Re: [PATCH v1 2/2] xen/kbdif: add multi-touch support

[Oleksandr Andrushchenko <andr2000@gmail.com>]

On 01/23/2017 09:49 PM, Stefano Stabellini wrote:
> On Sat, 21 Jan 2017, Oleksandr Andrushchenko wrote:
>> In the mail-thread you mentioned above there is a picture of
>> the xenstore entries and conclusion:
>>
>> 1. No change to the existing kbd+ptr:
>> 1.1. They still share a dedicated (single) connection
>> channel as they did before multi-touch: page-ref + event-channel:
>>
>> /local/domain/2/device/vkbd/0/page-ref = "1248025"
>> /local/domain/2/device/vkbd/0/page-gref = "336"
>> /local/domain/2/device/vkbd/0/event-channel = "43"
>>
>> 1.2. No multi-touch events via that connection
>> 1.3. Old backends/frontends will see no difference
>> after multi-touch
>>
>> 2. Each multi-touch device has its own:
>> 2.1. Configuration (width x height, num-contacts)
>> 2.2. Own connection channel (page-ref, event-channel)
>>
>> /local/domain/2/device/vkbd/0/mtouch/0/width = "3200"
>> /local/domain/2/device/vkbd/0/mtouch/0/height = "3200"
>> /local/domain/2/device/vkbd/0/mtouch/0/num-contacts = "5"
>> /local/domain/2/device/vkbd/0/mtouch/0/page-ref = "1252386"
>> /local/domain/2/device/vkbd/0/mtouch/0/page-gref = "335"
>> /local/domain/2/device/vkbd/0/mtouch/0/event-channel = "44"
>>
>> /local/domain/2/device/vkbd/0/mtouch/1/width = "6400"
>> /local/domain/2/device/vkbd/0/mtouch/1/height = "6400"
>> /local/domain/2/device/vkbd/0/mtouch/1/num-contacts = "10"
>> /local/domain/2/device/vkbd/0/mtouch/1/page-ref = "2376038"
>> /local/domain/2/device/vkbd/0/mtouch/1/page-gref = "337"
>> /local/domain/2/device/vkbd/0/mtouch/1/event-channel = "45"
>> So, for the example above: 1 kbd + 1 ptr + 2 mt devices
>> within a single driver, *3 connections*
>>
>>> There are no ring-ref and event-channel under mtouch, are there?
>> there are ring-refs and event-channels under mtouch *per multi-touch*
>> device
> Now, it is clear. Sorry it took so many back and forth.
np
> page-ref and event-channel should definitely be described under
> "Multi-touch Device Parameters". When I asked you to remove them, I
> meant not just from the description but also from the protocol. This is
> where the confusion originated: in the current patch the two parameters
> are not described, hence I assumed they didn't exist and you were
> reusing the existing ring (/local/domain/2/device/vkbd/0/page-ref).
understood
>
> Aside from new message structs, which look fine to me, we have two
> important choices to make:
>
> a) number of multitouch devices per PV frontend/backend connection
> (By frontend/backend connection, I mean by xenstore device,
> such as /local/domain/2/device/vkbd/0.)
>
> b) number of multitouch devices sharing the same ring
> (By ring I mean page-ref + event channel.)
>
> Both these choices need to be motivated. As I wrote in the past, I would
> go for 1 multitouch device per frontend/backend connection, reusing the
> existing ring (/local/domain/2/device/vkbd/0/page-ref and
> /local/domain/2/device/vkbd/0/event-channel), because I assume that
> there won't be many multitouch devices, less than 5,
yes, I have 2 mt devices in my use-case
> so we can afford to
> create new PV devices for each of them.
see at the bottom
>   I am also assuming that it is
> fine for them to share ring with the keyboard or pointer devices without
> effects on performance because they should be all low frequency devices.
sounds reasonable, but see below
> The current proposal supports multiple multitouch devices per
> frontend/backnend connection and allocates a new dedicated ring for each
> multitouch device. Your proposal might very well be the best solution,
> but why? The architectural choices need to be motivated. How many
> multitouch devices is reasonable to expect to be present on a platform?
> What is the multitouch events frequency we expect from them? The answer
> to the first question motivates choice a), the answer to the second
> question motivates choice b). Please add them both to the protocol
> description.  It is OK to add the explanation to kbdif.h. You also
> mentioned something about multiple PV devices causing troubles to Linux.
> If that is an issue somehow, please add info about that too.
Well, all the above sounds reasonable, some comments:
1. Event frequency: here we are talking about 60-120(?)Hz
scan frequency of a touch screen generating at least 2
events for each contact: frame (syn) and position.
So, we can estimate for 10 fingers the value to be
60 * 2 * 10 = 1200 events a second per multi-touch device.
Of course, this is rough estimate which I believe would
be smaller in real life.

2. "create new PV devices for each of them"
2.1. Driver
This is something which you are seeing in xenstore as
   /local/domain/2/device/vkbd/0
0 - being index of the *driver*.
And the challenge here is that if you want multiple *devices*
to be allocated in this way:
   /local/domain/2/device/vkbd/0
   /local/domain/2/device/vkbd/1
   /local/domain/2/device/vkbd/2
then you have to register multiple *DRIVERS*.
What we normally do in the PV front driver is we call
*xenbus_register_frontend*,passing a structure
*struct xenbus_driver* with *.ids* set to a single entry "vkbd".

I am a little bit confused here about front implementation:
how to dynamically register multiple "vkbd" *drivers* depending on
configuration, so we have the layout you mention.

2.2. Input device
You are virtually not limited on number of *DEVICES within a DRIVER*,
but they all are under "/local/domain/2/device/vkbd/0" entry.
That being said, under the same xenstore entry you can allocate
ptr+kbd+N*mt input devices.

Bottom line:
================
a) From (1), probably it would be fine to send these events
over the existing ring (kbd+ptr), but mt event structures
need an index to indicate which mt device is the owner of
the event
b) We may still want to have a dedicated ring either for all
mt devices (depending on event frequency we consider reasonable)
or per mt-device.
c) I see no simple way to support multiple drivers comparing to
multiple devices (*DISCLAIMER* which is natural to Linux).

All,
what do you think on the above? What is the approach we are going to
take?

Thank you,
Oleksandr

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