Re: [PATCH] drm/panel: add CONFIG_DRM_KMS_HELPER dependencies

From: Thomas Zimmermann
Date: Thu Mar 17 2022 - 15:15:48 EST

Hi Arnd

Am 16.03.22 um 21:59 schrieb Arnd Bergmann:
On Wed, Mar 16, 2022 at 8:31 PM Thomas Zimmermann <tzimmermann@xxxxxxx> wrote:
Am 16.03.22 um 20:12 schrieb Thomas Zimmermann:

Adding a dependency in all drivers that select DRM_MIPI_DBI avoids
the problem for now, adding the dependency in DRM_MIPI_DBI as well
should help make it easier to figure out why it breaks if someone
forgets the dependency the next time.

tristate
- depends on DRM
+ depends on DRM_KMS_HELPER

This symbol cannot be selected by users, so it's maybe not a good idea
to depend on it. In fact, I've had to remove such a statement because it
created a cyclic dependency. [1]

I tried to explain above what I was thinking here: the added dependency
is both a correct statement (DRM_MIPI_DBI depends on DRM_KMS_HELPER
because it cannot be built without DRM_KMS_HELPER) and helpful as
an indication what went wrong if we run into the same problem with a new
driver, instead of the cryptic link failure you get something like

WARNING: unmet direct dependencies detected for DRM_MIPI_DBI
Depends on [m]: HAS_IOMEM [=y] && DRM_KMS_HELPER [=m]
Selected by [y]:
- DRM_PANEL_WIDECHIPS_WS2401 [=y] && HAS_IOMEM [=y] && DRM [=y] &&
DRM_PANEL [=y] && SPI [=y] && GPIOLIB [=y] && BACKLIGHT_CLASS_DEVICE
[=y]
Selected by [m]:
- TINYDRM_ILI9225 [=m] && HAS_IOMEM [=y] && DRM [=y] && SPI [=y]

[1]
https://lore.kernel.org/dri-devel/20220315084559.23510-1-tzimmermann@xxxxxxx/

I was going for 'depends on' in the panel drivers because I saw the same being
done for other panel drivers, and mixing the two methods causes dependency
loops. I looked again now, and find that 'select DRM_KMS_HELPER' is more
common for other drivers, and makes sense here because it is generally
not user-selectable.

The easiest replacement for my patch would then be to just use 'select
DRM_KMS_HELPER' from CONFIG_DRM_MIPI_DBI, which makes it
safer and more consistent with your change. If you like, I'll send an updated
version.

MIPI DBI is another helper and select is not transitive IIRC. So drivers would still have to select KMS helpers as well. (?)

I just added my patch to drm-misc-fixes today and it should show up in upstream soon. Maybe just rebase your patch on top of it and if nothing breaks let's merge it as-is including the 'depends on'. You can add

Acked-by: Thomas Zimmermann <tzimmermann@xxxxxxx>

in this case.

More generally, I think you're right about making DRM helper libraries using 'depends on' to link to other libraries. Drivers would at least know which config symbols to select. A number of config rules would have to be adapted to make that happen, I guess.

One issue is that different submodules of DRM seem to use different logic for expressing such config dependencies. That's been an endless source of problems so far.


One thing I'm not sure about is whether there is still use for ever having
CONFIG_DRM without CONFIG_DRM_KMS_HELPER if it gets selected
by almost every driver anyway. Is this actually a configuration that
users rely on, or should we just remove the symbol completely and
build the KMS helpers unconditionally?

Best leave it as it is. i915 doesn't use it. And since it's a helper, it should not be lumped together with core DRM code simply for reasons of design.

For DRM_KMS_HELPER itself, the mid-term plan is to move some of the code into other modules. KMS helpers used to contain all kind of helpers, but recently there's interest in reducing the minimum size of a built-in DRM with minimal driver support. So the non-essential stuff needs to go into modules for the more-sophisticated DRM drivers.

Best regards
Thomas


Arnd

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

Attachment: OpenPGP_signature
Description: OpenPGP digital signature