Re: [v1,1/3] drm/panel: ili9341: Correct use of device property APIs
From: Sui Jingfeng
Date: Tue Apr 30 2024 - 13:24:56 EST
Hi,
On 2024/4/30 22:32, Andy Shevchenko wrote:
On Fri, Apr 26, 2024 at 04:43:18AM +0800, Sui Jingfeng wrote:
On 2024/4/26 03:10, Andy Shevchenko wrote:
On Fri, Apr 26, 2024 at 02:08:16AM +0800, Sui Jingfeng wrote:
On 2024/4/25 22:26, Andy Shevchenko wrote:
It seems driver missed the point of proper use of device property APIs.
Correct this by updating headers and calls respectively.
You are using the 'seems' here exactly saying that you are not 100% sure.
To add here, "seems" is used to show that I have no knowledge on what was
the idea behind this implementation by the original author. Plus my knowledge
in the firmware node / device property APIs and use cases in Linux kernel.
Please allow me to tell you the truth: This patch again has ZERO effect.
It fix nothing. And this patch is has the risks to be wrong.
Huh?! Really, stop commenting the stuff you do not understand.
I'm actually a professional display drivers developer at the downstream
in the past, despite my contribution to upstream is less. But I believe
that all panel driver developers know what I'm talking about. So please
have take a look at my replies.
Okay.
Simple because the "ili9341_probe() ---> ili9341_dbi_prob()" code path
is DT dependent.
First of all, the devm_of_find_backlight() is called in ili9341_dbi_probe()
under *non-DT* environment, devm_of_find_backlight() is just a just a
no-op and will return NULL. NULL is not an error code, so ili9341_dbi_probe()
won't rage quit. But the several side effect is that the backlight will
NOT works at all.
Is it a problem?
Yes, it is.
The core problem is that the driver you are modifying has *implicit* *dependency* on DT.
The implicit dependency is due to the calling of devm_of_find_backlight(). This function
is a no-op under non-DT systems.
Okay.
Therefore, before the devm_of_find_backlight() and
the device_get_match_data() function can truly DT independent.
True for the first part, not true for the second.
Removing the "OF" dependency just let the tigers run out from the jail.
It is not really meant to targeting at you, but I thinks, all of drm_panel drivers
that has the devm_of_find_backlight() invoked will suffer such concerns.
In short, the reason is that the *implicit* *dependency* populates and
the undefined behavior gets triggered.
Still no problem statement. My hardware works nicely on non-DT environment.
(And since it's Arduino-based one, I assume it will work on DT environments
the very same way.)
I'm sure you know that device_get_match_data() is same with of_device_get_match_data()
for DT based systems. For non DT based systems, device_get_match_data() is just *undefined*
Note that ACPI is not in the scope of the discussion here, as all of the drm bridges and
panels driver under drivers/gpu/drm/ hasn't the ACPI support yet.
This patch shows exactly how to bring back the ACPI support to one of them
(as it's done for tinyDRM cases).
Therefore, at present,
it safe to say that device_get_match_data() is *undefined* under no-DT environment.
This is not true.
Removing the "OF" dependency hints to us that it allows the driver to be probed as a
pure SPI device under non DT systems. When device_get_match_data() is called, it returns
NULL to us now. As a result, the drm driver being modified will tears down.
See bellow code snippet extracted frompanel-ilitek-ili9341.c:
```
ili->conf = of_device_get_match_data(dev);
if (!ili->conf) {
dev_err(dev, "missing device configuration\n");
return -ENODEV;
}
```
It is actually considered as fatal bug for *panels* if the backlight of
it is not light up, at least the brightness of *won't* be able to adjust.
What's worse, if there is no sane platform setup code at the firmware
or boot loader stage to set a proper initial state. The screen is complete
dark. Even though the itself panel is refreshing framebuffers, it can not
be seen by human's eye. Simple because of no backlight.
Can you imagine that I may have different hardware that considered
this is non-fatal error?
Yes, I can imagine.
I believe you have the hardware which make you patch correct to run
in 99.9% of all cases. But as long as there one bug happened, you patch
are going to be blamed.
Because its your patch that open the door, both from the perceptive of
practice and from the perceptive of the concept (static analysis).
Second, the ili9341_dbi_probe() requires additional device properties to
be able to works very well on the rotation screen case. See the calling
of "device_property_read_u32(dev, "rotation", &rotation)" in
ili9341_dbi_probe() function.
Yes, exactly, and how does it object the purpose of this patch?
Because under *non-DT* environment, your commit message do not give a
valid description, how does the additional device property can be acquired
is not demonstrated.
And it is exactly your patch open the non-DT code path (way or possibility).
It isn't has such risks before your patch is applied. In other words,
previously, the driver has the 'OF' dependency as the guard, all of the
potential risk(or problem) are suppressed. It is a extremely safe policy,
and it is also a extremely perfect defend.
And suddenly, you patch release the dangerous tiger from the cage.
So I think you can imagine...
No, I can't, sorry. I don't see how dangerous will be the use of DRM panel
in a wrong configuration. The same can very well happen on improperly working
hardware (backlight part) or simply when somebody didn't correctly set a DT
or manually use it when it should not be. But again I see *no* problem
statement, only some worries.
And on top of that I made tinyDRM drivers to be accessible on ACPI platforms
and so far I have none complains about the tigers that left the cage.
Combine with those two factors, it is actually can conclude that the
panel-ilitek-ili9394 driver has the *implicit* dependency on 'OF'.
Removing the 'OF' dependency from its Kconfig just trigger the
leakage of such risks.
What?!
Posting a patch is actually doing the defensive works, such a saying
may not sound fair for you, but this is just the hash cruel reality.
Sorry for saying that. :(
So, the summary of your message is that:
- there's no understanding how ACPI (or any other non-DT fwnode based
environment) can utilise the driver
- there's a worry about some problems which can't be stated clearly
- there's a neglecting of the previous successful cases specific for DRM
(tinyDRM drivers)
As a result of the false input, the non-constructive conclusion was given.
And note, I converted dozens if not hundredth of drivers that used to be
OF-only and haven't heart any negative feedback before this case. Maybe
we (reviewers of my patches and maintainers who applied them and end users)
miss a BIG DEAL here? Please, elaborate how dropping OF dependency can be
dangerous as a free walking tiger.
My software node related patches can help to reduce part of the potential
risks, but it still need some extra work. And it is not landed yet.
Your patch has nothing to do with this series.
I am not going to repeat the above.
With my patch applied, this is way to meet the gap under non-DT systems.
Users of this driver could managed to attach(complete) absent properties
to the SPI device with software node properties. Register the swnode
properties group into the system prior the panel driver is probed. There
may need some quirk. But at the least there has a way to go. When there
has a way to go, things become self-consistent. Viewed from both the
practice of viewpoint and the concept of viewpoint.
And the dangerous tiger will steer its way to the direction of "ACPI
support is missing". But both of will be safe then.
I have no obvious opinions then, code inside this patch seems no obvious problem
for majority applications. Sorry about the noise and thanks for reply.
--
Best regards,
Sui