Re: [PATCH] ACPI: video: Handle fetching EDID as ACPI_TYPE_PACKAGE

[Gergo Koteles]

Hi Mario,

Thanks for the suggestions!

On Fri, 2025-03-28 at 08:42 -0500, Mario Limonciello wrote:
> On 3/28/2025 06:12, Rafael J. Wysocki wrote:
> > CC: Hans
> > 
> > On Fri, Mar 28, 2025 at 3:51 AM Gergo Koteles <soyer@xxxxxx> wrote:
> > > 
> > > Some Lenovo laptops incorrectly return EDID as
> > > buffer in ACPI package (instead of just a buffer)
> > > when calling _DDC.
> > > 
> > > Calling _DDC generates this ACPI Warning:
> > > ACPI Warning: \_SB.PCI0.GP17.VGA.LCD._DDC: Return type mismatch - \
> > > found Package, expected Integer/Buffer (20240827/nspredef-254)
> > > 
> > > Use the first element of the package to get the EDID buffer.
> > > 
> > > The DSDT:
> > > 
> > > Name (AUOP, Package (0x01)
> > > {
> > >          Buffer (0x80)
> > >          {
> > >          ...
> > >          }
> > > })
> > > 
> > > ...
> > > 
> > > Method (_DDC, 1, NotSerialized)  // _DDC: Display Data Current
> > > {
> > >          If ((PAID == AUID))
> > >          {
> > >                  Return (AUOP) /* \_SB_.PCI0.GP17.VGA_.LCD_.AUOP */
> > >          }
> > >          ElseIf ((PAID == IVID))
> > >          {
> > >                  Return (IVOP) /* \_SB_.PCI0.GP17.VGA_.LCD_.IVOP */
> > >          }
> > >          ElseIf ((PAID == BOID))
> > >          {
> > >                  Return (BOEP) /* \_SB_.PCI0.GP17.VGA_.LCD_.BOEP */
> > >          }
> > >          ElseIf ((PAID == SAID))
> > >          {
> > >                  Return (SUNG) /* \_SB_.PCI0.GP17.VGA_.LCD_.SUNG */
> > >          }
> > > 
> > >          Return (Zero)
> > > }
> > > 
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Fixes: c6a837088bed ("drm/amd/display: Fetch the EDID from _DDC if available for eDP")
> > > Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4085
> > > Signed-off-by: Gergo Koteles <soyer@xxxxxx>
> 
> FWIW the ACPI spec is clear that this /should/ be an ACPI buffer.
> 
> 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
> 
> That being said this is production firmware and in the wild, I don't 
> personally see a problem with handling it this way.
> 
> Some other improvement suggestion though below.
> 
> > > ---
> > >   drivers/acpi/acpi_video.c | 5 ++++-
> > >   1 file changed, 4 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/acpi/acpi_video.c b/drivers/acpi/acpi_video.c
> > > index efdadc74e3f4..65cf36796506 100644
> > > --- a/drivers/acpi/acpi_video.c
> > > +++ b/drivers/acpi/acpi_video.c
> > > @@ -649,6 +649,9 @@ acpi_video_device_EDID(struct acpi_video_device *device, void **edid, int length
> > > 
> > >          obj = buffer.pointer;
> > > 
> > > +       if (obj && obj->type == ACPI_TYPE_PACKAGE && obj->package.count == 1)
> > > +               obj = &obj->package.elements[0];
> > > +
> 
> As the ACPI spec indicates this should be a buffer, I think it's a good 
> idea to emit a FW_BUG message here so that this can be detected by users 
> and tools like FWTS and the firmware can be improved in the future.
> 
> Something like this:
> 
> if (condition) {
> 	pr_info(FW_BUG "EDID was found in ACPI package instead of ACPI buffer");
> 	obj = &obj->package.elements[0];
> }
> 

An ACPI Warning is currently being generated:

ACPI Warning: \_SB.PCI0.GP17.VGA.LCD._DDC: Return type mismatch - found
Package, expected Integer/Buffer (20240827/nspredef-254)

This is also noticed by FWTS in the form of KlogAcpiReturnTypeMismatch
and may be noticed by users as well.

I think it is unnecessary to emit two warnings for the same problem.

However, some comments could make the code clearer. I will add some
comments to V2.


> > >          if (obj && obj->type == ACPI_TYPE_BUFFER) {
> > >                  *edid = kmemdup(obj->buffer.pointer, obj->buffer.length, GFP_KERNEL);
> > >                  ret = *edid ? obj->buffer.length : -ENOMEM;
> > > @@ -658,7 +661,7 @@ acpi_video_device_EDID(struct acpi_video_device *device, void **edid, int length
> > >                  ret = -EFAULT;
> > >          }
> > > 
> > > -       kfree(obj);
> > > +       kfree(buffer.pointer);
> 
> Any reason for this change?  obj is assigned to buffer.pointer already.
> 
> > 

In the case of an ACPI package, obj points to the first element of the
package. The buffer.pointer still points to the original location.

Thanks,
Gergo