Re: [PATCH v4 1/3] drm: Add drm_get_acpi_edid() helper

From: Mario Limonciello
Date: Sun Feb 11 2024 - 00:19:43 EST

On 2/9/2024 12:57, Daniel Vetter wrote:
On Fri, Feb 09, 2024 at 09:34:13AM -0600, Mario Limonciello wrote:
On 2/9/2024 05:07, Daniel Vetter wrote:
On Thu, Feb 08, 2024 at 11:57:11AM +0200, Jani Nikula wrote:
On Wed, 07 Feb 2024, Mario Limonciello <mario.limonciello@xxxxxxx> wrote:
Some manufacturers have intentionally put an EDID that differs from
the EDID on the internal panel on laptops. Drivers can call this
helper to attempt to fetch the EDID from the BIOS's ACPI _DDC method.

Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx>
drivers/gpu/drm/Kconfig | 5 +++
drivers/gpu/drm/drm_edid.c | 77 ++++++++++++++++++++++++++++++++++++++
include/drm/drm_edid.h | 1 +
3 files changed, 83 insertions(+)

diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index 6ec33d36f3a4..ec2bb71e8b36 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -21,6 +21,11 @@ menuconfig DRM
select KCMP
+ select ACPI_VIDEO if ACPI
+ select INPUT if ACPI
+ select X86_PLATFORM_DEVICES if ACPI && X86
+ select ACPI_WMI if ACPI && X86

I think I'll defer to drm maintainers on whether this is okay or
something to be avoided.

Uh yeah this is a bit much, and select just messes with everything. Just
#ifdef this in the code with a dummy alternative, if users configure their
kernel without acpi but need it, they get to keep all the pieces.

Alternatively make a DRM_ACPI_HELPERS symbol, but imo a Kconfig for every
function is also not great. And just using #ifdef in the code also works
for CONFIG_OF, which is exactly the same thing for platforms using dt to
describe hw.

Also I'd expect ACPI code to already provide dummy functions if ACPI is
provided, so you probably dont even need all that much #ifdef in the code.

What we defo cant do is select platform/hw stuff just because you enable

The problem was with linking. I'll experiment with #ifdef for the next

Ah yes, if e.g. acpi is a module but drm is built-in then it will compile,
but not link.

You need

depends on (ACPI || ACPI=n)

for this. Looks a bit funny but works for all combinations.

Nope; this fails at link time with this combination:


ld: drivers/gpu/drm/drm_edid.o: in function `drm_do_probe_acpi_edid':
drm_edid.c:(.text+0xd34): undefined reference to `acpi_video_get_edid'
make[5]: *** [scripts/Makefile.vmlinux:37: vmlinux] Error 1

So the logical solution is to try
depends on (ACPI_VIDEO || ACPI_VIDEO=n)

But that leads me back to the rabbit hole of why I had the selects moved to drm instead of drivers in the first place:

drivers/gpu/drm/Kconfig:8:error: recursive dependency detected!
drivers/gpu/drm/Kconfig:8: symbol DRM depends on ACPI_VIDEO
drivers/acpi/Kconfig:213: symbol ACPI_VIDEO depends on BACKLIGHT_CLASS_DEVICE
drivers/video/backlight/Kconfig:136: symbol BACKLIGHT_CLASS_DEVICE is selected by DRM_RADEON
drivers/gpu/drm/radeon/Kconfig:3: symbol DRM_RADEON depends on DRM

Since this gets mess it might be useful to have a DRM_ACPI_HELPERS Kconfig
that controls all this.

How about all those selects that I had in this patch moved to DRM_ACPI_HELPERS and keep the patch that drops from all the drivers then?


