RFC on upstreaming of a Mediatek DRM modesetting driver

[Frank Binns]

On 01/12/14 15:28, Daniel Vetter wrote:
> On Mon, Dec 01, 2014 at 10:01:37AM +0000, Frank Binns wrote:
>> Hi,
>>
>> We are currently in negotiations with one of our customers (Mediatek) on
>> a strategy that will allow them to push a DRM modesetting driver into
>> the upstream kernel. We are writing to get people's opinions and
>> feedback on our proposed approach.
>>
>> Currently, our driver is structured in such a way that the display
>> driver is more tightly integrated with the GPU driver than we would
>> like. Although our kernel driver has been shipped with a GPL license for
>> a long time, it is not in a form that would be considered acceptable
>> upstream. Unfortunately, it is going to be a long process to get this
>> part of the driver into a reasonable state. However, in the meantime, we
>> don't want to prevent customer portions of the driver from being
>> upstreamed. With the work done on recent kernels, and with a willing
>> partner in Mediatek, we now think that we can restructure our driver in
>> such a way as to allow this to happen.
>>
>> We see two basic approaches to achieving this:
>> 1) Two independent DRM drivers, i.e. modesetting and render node drivers
>> 2) A single componentised DRM driver
>>
>> Our (IMG's) preferred approach is to have a single componentised DRM
>> driver. This is due to the following reasons:
>>
>> - Existing user space is not fully prepared to handle render nodes.
>>
>> - There is concern that any IMG DRM render node driver will need
>> knowledge about multiple SoCs, each one being from a different vendor.
>> Would this be deemed acceptable?
>>
>> - There is a trend, at least for DRM SoC drivers, towards using the
>> component interface. Although there appears to be very few (one?)
>> examples of GPU component drivers.
>>
>> To give some high level details on how we expect the componentised DRM
>> driver model to work, each vendor (in this case Mediatek) will write
>> their own DRM driver (supporting modesetting, dumb buffers, GEM, prime,
>> etc) and IMG will provide an almost entirely independent component
>> driver that adds in GPU support. Until our GPU driver is in a suitable
>> state this will most likely necessitate a small kernel patch to wire up
>> support, e.g. GPU specific ioctls.
>>
>> Cross-device and cross-process memory allocations will be made using the
>> DRM driver. In order for this memory to be shared with the GPU component
>> driver it will be necessary, at least for the time being, to export it
>> via prime and import it via a GPU ioctl. Synchronisation between the
>> display and GPU will be performed via reservation objects.
>>
>> Does this sound like a sane approach? Questions and/or feedback is very
>> welcome.
> Rule of thumb is that if it's an externally licensed IP block it should be
> a separate driver. Which is the case here. The idea is that the mostly
> generic IMG driver could be reused on other platforms that ship the same
> IP-block, while linking up with the respective display controller driver.
> The end result is 2 drm drivers:
> - Display block drm driver which expose KMS objects for modesetting, but
>   only very basic gem (just enough to allocate dumb framebuffers and
>   import/export dma-bufs).
> - Full-blown gem driver for the img render IP block.
>
> For an example look at the tegra/nouveau combo which can run on TK1.
>
> Plugggin in an IMG driver into each display block like it's currently done
> with all the armsoc stuff on android is imo completely no-go.
>
> Note that the component interface is completely irrelevant wrt the
> interface you expose to userspace. It's just an driver-internal helper
> library useful in certain situation. Not even the drm core really cares
> whether you use component helpers or not.
>
> Thanks, Daniel

OK, so it seems the consensus is that IMG should provide a separate
render-node only DRM driver.

Having not worked directly on the core DRM code I'm not completely
familiar with it but it seems to me that the DRIVER_MODESET flag has a
dual meaning. Firstly it means that the driver supports KMS and secondly
it means that a lot of the legacy stuff isn't supported. It also changes
the way in which driver initialisation is performed. Would it make sense
for the DRIVER_RENDER flag to have a similar effect? In other words,
should it turn off legacy stuff and use the newer method of driver
initialisation?

Thanks
Frank