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@xxxxxxx> 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.
Honestly, I don't get the push-back.
Because what I am being asked to do here is implement some sort of
undefined behavior backward compatibility. For behavior which has no
users. Worse, for behavior which can only be triggered if your DT is
broken and does not even pass the YAML DT validation.
That kind of new requirement seems bonkers to me.
To fix this properly would require:
- to remove port@0 being mandatory
- to move the data-lanes property to the bridge node itself.
That does not work, data-lanes is port property.
That's it. It takes 5 minutes, 30 minutes with the test and the commit
log. We've spent more time arguing about it already.
How can I test this bug compatibility requirement ?
I don't have a broken DT, my DT does pass the YAML DT validation, so
should I explicitly break my DT to perform the test ?
Are we going to start asking people to implement support for randomly
broken DTs in their drivers during code review too ?
So if you want to continue debating on whether it's a regression or not,
or whether DT ABI stability is a good thing or not, go ahead. But as far
as I'm concerned, this isn't really up for debate.
As far as I can tell, undefined behavior can hardly be part of stable ABI.
[...]
