Den 24.05.2020 20.35, skrev Daniel Vetter:
On Sun, May 24, 2020 at 7:46 PM Noralf Trønnes noralf@tronnes.org wrote:
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.
Do we have drivers with dbi support that actually want to reuse the tinydrm drivers? Good clean is all good, but we need a solid reason for changing stuff. Plus we need to make sure we're not just rediscovering all the old reasons for why we ended up where we are right now in the first place.
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.
My initial tinydrm work used drm_panel, but I was not allowed to use it (at least not the way I had done it).
Hm, do we have a summary of all the discussions/reasons from back then? All I remember is that it's all that simple, you've done a lot of work exploring all the options, I'm fairly sure I suggested drm_panel even back then but somehow it didn't really work. Would be good if we make sure we don't at least repeat history too much :-)
Unfortunately I don't have the two RFC series in my inbox anymore. I can see from this coverletter that drm_panel was removed after RFC v2:
drm: Add support for tiny LCD displays https://patchwork.freedesktop.org/series/4520/
The problem is that there's no discussion in the relevant patch: https://patchwork.freedesktop.org/patch/80117/?series=4520&rev=2
What I remember is that someone said I couldn't use it, then I replied that someone suggested drm_panel to me in an earlier discussion, but I couldn't remember who. Then Emil chimed in and said he was the one that suggested it.
Anyways let's see what Paul comes up with, if he finds a way to move SPI DBI over to drm_panel than I'm all for it. If not we'll just have to live with a hybrid solution I guess, one for MIPI DBI parallel bus for his hardware type and one for MIPI DBI SPI.
The Pi also has a hw block for parallel DBI, downstream there's a driver that treats it as a generic parallel bus, since it's an 8080 compatible bus. If I'm not mistaken the BeagleBone Black also has a bus like this.
When I started on tinydrm I had the idea to try and add a parallel bus type to Linux (even had a prototype for bit banging gpio) that I could use with DBI. This would make the bus available for things like FPGA's also, not just displays. I gave up on the idea since parallel DBI uses a lot of pins to upload a frame to display GRAM, much better to drive the panel directly through MIPI DPI (better fps), which all these SoC's also support.
Noralf.
Cheers, Daniel
Noralf.
Cheers, -Paul
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