ABI. That downstream should fix its DT
>>> instead.
>>
>> Yes, agreed that ICN6211 DT bindings say port@0 is mandatory. However,
>> marking port@0 (after fixing DT binding) with non-I2C-ICN6211 is still
> 
> s/port@0/port@0 as optional/

The V4 requires the port@0 only for I2C bus option now, that should at 
least retain the "compatibility".