Le lun. 25 mai 2020 à 2:46, Noralf Trønnes noralf@tronnes.org a écrit :
Den 24.05.2020 23.33, skrev Paul Cercueil:
Le dim. 24 mai 2020 à 23:24, Noralf Trønnes noralf@tronnes.org a écrit :
Den 24.05.2020 22.42, skrev Paul Cercueil:
Le dim. 24 mai 2020 à 22:14, Noralf Trønnes noralf@tronnes.org a écrit :
Den 24.05.2020 21.54, skrev Paul Cercueil:
Hi Noralf,
Le dim. 24 mai 2020 à 19:46, Noralf Trønnes noralf@tronnes.org a écrit : > > > Den 24.05.2020 18.13, skrev Paul Cercueil: >> Hi list, >> >> I'd like to open a discussion about the current support of >> MIPI >> DSI and >> DBI panels. >> >> Both are standards from the MIPI alliance, both are >> communication >> protocols between a LCD controller and a LCD panel, they >> generally both >> use the same commands (DCS), the main difference is that >> DSI is >> serial >> and DBI is generally parallel. >> >> In the kernel right now, DSI is pretty well implemented. >> All the >> infrastucture to register a DSI host, DSI device etc. is >> there. DSI >> panels are implemented as regular drm_panel instances, and >> their >> drivers >> go through the DSI API to communicate with the panel, >> which makes >> them >> independent of the DSI host driver. >> >> DBI, on the other hand, does not have any of this. All (?) >> DBI >> panels >> are implemented as tinydrm drivers, which make them >> impossible to >> use >> with regular DRM drivers. Writing a standard drm_panel >> driver is >> impossible, as there is no concept of host and device. All >> these >> tinydrm >> drivers register their own DBI host as they all do DBI >> over SPI. >> >> I think this needs a good cleanup. Given that DSI and DBI >> are so >> similar, it would probably make sense to fuse DBI support >> into >> the >> current DSI code, as trying to update DBI would result in >> a lot >> of code >> being duplicated. With the proper host/device registration >> mechanism >> from DSI code, it would be possible to turn most of the >> tinydrm >> drivers >> into regular drm_panel drivers. >> >> The problem then is that these should still be available as >> tinydrm >> drivers. If the DSI/DBI panels can somehow register a >> .update_fb() >> callback, it would make it possible to have a >> panel-agnostic >> tinydrm >> driver, which would then probably open a lot of doors, and >> help a >> lot to >> clean the mess. >> >> I think I can help with that, I just need some guidance - >> I am >> fishing >> in exotic seas here. >> >> Thoughts, comments, are very welcome. > > I did look at this a few months back: > > drm/mipi-dbi: Support panel drivers > > > > https://lists.freedesktop.org/archives/dri-devel/2019-August/228966.html > > > > The problem with DBI is that it has reused other busses which > means we > don't have DBI drivers, we have SPI drivers instead > (6800/8080 > is not > avail. as busses in Linux yet). DSI and DPI on the other > hand has > dedicated hw controller drivers not shared with other > subsystems.
I don't think that should be much of a problem. You could have a DBI/SPI bridge, that wraps a SPI device into a DBI host, for instance. The panel drivers would just use the DBI API without having to know what's done behind the scene.
This will be a bridge implemented in software, are we allowed to have software devices in the Device Tree? I though it was just allowed to describe hardware.
It wouldn't appear in devicetree. If the panel is connected over SPI, then DBI is just the protocol it uses.
How do you attach a panel to the DBI device if it doesn't appear in DT?
When probed from a DBI host controller, the panel's devicetree binding would look like this:
&dbi_host {
panel { compatible = "my,dbi-device"; };};
When probed from SPI it would appear in DT like this:
&spi {
panel@0 { reg = <0>; compatible = "my,dbi-device-spi"; };};
In that case, the driver would create a SPI-DBI bridge, but that is an implementation detail that doesn't belong in devicetree.
You said that you want to turn the tinydrm drivers into regular drm_panel drivers. If this is a drm_panel driver, who calls drm_of_find_panel_or_bridge() to make use of it? Or is this drm_panel driver a full blown DRM driver?
What I had in mind was a generic tinydrm driver that fetched the drm_panel from devicetree. Which is what you were working on, right?
(btw. tinydrm.ko is gone now, all drivers in tiny/ are regular DRM drivers)
I'm curious, what kind of device is going to use this? It's a bit strange to spend so many pins on the display interface and choose DBI instead of DPI.
I'm not sure the number of pins changes that much between the two, does it? Here I have 16 pins for command/data, one pin for command/data signal, and the pixel clock.
DBI has advantages over DPI, e.g. you don't need a separate SPI/I2C to configure the panel, and data is only transferred when a new frame is available, which means power savings when displaying still images, or a variable refresh rate when displaying video.
-Paul
Another problem is that the DBI panel uses SPI both for framebuffer upload and controller initialization. How shall this be handled when the panel driver needs SPI for init and the DBI bridge needs SPI for frame upload?
Does the panel driver need SPI for init? I don't think so. It needs to send DBI commands over SPI, yes. Only the DBI-SPI bridge would control the SPI device.
-Paul
If probed as a SPI device driver, the panel's spi_driver would register an instance of the DBI/SPI host driver, then register itself as a dbi_driver. If probed from a DBI host it would just register itself as a dbi_driver.
-Paul
> My initial tinydrm work used drm_panel, but I was not > allowed to > use it > (at least not the way I had done it). > > Noralf. > >> >> Cheers, >> -Paul >> >>