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

[Noralf Trønnes]

(cc Dillon)

Den 03.07.2020 19.26, skrev Sam Ravnborg:
> 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 at 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.

In an ideal world I would have liked to see the MIPI DBI parallel
interface implemented using a new Linux parallel bus type. It could have
drivers for gpio bitbanging and mmio in addition to other hw specific
drivers. Now we could have a drm_mipi_dbi DRM driver that registers as a
SPI client driver and a Parallel bus client driver. Or it can be a
component driver for the existing DRM driver on the SoC.

I had plans to do this and made a prototype, but dropped it since it
would probably require a lot of work getting in a new Linux bus type.

However if we're going to treat this parallel bus only as a MIPI DBI
display interface but support gpio bitbanging and mmio as well, then we
could add DRM drivers for each MIPI DBI bus (that don't have special
parallel bus hw):
- mipi-dbi-spi
- mipi-dbi-gpio
- mipi-dbi-mmio

These drivers will register as a mipi_dsi_host adapted like Paul suggested.

The panel drivers will be mipi_dsi_drivers. Now the panels should work
regardless of bus type. They probably need to know about the bus type,
at least whether the parallell bus is 8-bit or 16-bit wide.

The current MIPI DBI SPI drivers (drm/tiny) will need to be treated
specially to keep working with old Device Trees when moved over to
drm/panel.

Noralf.


>>
>> 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
>