Message-ID: <55F7F5D2.4090403@codeaurora.org>
Date:	Tue, 15 Sep 2015 16:11:22 +0530
From:	Archit Taneja <architt@...eaurora.org>
To:	Thierry Reding <treding@...dia.com>,
	Mark Rutland <mark.rutland@....com>,
	Rob Herring <robh+dt@...nel.org>
Cc:	Andrzej Hajda <a.hajda@...sung.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org
Subject: Re: [RFC 0/2] drm/dsi: DSI for devices with different control bus

Hi Rob, Mark,

We've been trying to figure out the right way to represent a class
of display encoder devices in DT.

These devices have registers that are generally configured via i2c. Once 
the device is configured, it takes in video data from the mipi
dsi bus.

Until now, all the devices we've supported devices that can be are
configured by the dsi bus itself, and hence, we've been able to
represent them in DT as children under the dsi host.

For the above class of devices (using both i2c and dsi), we aren't
able to conclude upon what's the better approach among the two:

1. Represent the device via 2 different nodes in DT. One would be
a child under an i2c adapter, the other a child of a dsi host. We
would have two device drivers, one i2c client, and the other a
mipi dsi device.

2. Represent the device as an i2c client in DT. Provide an api
to create "dummy" dsi devices. The i2c client driver would use
this api to register a dsi device, and link itself with the dsi
host.

What do you think would be the way to go here? I guess you
might have faced something similar in other subsystems.

I've given a relatively brief description of the problem. There
were some more nitty gritties discussed in this thread before.

Thierry, Andrzej, Lucas,

Please feel free to add your comments if I have missed out on
something.