Kernel-level support for the Direct Rendering Infrastructure (DRI)
introduced in XFree86 4.0. If you say Y here, you need to select
diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 923c4423151c..c649b4f9fd8e 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -28,6 +28,7 @@
+#include <acpi/video.h>
#include <linux/bitfield.h>
#include <linux/cec.h>
#include <linux/hdmi.h>
@@ -2188,6 +2189,49 @@ drm_do_probe_ddc_edid(void *data, u8 *buf, unsigned int block, size_t len)
return ret == xfers ? 0 : -1;
+ * drm_do_probe_acpi_edid() - get EDID information via ACPI _DDC
+ * @data: struct drm_device
+ * @buf: EDID data buffer to be filled
+ * @block: 128 byte EDID block to start fetching from
+ * @len: EDID data buffer length to fetch
+ *
+ * Try to fetch EDID information by calling acpi_video_get_edid() function.
+ *
+ * Return: 0 on success or error code on failure.
+ */
+static int
+drm_do_probe_acpi_edid(void *data, u8 *buf, unsigned int block, size_t len)
+ struct drm_device *ddev = data;
+ struct acpi_device *acpidev = ACPI_COMPANION(ddev->dev);
+ unsigned char start = block * EDID_LENGTH;
+ void *edid;
+ int r;
+ if (!acpidev)
+ return -ENODEV;
+ /* fetch the entire edid from BIOS */
+ r = acpi_video_get_edid(acpidev, ACPI_VIDEO_DISPLAY_LCD, -1, &edid);
+ if (r < 0) {
+ DRM_DEBUG_KMS("Failed to get EDID from ACPI: %d\n", r);
+ return -EINVAL;
+ }
+ if (len > r || start > r || start + len > r) {
+ r = -EINVAL;
+ goto cleanup;
+ }
+ memcpy(buf, edid + start, len);
+ r = 0;
+ kfree(edid);
+ return r;
static void connector_bad_edid(struct drm_connector *connector,
const struct edid *edid, int num_blocks)
@@ -2643,6 +2687,39 @@ struct edid *drm_get_edid(struct drm_connector *connector,
+ * drm_get_acpi_edid - get EDID data, if available

I'd prefer all the new EDID API to be named drm_edid_*. Makes a clean
break from the old API, and is more consistent.

So perhaps drm_edid_read_acpi() to be in line with all the other struct
drm_edid based EDID reading functions.

+ * @connector: connector we're probing
+ *
+ * Use the BIOS to attempt to grab EDID data if possible.
+ *
+ * The returned pointer must be freed using drm_edid_free().
+ *
+ * Return: Pointer to valid EDID or NULL if we couldn't find any.
+ */
+const struct drm_edid *drm_get_acpi_edid(struct drm_connector *connector)
+ const struct drm_edid *drm_edid;
+ switch (connector->connector_type) {
+ break;
+ default:
+ return NULL;
+ }
+ if (connector->force == DRM_FORCE_OFF)
+ return NULL;
+ drm_edid = drm_edid_read_custom(connector, drm_do_probe_acpi_edid, connector->dev);
+ /* Note: Do *not* call connector updates here. */
+ return drm_edid;
* drm_edid_read_custom - Read EDID data using given EDID block read function
* @connector: Connector to use
diff --git a/include/drm/drm_edid.h b/include/drm/drm_edid.h
index 7923bc00dc7a..ca41be289fc6 100644
--- a/include/drm/drm_edid.h
+++ b/include/drm/drm_edid.h
@@ -410,6 +410,7 @@ struct edid *drm_do_get_edid(struct drm_connector *connector,
void *data);
struct edid *drm_get_edid(struct drm_connector *connector,
struct i2c_adapter *adapter);
+const struct drm_edid *drm_get_acpi_edid(struct drm_connector *connector);

There's a comment

/* Interface based on struct drm_edid */

towards the end of the file, gathering all the new API under it.

Other than that, LGTM,


u32 drm_edid_get_panel_id(struct i2c_adapter *adapter);
struct edid *drm_get_edid_switcheroo(struct drm_connector *connector,
struct i2c_adapter *adapter);

Jani Nikula, Intel