[PATCH V4 2/3] drm/panel: Add Samsung AMS495QA01 MIPI-DSI LCD panel

[Linus Walleij]

On Sat, Dec 3, 2022 at 8:01 PM Chris Morgan <macroalpha82 at gmail.com> wrote:

> Will do. I'll make the changes and resubmit. For what it's worth the
> documentation says this one is a Samsung AMS495QA01 panel on a
> Magnachip D53E6EA8966 controller IC.

I would name the driver panel-magnachip-d53e6ea8966.c and
KConfig PANEL_MAGNACHIP_D53E6EA8966 for now
but keep the Samsung compatible string & match.

Maybe this driver will match more magnachips in the future,
maybe not. Sometimes people get hold of datasheets and submit
proper code and #defines etc.

> > > +       if (db->reg_elvdd) {
> >
> > Do you really need to if() this? I thought the regulator
> > framework would just ignore the calls for an optional
> > regulator.
>
> I don't know for sure, but I'll make the change if you request it. I
> think other drivers had an if in this scenario which is why I did it.

Okay but I don't think that is necessary.

> > > +               mode->type = DRM_MODE_TYPE_DRIVER;
> > > +               if (panel_info->num_modes == 1)
> > > +                       mode->type |= DRM_MODE_TYPE_PREFERRED;
> >
> > I think you should probably set the preferred mode even
> > if there are several of them? But maybe just on the first
> > or something. (A bit unsure here, Sam?)
>
> I'll keep 60hz as the preferred. 50hz was added at the request of some
> userspace folks for running PAL based emulators and stuff.

OK

> This is the path that "works", but I'll happily change to something
> else.
>
> > > +       db->dsi_dev->lanes = 2;
> > > +       db->dsi_dev->format = MIPI_DSI_FMT_RGB888;
> > > +       db->dsi_dev->mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_VIDEO_BURST |
> > > +                         MIPI_DSI_MODE_LPM | MIPI_DSI_MODE_NO_EOT_PACKET;
> > > +
> > > +       drm_panel_init(&db->panel, dev, &ams495qa01_drm_funcs,
> > > +                      DRM_MODE_CONNECTOR_DSI);
> >
> > Pixel data passes to the display using DSI but all display control
> > is done over SPI, and the core will not help with this.
> >
> > So from the display controller POV this is a DSI display
> > and from the display POV this is an SPI-controlled display.
> > So it sits on two buses. Data path is on DSI, control path
> > is on SPI.
> >
> > This would be kind of odd actually, normally DSI displays
> > do the display control over DSI as well. SPI control is usually
> > used on DPI displays. But I'm not surprised.
> >
> > If this is necessary, isn't this something we need to teach the
> > core to handle instead of adding quirks like this to all drivers that
> > have this characteristic?
> >
>
> You are correct, this panel is controlled via 3-wire SPI in my example.
> The panel can be controlled either by 3-wire SPI or DSI commands
> depending on whether or not pin 15 is driven high or low. Unfortunately
> in my case it's hardwired high, so I am forced to do it via 3-wire SPI.
> I have no way of testing it with pure DSI but that would simplify
> things quite a bit. Pixel data is transmitted soley through DSI.

OK

> The way I have it implemented currently is to put the panel on the SPI
> bus as a DBI panel; traverse through the DT bindings to find the
> associated DSI controller, then attach it as a DSI device so the DSI
> bus can transmit the pixel data.
>
> I'm absolutely cool with making those functions part of the core and
> not just specific to this panel, only I might need a bit of help on
> that part to make sure I do it the right way. I just wasn't sure how
> often that would be needed since this is the only panel I've ever seen
> driven this way, especially since it seems like any sane person would
> just want to do the whole control data/pixel data over DSI to keep
> things simple.

It doesn't seem all that unique.

Can you put some helper in drivers/gpu/drm/drm_of.c with the rest
and it will (hopefully) not be linked in unless used anyway.

Yours,
Linus Walleij