linux-kernel - [PATCH v2 1/4] ACPI: video: Handle fetching EDID that is longer than 256 bytes

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Message-ID: <20240130192608.11666-2-mario.limonciello@amd.com>
Date: Tue, 30 Jan 2024 13:26:05 -0600
From: Mario Limonciello <mario.limonciello@....com>
To: <amd-gfx@...ts.freedesktop.org>, Alex Deucher <alexander.deucher@....com>,
	Harry Wentland <harry.wentland@....com>, "Rafael J . Wysocki"
	<rafael@...nel.org>, Hans de Goede <hdegoede@...hat.com>
CC: "open list:ACPI" <linux-acpi@...r.kernel.org>, open list
	<linux-kernel@...r.kernel.org>, "open list:DRM DRIVERS"
	<dri-devel@...ts.freedesktop.org>, Melissa Wen <mwen@...lia.com>, "Mark
 Pearson" <mpearson-lenovo@...ebb.ca>, Mario Limonciello
	<mario.limonciello@....com>
Subject: [PATCH v2 1/4] ACPI: video: Handle fetching EDID that is longer than 256 bytes

The ACPI specification allows for an EDID to be up to 512 bytes but
the _DDC EDID fetching code will only try up to 256 bytes.

Modify the code to instead start at 512 bytes and work it's way
down instead.

As _DDC is now called up to 4 times on a machine debugging messages
are noisier than necessary.  Decrease from info to debug.

Link: https://uefi.org/htmlspecs/ACPI_Spec_6_4_html/Apx_B_Video_Extensions/output-device-specific-methods.html#ddc-return-the-edid-for-this-device
Signed-off-by: Mario Limonciello <mario.limonciello@....com>
---
v1->v2:
 * Use for loop for acpi_video_get_edid()
 * Use one of Rafael's suggestions for acpi_video_device_EDID()
 * Decrease message level too

I was going to split this separately, but decided to keep it in the same
series in case there is any decision to change the interface to
acpi_video_get_edid() at the same series.
---
 drivers/acpi/acpi_video.c | 25 +++++++++----------------
 1 file changed, 9 insertions(+), 16 deletions(-)

diff --git a/drivers/acpi/acpi_video.c b/drivers/acpi/acpi_video.c
index 4afdda9db019..3bfd013e09d2 100644
--- a/drivers/acpi/acpi_video.c
+++ b/drivers/acpi/acpi_video.c
@@ -625,12 +625,9 @@ acpi_video_device_EDID(struct acpi_video_device *device,
 
 	if (!device)
 		return -ENODEV;
-	if (length == 128)
-		arg0.integer.value = 1;
-	else if (length == 256)
-		arg0.integer.value = 2;
-	else
+	if (!length || (length % 128))
 		return -EINVAL;
+	arg0.integer.value = length / 128;
 
 	status = acpi_evaluate_object(device->dev->handle, "_DDC", &args, &buffer);
 	if (ACPI_FAILURE(status))
@@ -641,7 +638,8 @@ acpi_video_device_EDID(struct acpi_video_device *device,
 	if (obj && obj->type == ACPI_TYPE_BUFFER)
 		*edid = obj;
 	else {
-		acpi_handle_info(device->dev->handle, "Invalid _DDC data\n");
+		acpi_handle_debug(device->dev->handle,
+				 "Invalid _DDC data for length %ld\n", length);
 		status = -EFAULT;
 		kfree(obj);
 	}
@@ -1447,7 +1445,6 @@ int acpi_video_get_edid(struct acpi_device *device, int type, int device_id,
 
 	for (i = 0; i < video->attached_count; i++) {
 		video_device = video->attached_array[i].bind_info;
-		length = 256;
 
 		if (!video_device)
 			continue;
@@ -1478,18 +1475,14 @@ int acpi_video_get_edid(struct acpi_device *device, int type, int device_id,
 			continue;
 		}
 
-		status = acpi_video_device_EDID(video_device, &buffer, length);
-
-		if (ACPI_FAILURE(status) || !buffer ||
-		    buffer->type != ACPI_TYPE_BUFFER) {
-			length = 128;
+		for (length = 512; length > 0; length -= 128) {
 			status = acpi_video_device_EDID(video_device, &buffer,
 							length);
-			if (ACPI_FAILURE(status) || !buffer ||
-			    buffer->type != ACPI_TYPE_BUFFER) {
-				continue;
-			}
+			if (ACPI_SUCCESS(status))
+				break;
 		}
+		if (!length)
+			continue;
 
 		*edid = buffer->buffer.pointer;
 		return length;
-- 
2.34.1