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Message-ID: <ad625d5f-6b4b-41bb-a9c0-edb513697802@amd.com>
Date: Mon, 29 Jan 2024 10:12:13 -0600
From: Mario Limonciello <mario.limonciello@....com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: amd-gfx@...ts.freedesktop.org, Alex Deucher <alexander.deucher@....com>,
Harry Wentland <harry.wentland@....com>, Hans de Goede
<hdegoede@...hat.com>, "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>
Subject: Re: [PATCH 1/2] ACPI: video: Handle fetching EDID that is longer than
256 bytes
On 1/29/2024 07:54, Rafael J. Wysocki wrote:
> On Fri, Jan 26, 2024 at 7:55 PM Mario Limonciello
> <mario.limonciello@....com> 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@....com>
>> ---
>> 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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