Re: [PATCH 1/2] ACPI: video: Handle fetching EDID that is longer than 256 bytes

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On 1/29/2024 07:54, Rafael J. Wysocki wrote:
On Fri, Jan 26, 2024 at 7:55 PM Mario Limonciello
<mario.limonciello@xxxxxxx> wrote:

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.

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@xxxxxxx>
---
  drivers/acpi/acpi_video.c | 23 ++++++++++++++++-------
  1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/drivers/acpi/acpi_video.c b/drivers/acpi/acpi_video.c
index 62f4364e4460..b3b15dd4755d 100644
--- a/drivers/acpi/acpi_video.c
+++ b/drivers/acpi/acpi_video.c
@@ -624,6 +624,10 @@ acpi_video_device_EDID(struct acpi_video_device *device,
                 arg0.integer.value = 1;
         else if (length == 256)
                 arg0.integer.value = 2;
+       else if (length == 384)
+               arg0.integer.value = 3;
+       else if (length == 512)
+               arg0.integer.value = 4;

It looks like switch () would be somewhat better.

Or maybe even

arg0.integer.value = length / 128;

The validation could be added too:

if (arg0.integer.value > 4 || arg0.integer.value * 128 != length)
         return -EINVAL;

but it is pointless, because the caller is never passing an invalid
number to it AFAICS.


Thanks.  I'll swap over to one of these suggestions.

I will also split this patch separately from the other as the other will take some time with refactoring necessary in DRM that will take a cycle or two.

         else
                 return -EINVAL;

@@ -1443,7 +1447,7 @@ 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;
+               length = 512;

                 if (!video_device)
                         continue;
@@ -1478,13 +1482,18 @@ int acpi_video_get_edid(struct acpi_device *device, int type, int device_id,

                 if (ACPI_FAILURE(status) || !buffer ||
                     buffer->type != ACPI_TYPE_BUFFER) {
-                       length = 128;
-                       status = acpi_video_device_EDID(video_device, &buffer,
-                                                       length);
-                       if (ACPI_FAILURE(status) || !buffer ||
-                           buffer->type != ACPI_TYPE_BUFFER) {
-                               continue;
+                       while (length) {

I would prefer a do {} while () loop here, which could include the
first invocation of acpi_video_device_EDID() too (and reduce code
duplication a bit).

+                               length -= 128;
+                               status = acpi_video_device_EDID(video_device, &buffer,
+                                                               length);

No line break, please.

+                               if (ACPI_FAILURE(status) || !buffer ||
+                                   buffer->type != ACPI_TYPE_BUFFER) {
+                                       continue;
+                               }
+                               break;
                         }
+                       if (!length)
+                               continue;
                 }

                 *edid = buffer->buffer.pointer;
--




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