Re: [RFC PATCH 0/4] DSI/DBI and TinyDRM driver

[Sam Ravnborg <sam@ravnborg.org>]

Hi Noralf/Paul.

Trying to stir up this discussion again.

On Sun, Jun 14, 2020 at 06:36:22PM +0200, Noralf Trønnes wrote:
> 
> 
> Den 07.06.2020 15.38, skrev Paul Cercueil:
> > Hi,
> > 
> > Here's a follow-up on the previous discussion about the current state of
> > DSI/DBI panel drivers, TinyDRM, and the need of a cleanup.
> > 
> > This patchset introduces the following:
> > * It slightly tweaks the MIPI DSI code so that it supports MIPI DBI over
> >   various buses. This patch has been tested with a non-upstream DRM
> >   panel driver for a ILI9331 DBI/8080 panel, written with the DSI
> >   framework (and doesn't include <drm/drm_mipi_dbi.h>), and non-upstream
> >   DSI/DBI host driver for the Ingenic SoCs.
> > 
> > * A SPI DBI host driver, using the current MIPI DSI framework. It allows
> >   MIPI DSI/DBI drivers to be written with the DSI framework, even if
> >   they are connected over SPI, instead of registering as SPI device
> >   drivers. Since most of these panels can be connected over various
> >   buses, it permits to reuse the same driver independently of the bus
> >   used.
> > 
> > * A TinyDRM driver for DSI/DBI panels, once again independent of the bus
> >   used; the only dependency (currently) being that the panel must
> >   understand DCS commands.
> > 
> > * A DRM panel driver to test the stack. This driver controls Ilitek
> >   ILI9341 based DBI panels, like the Adafruit YX240QV29-T 320x240 2.4"
> >   TFT LCD panel. This panel was converted from
> >   drivers/gpu/drm/tiny/ili9341.c.
> > 
> > I would like to emphasize that while it has been compile-tested, I did
> > not test it with real hardware since I do not have any DBI panel
> > connected over SPI. I did runtime-test the code, just without any panel
> > connected.
> > 
> > Another thing to note, is that it does not break Device Tree ABI. The
> > display node stays the same:
> > 
> > display@0 {
> > 	compatible = "adafruit,yx240qv29", "ilitek,ili9341";
> > 	reg = <0>;
> > 	spi-max-frequency = <32000000>;
> > 	dc-gpios = <&gpio0 9 GPIO_ACTIVE_HIGH>;
> > 	reset-gpios = <&gpio0 8 GPIO_ACTIVE_HIGH>;
> > 	rotation = <270>;
> > 	backlight = <&backlight>;
> > };
> > 
> > The reason it works, is that the "adafruit,yx240qv29" device is probed
> > on the SPI bus, so it will match with the SPI/DBI host driver. This will
> > in turn register the very same node with the DSI bus, and the ILI9341
> > DRM panel driver will probe. The driver will detect that no controller
> > is linked to the panel, and eventually register the DBI/DSI TinyDRM
> > driver.
> > 
> > I can't stress it enough that this is a RFC, so it still has very rough
> > edges.
> > 
> 
> I don't know bridge and dsi drivers so I can't comment on that, but one
> thing I didn't like is that the DT compatible string has to be added to
> 2 different modules.
> 
> As an example, a MI0283QT panel (ILI9341) supports these interface options:
> 
> 1. SPI
>    Panel setup/control and framebuffer upload over SPI
> 
> 2. SPI + DPI
>    Panel setup/control over SPI, framebuffer scanout over DPI
> 
> 3. Parallel bus
>    Panel setup/control and framebuffer upload over parallel bus

To continue the configurations we should support:
- Panels where the chip can be configured to SPI, SPI+DPI, Parallel bus
  (as detailed by Noralf above)
- Panels that supports only 6800 or 8080 - connected via GPIO pins or
  memory mapped (maybe behind some special IP to support this)
  Command set is often special.

We will see a number of chips with many different types of displays.
So the drivers should be chip specific with configuration depending on
the connected display.

What I hope we can find a solution for is a single file/driver that can
support all the relevant interface types for a chip.
So we would end up with a single file that included the necessary
support for ili9341 in all interface configurations with the necessary
support for the relevant displays.

I do not know how far we are from this as I have not dived into the
details of any of the proposals.
> 
> My suggestion is to have one panel driver module that can support all of
> these like this:
So I think we agree here.

> 
> For 1. and 2. a SPI driver is registered and if I understand your
> example correctly of_graph_get_port_by_id() can be used during probe to
> distinguish between the 2 options and register a full DRM driver for 1.
> and add a DRM panel for 2.
> 
> For 3. a DSI driver is registered (adapted for DBI use like you're
> suggesting).
> 
> Note that the interface part of the controller initialization will
> differ between these, the panel side init will be the same I assume.

	Sam
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