Re: [RFC 0/2] drm/dsi: DSI for devices with different control bus

From: Archit Taneja
Date: Thu Aug 20 2015 - 00:16:32 EST

Hi Thierry, Lucas,


On 08/19/2015 08:32 PM, Thierry Reding wrote:
On Wed, Aug 19, 2015 at 04:52:24PM +0200, Lucas Stach wrote:
Am Mittwoch, den 19.08.2015, 16:34 +0200 schrieb Thierry Reding:
On Wed, Aug 19, 2015 at 04:17:08PM +0200, Lucas Stach wrote:
Hi Thierry, Archit,

[...]
Perhaps a better way would be to invert this relationship. According to
your proposal we'd have to have DT like this:

i2c@... {
...

dsi-device@... {
...
dsi-bus = <&dsi>;
...
};

...
};

dsi@... {
...
};

Inversing the relationship would become something like this:

i2c@... {
...
};

dsi@... {
...

peripheral@... {
...
i2c-bus = <&i2c>;
...
};

...
};

Both of those aren't fundamentally different, and they both have the
disavantage of lacking ways to transport configuration data that the
other bus needs to instantiate the dummy device (such as the reg
property for example, denoting the I2C slave address or the DSI VC).

So how about we create two devices in the device tree and fuse them at
the driver level:

i2c@... {
...

i2cdsi: dsi-device@... {
...
};

...
};

dsi@... {
...

peripheral@... {
...
control = <&i2cdsi>;
...
};

...
};

This way we'll get both an I2C device and a DSI device that we can fully
describe using the standard device tree bindings. At driver time we can
get the I2C device from the phandle in the control property of the DSI
device and use it to execute I2C transactions.

I don't really like to see that you are inventing yet-another-way to
handle devices connected to multiple buses.

Devicetree is structured along the control buses, even if the devices
are connected to multiple buses, in the DT they are always children of
the bus that is used to control their registers from the CPUs
perspective. So a DSI encoder that is controlled through i2c is clearly
a child of the i2c master controller and only of that one.

I think that's a flawed interpretation of what's going on here. The
device in fact has two interfaces: one is I2C, the other is DSI. In my
opinion that's reason enough to represent it as two logical devices.

Does it really have 2 control interfaces that are used at the same time?
Or is the DSI connection a passive data bus if the register control
happens through i2c?

The interfaces may not be used at the same time, and the DSI interface
may even be crippled, but the device is still addressable from the DSI
host controller, if for nothing else than for routing the video stream.

If you need to model connections between devices that are not reflected
through the control bus hierarchy you should really consider using the
standardized of-graph bindings.
(Documentation/devicetree/bindings/graph.txt)

The problem is that the original proposal would instantiate a dummy
device, so it wouldn't be represented in DT at all. So unless you do add
two logical devices to DT (one for each bus interface), you don't have
anything to glue together with an OF graph.

I see that the having dummy device is the least desirable solution. But
if there is only one control bus to the device I think it should be one
device sitting beneath the control bus.

You can then use of-graph to model the data path between the DSI encoder
and device.

But you will be needing a device below the DSI host controller to
represent that endpoint of the connection. The DSI host controller
itself is in no way connected to the I2C adapter. You would have to
add some sort of quirk to the DSI controller binding to allow it to

Thanks for the review.

I implemented this to support ADV7533 DSI to HDMI encoder chip, which
has a DSI video bus and an i2c control bus.

There weren't any quirks as such in the device tree when I tried to
implement this. The DT seems to manage fine without a node
corresponding to a mipi_dsi_device:

i2c_adap@.. {
adv7533@.. {

port {
adv_in: endpoint {
remote-endpoint = <&dsi_out>;
};
};
};
};

dsi_host@.. {
...
...

port {
dsi_out: endpoint {
remote-endpoint = <&adv_in>;
}
};
};

It's the i2c driver's job to parse the graph and retrieve the
phandle to the dsi host. Using this, it can proceed with
registering itself to this host using the new dsi funcs. This
patch does the same for the adv7533 i2c driver:

http://www.spinics.net/lists/dri-devel/msg86840.html

hook up with a control endpoint. And then you'll need more quirks
to describe what kind of DSI device this is.

Could you explain what you meant by this? I.e. describing the kind
of DSI device?

The dsi device created isn't really a dummy device as such. It's
dummy in the sense that there isn't a real dsi driver associated
with it. The dsi device is still used to attach to a mipi dsi host,
the way normal dsi devices do.


On the other hand if you properly instantiate the DSI device you can
easily write a driver for it, and the driver will set up the correct
properties as implied by the compatible string. Once you have that you
can easily hook it up to the I2C control interface in whatever way you
like, be that an OF graph or just a simple unidirectional link by
phandle.


With the fused approach you suggested, we would have 2 drivers: one i2c
and the other dsi. The i2c client driver would be more or less minimal,
preparing an i2c_client device for the dsi driver to use. Is my
understanding correct?

We can do without dummy dsi devices with this method. But, representing
a device with 2 DT nodes seems a bit off. We'd also need to compatible
strings for the same device, one for the i2c part, and the other for
the dsi part.

From an adv75xx driver perspective, it should also support the ADV7511
chip, which is a RGB/DPI to HDMI encoder. For adv7511, we don't need a
DSI DT node. It would be a bit inconsistent to have the bindings
require both DSI and I2C nodes for one chip, and only I2C node for the
other, with both chips being supported by the same driver.

Thanks,
Archit

--
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/