Hi,
first of all, thanks for the detailed feedback.
Am 26.03.19 um 15:53 schrieb Daniel Vetter:
On Tue, Mar 26, 2019 at 10:17:33AM +0100, Thomas Zimmermann wrote:
Hi,
this RFC patch set implements fbdevdrm, a DRM driver on top of fbdev drivers. I'd appreciate feedback on the code and the idea in general.
The fbdev subsystem is considered legacy and will probably be removed at some point. This would mean the loss of a signifanct number of drivers. Some of the affected hardware is probably not in use any longer, but some hardware is still around and provides good(-enough) framebuffers.
OTOH, userspace programs that want to support a wide range of graphics hardware have to implement support for both DRM and fbdev interfaces. Such software would benefit from a single interface.
The fbdevdrm driver provides a way of running drivers for old and new hardware from the DRM subsystem and interfaces.
It's not intended to add new features or drivers to fbdev. Instead fbdevdrm is supposed to be a template for converting fbdev drivers to DRM. It contains a number of comments (labeled 'DRM porting note') that explain the required steps. The license is fairly liberal to allow for combination with existing fbdev code.
I tested the current patch set with the following drivers: atyfb, aty128fb, matroxfb, pm2fb, s3fb, savagefb, sisfb, tdfxfb and tridentfb. I was able to successfully start with fbcon enabled and then run weston or X.
Thomas Zimmermann (11): drm/fbdevdrm: Add driver skeleton drm/fbdevdrm: Add fbdevdrm device drm/fbdevdrm: Add memory management drm/fbdevdrm: Add file operations drm/fbdevdrm: Add GEM and dumb interfaces drm/fbdevdrm: Add modesetting infrastructure drm/fbdevdrm: Add DRM <-> fbdev pixel-format conversion drm/fbdevdrm: Add mode conversion DRM <-> fbdev drm/fbdevdrm: Add primary plane drm/fbdevdrm: Add CRTC drm/fbdevdrm: Detect and validate display modes
Looks surprisingly clean, at least from a quick read. Only big thing I noticed on the implementation side is that you probably want to use the simple display helpers. At least that's a much better fit for simple display hw supported by these fbdev drivers.
I thought about using the simple-DRM helpers, but found that a full DRM driver would be more helpful for porting over fbdev drivers. Unless simple DRM is a hard requirement, I'd prefer to leave it this way.
For those devices that only support a single pipeline, the conversion to simple DRM should then be mandatory during the porting process.
What I'm not sure at all on is whether this is a good idea. It's a quick way of supporting a few drivers we might need from fbdev. But I fear for long-term ecosystem health it'll be a loss: Much less motivation to port the drivers to be native kms ones, and as a result, much less opportunities to extract helpers to make driver writing for such hw easier for everyone.
And the latter is really important, if you look at all the work people have done to improve the modeset helpers. Nowadays we have some examples where the kms native port of an fbdev driver turned out to be 2-3x smaller.
I don't disagree with any of this. My intention is to simplify the graphics stack *without* loosing support for all the old hardware. I see fbdevdrm as help for porting drivers, or short-term solution for getting old framebuffer to work.
But I also see the benefit of making the fbdev->kms transition smoother. For discussion here's a pretty wild idea that we might want to consider:
move the ttm based gem code into a helper library like the cma gem helpers. Only special piece we need to pass it is the fb_info structure, for that we can use the same hooks the cma helpers use to allow drivers to specify the right struct device to use for cma allocations. plus ofc an init function for the memory manager and all that, which drivers can put into their driver private device structure
move the modeset compat code into a helper based on top of the simple display pipe helpers.
for each fbdev driver we care about:
- create bare-bones kms driver
- copypaste the entire fbdev driver code into the drm driver
- hook up the above two helper libraries, remove the
register_framebuffer - I think we can still allocate the fb_info and all that for transition 4. transition the driver over. If we have the helpers a bit split up between display and buffer management side that should be a lot more gradual, and with the fbdevdrm midlayer inserted in the middle.
I use the current approach because it does not require modifications to DRM or fbdev. Not copying the fbdev driver code has the advantage that any fix that goes into fbdev is also used by fbdevdrm.
OTOH, that has some problems. At least the event-reporting hooks appear to be fragile. You mentioned that you have patches for replacing it. I'd be happy to use something else. Filtering out generic and DRM-based drivers is also not optimal.
Instead of adding the porting helpers to the DRM core, I'd suggest to add them to fbdevdrm. Fbdevdrm would have to duplicate some of the fbdev functionality and provide a replacement for register_framebuffer.
