Multiple parents in device driver model and Common Display Framework (CDF)

[Marcus Lorentzon]

On 02/12/2013 04:53 PM, Tomi Valkeinen wrote:
> On 2013-02-12 17:04, Marcus Lorentzon wrote:
>
>> Now we have some different types of panels which are attached on a
>> combination of busses:
>>
>> a) control:DBI, data:DBI
>> b) control:I2C/SPI, data:I2C/SPI
>> c) control:static setup, data:DPI
>> d) control:I2C/SPI, data:DPI
>> e) control:DSI, data:DSI
>> f) control:I2C, data:DSI
>> g) control:DSI+I2C, data:DSI
>>
>> As you could guess, g) is our issue. We have a device family that has
>> two control bus attachments and one data bus. The kernel device/driver
>> model only supports a single parent device which is normally the bus
>> device.
>>
>> We will write drivers for all device types a-g implementing the CDF API.
>> Those with only I2C/SPI bus attachemnt will of course be I2C drivers
>> registered to CDF, same probably goes for DBI and DSI panels if we
>> create a new bus type for them (if not - platform devices). But the CDF
>> drivers still need some way to access the bus/host operations. So we
>> will have to create an API for the exposed operations possible for the
>> MIPI type busses (DBI/DSI/DPI), some control and some data bus related.
>> For this problem we have discussed a few solutions, which is where we
>> need your guidance:
>>
>> 1) Due to the fact there is no support for multiple parents in the
>> device driver model and the fact that there are no DSI/DBI busses in the
>> kernel, Tomi has come up with a sort of logical parent device for
>> displays (see video source section, top section is "CDF API"):
>> http://gitorious.org/linux-omap-dss2/linux/blobs/work/dss-dev-model-cdf/include/video/display.h
> When I made that, I didn't even have in mind the case g).
>
> I made it because I think we have issues with case f) also (and, well,
> in some sense we have issues with all cases. see below). If we have a
> full linux bus for DSI and DBI, I don't quite understand how we should
> manage f), because we have both I2C and DSI busses to which the display
> device should belong.

You are right that any display where control bus is different from data 
bus have the same issue of two parents/busses.
>
> I also had these points in my mind why I chose the video_source approach
> in my version:
>
> - The display busses are very specialized, point-to-point busses, so a
> real linux bus doesn't give very much, I think.

Maybe in most cases, like a modern mobile phone. But for DBI and DSI you 
can have many devices or endpoints on the same bus. On DSI you can even 
probe for devices. So I don't think they are that far off Linux busses. 
Agreed that DPI is not one of those, but even that can be used for 
multiple panels with a scanline buffered panel and scanline interleaving 
(think interlaced). But the latter DPI is not a use case I think we 
should target, it could be simulated as two point to point connections.
In all cases I would like to use normal pm, enumeration etc, but then we 
need a way to describe multiple parents so I2C is not turned off before 
DPI etc. Not all platforms have dedicated I2C master under graphics sub 
system control. And even point-to-point will have a parent. A DPI bus 
could be extremely simple, the bus type struct, the dispc as bus device 
(display parent), and some DPI ops for the device, not the parent 
device. I assume video sources is a point to point reference (maybe 
video pipe is more correct), that is, it encapsulates both the host and 
the client in one object, since there is no reference to the actual 
device you operate on in the video source ops.
> - You never have a video bus used only for control, for example, a panel
> controlled via DSI but video data sent via DPI. Yes, possible in theory,
> but that would be rather insane.
Agreed, insane :)
> - We anyway need some kind of non-bus approach for simple video data
> busses like DPI. And if we have that, the slightly more complex video
> busses like DSI fit quite easily in.

Agreed, but that doesn't mean we need to bake them all into one. There 
is very little overlap between the different bus APIs.
> - We need something to represent all the data busses (see below).
>
>> Pros: Simple, easy to implement, merging all bus types into one logical
>> bus (simplicity)
>> Cons: Diverging from device driver model, merging all bus types into one
>> logical bus (scalability, maintainability), has to make a copy of many
>> things already in device driver model (pm, enumeration, registration,
>> relations, ...), solution only really needed for one special type (g)
> It's not only for g). We need something similar for all cases. We need
> to represent the chain of display devices with something, which is based
> on the data busses. The control bus plays no role in this chain (except
> when the data and control busses happen to be the same).

The idea is to keep things simple. If you have video bridge chips, you 
can create a "bridge-CDF" driver that aggregate the next device in the 
chain. That way we never expose the complexity in those special cases 
with chains of encoders. This would also be easy to describe using the 
dev->parent relationship.
> My video_source really represents the data bus, but offers some extended
> features so that it also offers control bus operations for those video
> busses that have control and data in the same bus.

For MIPI type interfaces yes. But not for I2C+DSI for example. I think 
it is this idea to handle some interfaces differently from others that 
my pattern matching brain don't accept ;).
> If we go for a full DSI/DBI linux bus, we still need something to
> represent the video bus. Then we'll have two separate entities for DSI
> control (the real bus) and for DSI data (video_source or similar), which
> in the end go via the same physical DSI bus.
>

That depends on what ops we put in the bus. If we put both data and 
control ops as bus ops, we don't need that. The only thing I don't like 
with the full bus solutions is the idea of having two struct device for 
one physical device. But that is only for displays with multiple busses, 
which are kind of rare compared to the main target embedded panels which 
normally only use DSI/DBI bus for both data and control. Of course there 
are some DPI panels which would need dual busses. But most DPI panels I 
have used in products (if not all) never use I2C at all, only power 
on/off regulator and nothing more.

BTW. It should be noted that my own SoC driver use a design similar to 
"video source" currently :). Except it is called port in driver and 
formatter in dispc HW.

/BR
/Marcus