Re: [PATCH v2 1/3] drm: Add panel backlight quirks

[Hans de Goede <hdegoede@redhat.com>]

Hi Thomas,

On 6/23/24 10:51 AM, Thomas Weißschuh wrote:
> Panels using a PWM-controlled backlight source without an do not have a
> standard way to communicate their valid PWM ranges.
> On x86 the ranges are read from ACPI through driver-specific tables.
> The built-in ranges are not necessarily correct, or may grow stale if an
> older device can be retrofitted with newer panels.
> 
> Add a quirk infrastructure with which the valid backlight ranges can be
> maintained as part of the kernel.
> 
> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> ---
>  Documentation/gpu/drm-kms-helpers.rst        |  3 ++
>  drivers/gpu/drm/Kconfig                      |  4 ++
>  drivers/gpu/drm/Makefile                     |  1 +
>  drivers/gpu/drm/drm_panel_backlight_quirks.c | 67 ++++++++++++++++++++++++++++
>  include/drm/drm_utils.h                      | 11 +++++
>  5 files changed, 86 insertions(+)
> 
> diff --git a/Documentation/gpu/drm-kms-helpers.rst b/Documentation/gpu/drm-kms-helpers.rst
> index 59cfe8a7a8ba..1998a2675210 100644
> --- a/Documentation/gpu/drm-kms-helpers.rst
> +++ b/Documentation/gpu/drm-kms-helpers.rst
> @@ -224,6 +224,9 @@ Panel Helper Reference
>  .. kernel-doc:: drivers/gpu/drm/drm_panel_orientation_quirks.c
>     :export:
>  
> +.. kernel-doc:: drivers/gpu/drm/drm_panel_backlight_quirks.c
> +   :export:
> +
>  Panel Self Refresh Helper Reference
>  ===================================
>  
> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
> index 959b19a04101..50ccb43315bf 100644
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -443,6 +443,10 @@ config DRM_EXPORT_FOR_TESTS
>  config DRM_PANEL_ORIENTATION_QUIRKS
>  	tristate
>  
> +# Separate option as not all DRM drivers use it
> +config DRM_PANEL_BACKLIGHT_QUIRKS
> +	tristate
> +
>  config DRM_LIB_RANDOM
>  	bool
>  	default n
> diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> index f9ca4f8fa6c5..6669913b907e 100644
> --- a/drivers/gpu/drm/Makefile
> +++ b/drivers/gpu/drm/Makefile
> @@ -92,6 +92,7 @@ drm-$(CONFIG_DRM_PANIC) += drm_panic.o
>  obj-$(CONFIG_DRM)	+= drm.o
>  
>  obj-$(CONFIG_DRM_PANEL_ORIENTATION_QUIRKS) += drm_panel_orientation_quirks.o
> +obj-$(CONFIG_DRM_PANEL_BACKLIGHT_QUIRKS) += drm_panel_backlight_quirks.o
>  
>  #
>  # Memory-management helpers
> diff --git a/drivers/gpu/drm/drm_panel_backlight_quirks.c b/drivers/gpu/drm/drm_panel_backlight_quirks.c
> new file mode 100644
> index 000000000000..a89b5fd1940e
> --- /dev/null
> +++ b/drivers/gpu/drm/drm_panel_backlight_quirks.c
> @@ -0,0 +1,67 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/array_size.h>
> +#include <linux/dmi.h>
> +#include <linux/mod_devicetable.h>
> +#include <linux/module.h>
> +#include <drm/drm_connector.h>
> +#include <drm/drm_utils.h>
> +
> +struct drm_panel_backlight_entry {
> +	struct {
> +		enum dmi_field field;
> +		const char * const value;
> +	} dmi_match;

Matching on a single DMI string is not always enough to uniquely identify
a machine. I would change this to a dmi_system_id struct and then add
an array with 2 dmi_system_id structs in drm_panel_backlight_entry_matches()
and copy the struct to the first array entry + zero out the second entry
(terminator) and then use dmi_check_system().

> +	struct drm_edid_ident ident;

Hmm, what about DSI panels? These do not (always) have EDID info AFAIK.

drivers/gpu/drm/drm_panel_orientation_quirks.c is using a resolution match
so as to hopefully not match external screens, but that is also so that it can
be used from efifb / simpledrm and here you really do want to differentiate
between different panels by panel model.

So I guess that the EDId match is fine and if we ever need to match DSI panels
without EDID we figure something out then.

Thinking more about this I have a question about the approach as a whole though,
I'll reply to the cover-letter with this.

Regards,

Hans



> +	struct drm_panel_backlight_quirk quirk;
> +};
> +
> +static const struct drm_panel_backlight_entry drm_panel_backlight_entries[] = {
> +};
> +
> +static bool drm_panel_backlight_entry_matches(const struct drm_panel_backlight_entry *entry,
> +					      const struct drm_edid *edid)
> +{
> +	if (!dmi_match(entry->dmi_match.field, entry->dmi_match.value))
> +		return false;
> +
> +	if (!drm_edid_match(edid, &entry->ident))
> +		return false;
> +
> +	return true;
> +}
> +
> +/**
> + * drm_get_panel_panel_quirk - Check for panel backlight quirks
> + * @edid: EDID of the panel to check
> + *
> + * This function checks for platform specific (e.g. DMI based) quirks
> + * providing info on backlight control for systems where this cannot be
> + * probed from the hard-/firm-ware.
> + *
> + * Returns:
> + * A struct drm_panel_backlight_quirk if a quirk is found or NULL otherwise.
> + */
> +const struct drm_panel_backlight_quirk *drm_get_panel_backlight_quirk(const struct drm_edid *edid)
> +{
> +	const struct drm_panel_backlight_entry *entry;
> +	size_t i;
> +
> +	if (!IS_ENABLED(CONFIG_DMI))
> +		return NULL;
> +
> +	if (!edid)
> +		return NULL;
> +
> +	for (i = 0; i < ARRAY_SIZE(drm_panel_backlight_entries); i++) {
> +		entry = &drm_panel_backlight_entries[i];
> +
> +		if (drm_panel_backlight_entry_matches(entry, edid))
> +			return &entry->quirk;
> +	}
> +
> +	return NULL;
> +}
> +EXPORT_SYMBOL(drm_get_panel_backlight_quirk);
> +
> +MODULE_LICENSE("GPL");
> diff --git a/include/drm/drm_utils.h b/include/drm/drm_utils.h
> index 70775748d243..37cc6de1a01a 100644
> --- a/include/drm/drm_utils.h
> +++ b/include/drm/drm_utils.h
> @@ -11,9 +11,20 @@
>  #define __DRM_UTILS_H__
>  
>  #include <linux/types.h>
> +#include <drm/drm_edid.h>
> +
> +struct drm_panel_backlight_quirk {
> +	struct {
> +		bool pwm_min_brightness:1;
> +	} overrides;
> +
> +	u8 pwm_min_brightness; /* min_brightness/255 of max */
> +};
>  
>  int drm_get_panel_orientation_quirk(int width, int height);
>  
> +const struct drm_panel_backlight_quirk *drm_get_panel_backlight_quirk(const struct drm_edid *edid);
> +
>  signed long drm_timeout_abs_to_jiffies(int64_t timeout_nsec);
>  
>  #endif
>