Thanks,
Archit

On 09/10/2015 01:02 PM, Thierry Reding wrote:
> On Thu, Sep 10, 2015 at 11:45:35AM +0530, Archit Taneja wrote:
>>
>>
>> On 09/08/2015 03:57 PM, Andrzej Hajda wrote:
>>> On 09/07/2015 01:46 PM, Archit Taneja wrote:
>>>> Thierry,
>>>>
>>>> On 08/21/2015 11:39 AM, Archit Taneja wrote:
>>>>>
>>>>>
>>>>> On 08/20/2015 05:18 PM, Thierry Reding wrote:
>>>>>> On Thu, Aug 20, 2015 at 09:46:14AM +0530, Archit Taneja wrote:
>>>>>>> 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?
>>>>>>
>>>>>> Describing the number of lanes, specifying the virtual channel, mode
>>>>>> flags, etc. You could probably set the number of lanes and mode flags
>>>>>> via the I2C driver, but especially the virtual channel cannot be set
>>>>>> because it isn't known to the I2C DT branch of the device.
>>>>>
>>>>> I agree with the VC part. It could be a DT entry within the i2c client
>>>>> node, but that does make it seem like a quirk. The 'reg' way under the
>>>>> DSI host is definitely better to populate the virtual channel.
>>>>>
>>>>>>
>>>>>>> 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.
>>>>>>
>>>>>> I understand, but I don't see why it has to be instantiated by the I2C
>>>>>> driver, that's what I find backwards. There is already a standard way
>>>>>> for instantiating DSI devices, why not use it?
>>>>>
>>>>> I assumed we could either represent the device using an i2c driver, or
>>>>> dsi, but not both. Hence, I came up with this approach.
>>>>>
>>>>>>
>>>>>>>> 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?
>>>>>>
>>>>>> Correct. That's kind of similar to the way an HDMI encoder driver would
>>>>>> use an I2C adapter to query EDID. The i2c_client device would be a means
>>>>>> for the DSI driver to access the control interface.
>>>>>
>>>>> Okay.
>>>>>
>>>>> Although, I'm not sure about the HDMI encoder example. An HDMI
>>>>> encoder would read off edid directly from the adapter(with an address
>>>>> specified), it wouldn't need to create an i2c client device. Therefore,
>>>>> an HDMI encoder wouldn't need to have a separate node for i2c in DT.
>>>>>
>>>>>>
>>>>>>> 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.
>>>>>>
>>>>>> I agree that this somewhat stretches the capabilities of device tree.
>>>>>> Another alternative I guess would be to not have a compatible string for
>>>>>> the I2C device at all (that's technically not valid, I guess) because we
>>>>>> really don't need an I2C driver for the device. What we really need is a
>>>>>> DSI driver with a means to talk over some I2C bus with some other part
>>>>>> of its device.
>>>>>
>>>>> I think what the driver should 'really' be is a bit subjective, and can
>>>>> vary based on what the buses are used for in the device. For the Toshiba
>>>>> chip that Jani mentioned, it tends more towards a DSI driver. Whereas,
>>>>> for an ADV75xx chip, it's closer to an I2C driver since only I2C can be
>>>>> used to configure the chip registers.
>>>>>
>>>>> Although, I agree with the point you made about the DSI bus here:
>>>>>
>>>>> "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."
>>>>>
>>>>> The fact that the data on the DSI bus contains routing information (i.e,
>>>>> virtual channel number) always gives it some 'control' aspect.
>>>>>
>>>>>>
>>>>>>>   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.
>>>>>>
>>>>>> Why would that be inconsistent? That sounds like the most accurate
>>>>>> representation of the hardware to me.
>>>>>
>>>>> Inconsistent wasn't the right term. I should have used 'uncommon' :)
>>>>> It's common for two chips of the same family to have a different set
>>>>> optional properties in DT, but it's not common for two chips of the
>>>>> same family to be represented by a different number of devices in
>>>>> DT.
>>>>>
>>>>> I don't have an issue with the fused approach you suggested, as long
>>>>> as people are okay with the DT parts. Especially the part of needing 2
>>>>> compatible strings in the DT.
>>>>
>>>> I implemented the ADV7533 driver with the approach you suggested above
>>>> (2 drivers for 2 different components of the chip). I posted it out
>>>> just a while back (with you in loop).
>>>>
>>>> The DT node with this apporach would look like this:
>>>>
>>>> https://github.com/boddob/linux/blob/c24cbf63a6998d00095c10122ce5e37b764c7dba/arch/arm64/boot/dts/qcom/apq8016-sbc.dtsi#L162
>>>>
>>>> The main irritant with the '2 driver' approach is that we need to
>>>> share the per-device driver data with them. For ADV7533, I've made
>>>> the i2c driver allocate driver data (struct adv7511).
>>>>
>>>> The dsi driver gets the driver data in the following way:
>>>>
>>>> - The i2c driver sets the driver data as its client data using
>>>>     i2c_set_clientdata()
>>>> - Parse the i2c-control phandle to get the corresponding i2c client.
>>>> - Extract the adv7511 struct by getting i2c_get_clientdata()
>>>>
>>>> This way of getting the same driver data is a bit strange, but it
>>>> works. For this, we do need to ensure that the dsi driver defers
>>>> as long as the i2c driver isn't probed.
>>>>
>>>> I've now implemented both approaches for the driver. The first using
>>>> a dummy dsi device, and this one using 2 drivers (with both being
>>>> represented in DT). The advantage of the latter is that we don't need
>>>> to create any dummy device stuff, the disadvantage is that DT is a bit
>>>> uncommon.
>>>>
>>>> Can we now come to a conclusion on what approach is better?
>>>
>>> DSI by design is data bus which can be used additionally as a control bus, but
>>> in this particular case it is purely data bus. So of-graph bindings seem to be
>>> better choice. As already Lucas Stach said DT hierarchy should describe control
>>> buses and of-graph bindings should describe data bus. Argument that hw has two
>>> interfaces does not seem to be valid here - it has only one control interface.
>>> The other one is only for data, representing every data interface using DT
>>> hierarchy would lead to inflation of pseudo devices.
>>>
>>> On the other side I do not see dummy device approach ideal solution, I guess
>>> lightweight framework providing DSI hosts detached from Linux device model could
>>> work better here.
>>> The only problem here is that it should coexist somehow with dsi bus used to
>>> control devices. Anyway implementing it shouldn't be hard, question is if it
>>> would be eventually accepted :) I guess we can live for now with dummy devs.
>>>
>>> Summarizing I would prefer version with dummy devices, as it seems more
>>> compatible with DT design.
>>
>> Thanks for the feedback. I'll spin a newer version of the dummy dsi dev
>> patches after waiting for some more comments.
>
> Let's wait for someone from the device tree maintainers to comment
> instead of going around in circles.
>
> Thierry
>

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