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

[Mario Limonciello]

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;
> > --