Hi,
On 22/10/2024 02:29, Saravana Kannan wrote:
Hi Tomi,
Sorry it took a while to get back.
On Mon, Sep 16, 2024 at 4:52 AM Tomi Valkeinen
<tomi.valkeinen@xxxxxxxxxxxxxxxx> wrote:
Hi,
We have an issue where two devices have dependencies to each other,
according to drivers/base/core.c's fw_devlinks, and this prevents them
from probing. I've been adding debugging to the core.c, but so far I
don't quite grasp the issue, so I thought to ask. Maybe someone can
instantly say that this just won't work...
So, we have two devices, DSS (display subsystem) and an LVDS panel. The
DSS normally outputs parallel video from its video ports (VP), but it
has an integrated LVDS block (OLDI, Open LVDS Display Interface). The
OLDI block takes input from DSS's parallel outputs. The OLDI is not
modeled as a separate device (neither in the DT nor in the Linux device
model) as it has no register space, and is controlled fully by the DSS.
To support dual-link LVDS, the DSS has two OLDI instances. They both
take their input from the same parallel video port, but each OLDI sends
alternate lines forward. So for a dual-link setup the connections would
be like this:
+-----+-----+ +-------+ +----------+
| | | | | | |
| | VP1 +----+--->| OLDI0 +-------->| |
| | | | | | | |
| DSS +-----+ | +-------+ | Panel |
| | | | | | | |
| | VP2 | +--->| OLDI1 +-------->| |
| | | | | | |
+-----+-----+ +-------+ +----------+
As the OLDI is not a separate device, it also does not have an
independent device tree node, but rather it's inside DSS's node. The DSS
parallel outputs are under a normal "ports" node, but OLDI ports are
under "oldi-txes/ports" (see below for dts to clarify this).
And I think (guess...) this is the root of the issue we're seeing, as it
means the following, one or both of which might be the reason for this
issue:
- OLDI fwnodes don't have an associated struct device *. I think the
reason is that the OLDI media graph ports are one level too deep in the
hierarchy. So while the DSS ports are associated with the DSS device,
OLDI ports are not.
This is the root cause of the issue in some sense. fw_devlink doesn't
know that DSS depends on the VP. In the current DT, it only appears as
if the OLDI depends on VP. See further below for the fix.
- The VP ports inside the DSS point to OLDI ports, which are also inside
DSS. So ports from a device point to ports in the same device (and back).
If I understand the fw_devlink code correctly, in a normal case the
links formed with media graphs are marked as a cycle
(FWLINK_FLAG_CYCLE), and then ignored as far as probing goes.
What we see here is that when using a single-link OLDI panel, the panel
driver's probe never gets called, as it depends on the OLDI, and the
link between the panel and the OLDI is not a cycle.
The DSS driver probes, but the probe fails as it requires all the panel
devices to have been probed (and thus registered to the DRM framework)
before it can finish its setup.
With dual-link, probing does happen and the drivers work. But I believe
this is essentially an accident, in the sense that the first link
between the panel and the OLDI still blocks the probing, but the second
links allows the driver core to traverse the devlinks further, causing
it to mark the links to the panel as FWLINK_FLAG_CYCLE (or maybe it only
marks one of those links, and that's enough).
If I set fw_devlink=off as a kernel parameter, the probing proceeds
successfully in both single- and dual-link cases.
Now, my questions is, is this a bug in the driver core, a bug in the DT
bindings, or something in between (DT is fine-ish, but the structure is
something that won't be supported by the driver core).
And a follow-up question, regardless of the answer to the first one:
which direction should I go from here =).
The device tree data (simplified) for this is as follows, first the
dual-link case, then the single-link case:
/* Dual-link */
dss: dss@30200000 {
compatible = "ti,am625-dss";
oldi-txes {
oldi0: oldi@0 {
oldi0_ports: ports {
port@0 {
oldi_0_in: endpoint {
remote-endpoint = <&dpi0_out0>;
};
};
port@1 {
oldi_0_out: endpoint {
remote-endpoint = <&lcd_in0>;
};
};
};
};
oldi1: oldi@1 {
oldi1_ports: ports {
port@0 {
oldi_1_in: endpoint {
remote-endpoint = <&dpi0_out1>;
};
};
port@1 {
oldi_1_out: endpoint {
remote-endpoint = <&lcd_in1>;
};
};
};
};
};
dss_ports: ports {
port@0 {
dpi0_out0: endpoint@0 {
remote-endpoint = <&oldi_0_in>;
};
dpi0_out1: endpoint@1 {
remote-endpoint = <&oldi_1_in>;
};
};
};
};
display {
compatible = "microtips,mf-101hiebcaf0", "panel-simple";
In here, add this new property that I added some time back.
post-init-providers = <&oldi-txes>;
Thanks! This helps:
post-init-providers = <&oldi0>;
or for dual-link:
post-init-providers = <&oldi0>, <&oldi1>;
This tells fw_devlink that VP doesn't depend on this node for
initialization/probing. This property is basically available to break
cycles in DT and mark one of the edges of the cycles as "not a real
init dependency".
You should do the same for the single link case too.
While this helps, it's not very nice... Every new DT overlay that uses
OLDI display needs to have these.
I'm still confused about why this is needed. OF graphs are _always_
two-way links. Doesn't that mean that OF graphs never can be used for
dependencies, as they go both ways? If so, shouldn't we just always
ignore all OF graphs for dependency checking?
Tomi
Hope that helps. Let me know.
Thanks,
Saravana
ports {
port@0 {
lcd_in0: endpoint {
remote-endpoint = <&oldi_0_out>;
};
};
port@1 {
lcd_in1: endpoint {
remote-endpoint = <&oldi_1_out>;
};
};
};
};
/* Single-link */
dss: dss@30200000 {
compatible = "ti,am625-dss";
oldi-txes {
oldi0: oldi@0 {
oldi0_ports: ports {
port@0 {
oldi_0_in: endpoint {
remote-endpoint = <&dpi0_out0>;
};
};
port@1 {
oldi_0_out: endpoint {
remote-endpoint = <&lcd_in0>;
};
};
};
};
};
dss_ports: ports {
port@0 {
dpi0_out0: endpoint@0 {
remote-endpoint = <&oldi_0_in>;
};
};
};
};
display {
compatible = "microtips,mf-101hiebcaf0", "panel-simple";
ports {
port@0 {
lcd_in0: endpoint {
remote-endpoint = <&oldi_0_out>;
};
};
};
};
Tomi
