[RFC PATCH 04/10] drm/panel_helper: Introduce drm_panel_helper

[Maxime Ripard]

On Mon, Aug 28, 2023 at 09:06:29AM -0700, Doug Anderson wrote:
> > > Shutdown is called any time you reboot a device. That means that if a
> > > DRM driver is _not_ calling drm_atomic_helper_shutdown() on the
> > > panel's behalf at shutdown time then the panel won't be powered off
> > > properly. This feels to me like something that might actually matter.
> >
> > It does matter. What I disagree on is that you suggest working around
> > that brokenness in the core framework. What I'm saying is driver is
> > broken, we should keep the core framework sane and fix it in the driver.
> >
> > It should be fairly easy with a coccinelle script to figure out which
> > panels are affected, and to add that call in remove.
> 
> I think I'm confused here. I've already figured out which panels are
> affected in my patch series, right? It's the set of panels that today
> try to power the panel off in their shutdown call, right? ...but I
> think we can't add the call you're suggesting,
> drm_atomic_helper_shutdown(), to the _panel_'s shutdown callback, can
> we? We need to add it to the shutdown callback of the top-level DRM
> driver, right?

I have no idea what happens if we just unbind the panel device from its
driver.

If we can't, then it's all fine. If we can, then we need figure out how
to unregister the DRM device (or block the unbinding from happening).

> > > Panels tend to be one of those things that really care about their
> > > power sequencing and can even get damaged (or not turn on properly
> > > next time) if sequencing is not done properly, so just removing this
> > > code and putting the blame on the DRM driver seems scary to me.
> >
> > Sure, it's bad. But there's no difference compared to the approach you
> > suggest in that patch: you created a helper, yes, but every driver will
> > still have to call that helper and if they don't, the panel will still
> > be called and it's a bug. And we would have to convert everything to
> > that new helper.
> >
> > It's fundamentally the same discussion than what you were opposed to
> > above.
> 
> I think the key difference here is that, if I understand correctly,
> drm_atomic_helper_shutdown() needs to be added to the top-level DRM
> driver, not to the panel itself. I guess I'm worried that I'll either
> miss a case or that simply adding a call to
> drm_atomic_helper_shutdown() in the top-level DRM driver will break
> something. Well, I suppose I can try it and see what happens...

The more I think about this discussion, the more I think that the
original intent of the prepared/enabled flags were precisely there to
prevent a double-disable for drivers with drm_atomic_helper_shutdown(),
while still shutting down the panel resources when the panel is used
with a driver that doesn't call it.

Honestly, I think the right thing to do here is to make every driver
call shutdown, and then you don't need the reference counting anymore.

I had a shot at a (possibly very suboptimal) coccinelle script to look
for drivers that are KMS drivers but don't call
drm_atomic_helper_shutdown() at shutdown.

https://paste.ack.tf/bb42e6@raw

The result is:

$ make coccicheck COCCI=./test-drm-shutdown.cocci MODE=report

...

./drivers/gpu/drm/sti/sti_drv.c:262:30-49: ERROR: KMS driver sti_platform_driver is missing shutdown implementation
./drivers/gpu/drm/armada/armada_drv.c:245:30-56: ERROR: KMS driver armada_drm_platform_driver is missing shutdown implementation
./drivers/gpu/drm/vmwgfx/vmwgfx_drv.c:1637:25-39: ERROR: KMS driver vmw_pci_driver is missing shutdown implementation
./drivers/gpu/drm/vboxvideo/vbox_drv.c:163:25-40: ERROR: KMS driver vbox_pci_driver is missing shutdown implementation
./drivers/gpu/drm/tiny/arcpgu.c:424:30-52: ERROR: KMS driver arcpgu_platform_driver is missing shutdown implementation
./drivers/gpu/drm/omapdrm/omap_drv.c:856:30-34: ERROR: KMS driver pdev is missing shutdown implementation
./drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_drv.c:361:30-57: ERROR: KMS driver fsl_dcu_drm_platform_driver is missing shutdown implementation
./drivers/gpu/drm/tve200/tve200_drv.c:261:30-43: ERROR: KMS driver tve200_driver is missing shutdown implementation
./drivers/gpu/drm/stm/drv.c:235:30-53: ERROR: KMS driver stm_drm_platform_driver is missing shutdown implementation
./drivers/gpu/drm/qxl/qxl_drv.c:267:25-39: ERROR: KMS driver qxl_pci_driver is missing shutdown implementation
./drivers/gpu/drm/logicvc/logicvc_drm.c:494:30-57: ERROR: KMS driver logicvc_drm_platform_driver is missing shutdown implementation
./drivers/gpu/drm/sun4i/sun4i_drv.c:439:30-55: ERROR: KMS driver sun4i_drv_platform_driver is missing shutdown implementation
./drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_dc.c:827:30-60: ERROR: KMS driver atmel_hlcdc_dc_platform_driver is missing shutdown implementation
./drivers/gpu/drm/mcde/mcde_drv.c:471:30-41: ERROR: KMS driver mcde_driver is missing shutdown implementation
./drivers/gpu/drm/hisilicon/hibmc/hibmc_drm_drv.c:366:25-41: ERROR: KMS driver hibmc_pci_driver is missing shutdown implementation
./drivers/gpu/drm/aspeed/aspeed_gfx_drv.c:365:30-56: ERROR: KMS driver aspeed_gfx_platform_driver is missing shutdown implementation
./drivers/gpu/drm/ingenic/ingenic-drm-drv.c:1607:30-48: ERROR: KMS driver ingenic_drm_driver is missing shutdown implementation
./drivers/gpu/drm/arm/malidp_drv.c:982:30-52: ERROR: KMS driver malidp_platform_driver is missing shutdown implementation
./drivers/gpu/drm/arm/hdlcd_drv.c:400:30-51: ERROR: KMS driver hdlcd_platform_driver is missing shutdown implementation
./drivers/gpu/drm/kmb/kmb_drv.c:622:30-49: ERROR: KMS driver kmb_platform_driver is missing shutdown implementation
./drivers/gpu/drm/exynos/exynos_drm_drv.c:356:30-56: ERROR: KMS driver exynos_drm_platform_driver is missing shutdown implementation
./drivers/gpu/drm/tiny/bochs.c:718:25-41: ERROR: KMS driver bochs_pci_driver is missing shutdown implementation
./drivers/gpu/drm/tiny/cirrus.c:746:25-42: ERROR: KMS driver cirrus_pci_driver is missing shutdown implementation
./drivers/gpu/drm/mediatek/mtk_drm_drv.c:954:30-53: ERROR: KMS driver mtk_drm_platform_driver is missing shutdown implementation

It's a significant number of drivers, but it's not the end of the world,
really.

Then, once the expectation is that all drivers are calling shutdown, we
don't have to worry about the refcounting at all in the panels, or
resources not being free'd anymore. And we have a single path to test
(disable) instead of two including one that might be difficult to test
properly.

> I'll try to cook up a v2 and we'll see what people say. I might keep
> the RFC tag for v2 just because I expect it to still be touching a lot
> of stuff.

Awesome, thanks

Maxime