On 11-07-18 14:08, Laurent Pinchart wrote:
Hi Carlos,

On Wednesday, 11 July 2018 14:36:48 EEST Carlos Garnacho wrote:
On Wed, Jul 11, 2018 at 1:00 PM, Laurent Pinchart wrote:
On Wednesday, 11 July 2018 11:37:14 EEST Hans de Goede wrote:
Hi Laurent,

At Guadec Carlos (in the Cc) told me that on his Acer 2-in-1 only
the frontcam is working and it seems both are represented by a
single UVC USB device. I've told him to check for some v4l control
to flip between front and back.

Carlos, as I mentioned you can try gtk-v4l
("sudo dnf install gtk-v4l") or qv4l2
("sudo dnf install qv4l2") these will both show
you various controls for the camera. One of those might do the trick.

But I recently bought a 2nd second hand Cherry Trail based HP
X2 2-in-1 and much to my surprise that is actually using an UVC
cam, rather then the usual ATOMISP crap and it has the same issue.

This device does not seem to have a control to flip between the
2 cams, instead it registers 2 /dev/video? nodes but the second
node does not work

The second node is there to expose metadata to userspace, not image data.
That's a recent addition to the uvcvideo driver.

and dmesg contains:

[   26.079868] uvcvideo: Found UVC 1.00 device HP TrueVision HD
[   26.095485] uvcvideo 1-4.2:1.0: Entity type for entity Extension 4 was
not initialized!
[   26.095492] uvcvideo 1-4.2:1.0: Entity type for entity Processing 2
not initialized!
[   26.095496] uvcvideo 1-4.2:1.0: Entity type for entity Camera 1 was

You can safely ignore those messages. I need to submit a patch to get rid
of them.

Laurent, I've attached lsusb -v output so that you can check the

Thank you.

It's funny how UVC specifies a standard way to describe a device with two
camera sensors with dynamic selection of one of them at runtime, and
vendors instead implement vendor-specific crap :-(

The interesting part in the descriptors is

       VideoControl Interface Descriptor:
         bLength                27
         bDescriptorType        36
         bDescriptorSubtype      6 (EXTENSION_UNIT)
         bUnitID                 4
         guidExtensionCode         {1229a78c-47b4-4094-b0ce-db07386fb938}
         bNumControl             2
         bNrPins                 1
         baSourceID( 0)          2
         bControlSize            2
         bmControls( 0)       0x00
         bmControls( 1)       0x06
         iExtension              0

The extension unit exposes two controls (bmControls is a bitmask). They
can be accessed from userspace through the UVCIOC_CTRL_QUERY ioctl, or
mapped to V4L2 controls through the UVCIOC_CTRL_MAP ioctl, in which case
they will be exposed to standard V4L2 applications.

If you want to experiment with this, I would advise querying both controls
UVC_GET_MAX, UVC_GET_DEF and UVC_GET_RES requests to get the control
current, minimum, maximum, default and resolution values, and UVC_GET_LEN
and UVC_GET_INFO to get the control size (in bytes) and flags. Based on
that you can start experimenting with UVC_SET_CUR to set semi-random

I'm however worried that those two controls would be a register address
and a register value, for indirect access to all hardware registers in
the device. In that case, you would likely need information from the
device vendor, or possibly a USB traffic dump from a Windows machine when
switching between the front and back cameras.

Carlos, it might be good to get Laurent your descriptors too, to do
this do "lsusb", note what is the <vid>:<pid> for your camera and then

sudo lsusb -v -d <vid>:<pid>  > lsusb.log

And send Laurent a mail with the generated lsusb

That would be appreciated, but I expect the same issue :-(

Please find it attached. IIUC your last email, it might not be the
exact same issue, but you can definitely judge better.

Your device is similar in the sense that it doesn't use the standard UVC
support for multiple camera sensors. It instead exposes two extension units:

       VideoControl Interface Descriptor:
         bLength                27
         bDescriptorType        36
         bDescriptorSubtype      6 (EXTENSION_UNIT)
         bUnitID                 4
         guidExtensionCode         {1229a78c-47b4-4094-b0ce-db07386fb938}
         bNumControl             2
         bNrPins                 1
         baSourceID( 0)          2
         bControlSize            2
         bmControls( 0)       0x00
         bmControls( 1)       0x06
         iExtension              0
       VideoControl Interface Descriptor:
         bLength                29
         bDescriptorType        36
         bDescriptorSubtype      6 (EXTENSION_UNIT)
         bUnitID                 6
         guidExtensionCode         {26b8105a-0713-4870-979d-da79444bb68e}
         bNumControl             9
         bNrPins                 1
         baSourceID( 0)          4
         bControlSize            4
         bmControls( 0)       0x1f
         bmControls( 1)       0x01
         bmControls( 2)       0x38
         bmControls( 3)       0x00
         iExtension              6 Realtek Extended Controls Unit

The first one is identical to Hans', and I expect it to offer indirect access
to internal device registers. The second one exposes 9 controls, and I expect
at least some of those to have direct effects on the device. What they do and
how they operate is unfortunately unknown.

Laurent, thank you for your input on this. I thought it was a bit weird that
the cam on my HP X2 only had what appears to be the debug controls, so I
opened it up and as I suspect (after your analysis) it is using a USB module
for the front camera, but the back camera is a sensor directly hooked with its
CSI/MIPI bus to the PCB, so very likely it is using the ATOMISP stuff.

So I think that we can consider this "solved" for my 2-in-1.

Carlos, your Acer is using a Core CPU (not an Atom) right ? And the front and
rear cams are both centered at the same edge I guess ?  (mine had one in the
corner and the other centered which already was a hint)



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