Porting would then mean to
1) create a new DRM driver by copying fbdevdrm, and 2) copy the fbdev driver code to the new DRM driver and rename the function calls accordingly. At this point the new driver should already work to some extend. 3) Finally, do the refactoring and get it out of staging.
If the fbdev subsystem is ever going to be removed, the remaining drivers could be moved under fbdevdrm and have their function calls adapted. If anyone cares about a driver, it'd be available for refactoring; otherwise it'd just sit there doing nothing. It's all self-contained and doesn't pollute the DRM core.
- Improve the other helpers we have as needed - there's probably room for a simple ttm version, inspired by all these simple display chips (e.g. ast/cirrus/bochs/mga all solve similar problems like this).
Makes sense, but appears to be unrelated.
Best regards Thomas
- Treat any such drivers as CONFIG_STAGING until they've become fully native, i.e. if no one bothers to convert them, we'll drop them again.
The above is kinda similar in spirit to the transitional helpers we've used to upgrade the big drivers from legacy kms to atomic.
Thoughts?
Cheers, Daniel
drivers/gpu/drm/Kconfig | 2 + drivers/gpu/drm/Makefile | 1 + drivers/gpu/drm/fbdevdrm/Kconfig | 13 + drivers/gpu/drm/fbdevdrm/Makefile | 11 + drivers/gpu/drm/fbdevdrm/fbdevdrm_bo.c | 276 ++++++++ drivers/gpu/drm/fbdevdrm/fbdevdrm_bo.h | 58 ++ drivers/gpu/drm/fbdevdrm/fbdevdrm_device.c | 96 +++ drivers/gpu/drm/fbdevdrm/fbdevdrm_device.h | 55 ++ drivers/gpu/drm/fbdevdrm/fbdevdrm_drv.c | 347 +++++++++ drivers/gpu/drm/fbdevdrm/fbdevdrm_format.c | 441 ++++++++++++ drivers/gpu/drm/fbdevdrm/fbdevdrm_format.h | 26 + drivers/gpu/drm/fbdevdrm/fbdevdrm_modes.c | 195 +++++ drivers/gpu/drm/fbdevdrm/fbdevdrm_modes.h | 53 ++ drivers/gpu/drm/fbdevdrm/fbdevdrm_modeset.c | 746 ++++++++++++++++++++ drivers/gpu/drm/fbdevdrm/fbdevdrm_modeset.h | 38 + drivers/gpu/drm/fbdevdrm/fbdevdrm_primary.c | 498 +++++++++++++ drivers/gpu/drm/fbdevdrm/fbdevdrm_primary.h | 27 + drivers/gpu/drm/fbdevdrm/fbdevdrm_ttm.c | 202 ++++++ drivers/gpu/drm/fbdevdrm/fbdevdrm_ttm.h | 35 + 19 files changed, 3120 insertions(+) create mode 100644 drivers/gpu/drm/fbdevdrm/Kconfig create mode 100644 drivers/gpu/drm/fbdevdrm/Makefile create mode 100644 drivers/gpu/drm/fbdevdrm/fbdevdrm_bo.c create mode 100644 drivers/gpu/drm/fbdevdrm/fbdevdrm_bo.h create mode 100644 drivers/gpu/drm/fbdevdrm/fbdevdrm_device.c create mode 100644 drivers/gpu/drm/fbdevdrm/fbdevdrm_device.h create mode 100644 drivers/gpu/drm/fbdevdrm/fbdevdrm_drv.c create mode 100644 drivers/gpu/drm/fbdevdrm/fbdevdrm_format.c create mode 100644 drivers/gpu/drm/fbdevdrm/fbdevdrm_format.h create mode 100644 drivers/gpu/drm/fbdevdrm/fbdevdrm_modes.c create mode 100644 drivers/gpu/drm/fbdevdrm/fbdevdrm_modes.h create mode 100644 drivers/gpu/drm/fbdevdrm/fbdevdrm_modeset.c create mode 100644 drivers/gpu/drm/fbdevdrm/fbdevdrm_modeset.h create mode 100644 drivers/gpu/drm/fbdevdrm/fbdevdrm_primary.c create mode 100644 drivers/gpu/drm/fbdevdrm/fbdevdrm_primary.h create mode 100644 drivers/gpu/drm/fbdevdrm/fbdevdrm_ttm.c create mode 100644 drivers/gpu/drm/fbdevdrm/fbdevdrm_ttm.h
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