Re: [PATCH v4 1/3] drm/uapi: Add USB connector type

["Noralf Trønnes" <noralf@tronnes.org>]

Den 22.01.2021 13.47, skrev Thomas Zimmermann:
> Hi
> 
> Am 22.01.21 um 12:44 schrieb Noralf Trønnes:
>>
>> And wrt PCI it wouldn't be a PCI connector if the card has some other
>> connector for the display, but if it was possible to connect a display
>> directly to the PCI connector, then yes I would call that a PCI
>> connector.
> 
> You're not connecting a display to the computer. You're connecting an
> RPi and then connect the display to the RPi. The RPi acts like an
> external graphics card.
> 
>> This begs the question: Why does the kernel provide info to userspace
>> about the connector type?
>>
>> My take is that it is so the user can know which display is connected to
>> which port on the computer.
> 
> This exactly illustrates the problem with the current naming. For a
> single output the distinction between bus and connector might be fuzzy.
> As soon as a connected SoC contains multiple connectors. The user then
> sees names such as card1-USB-0 and card1-USB-1, which makes no sense.
> 

If you look at the code I pasted in, you'll see that the SoC connector
types are passed through to the host driver as-is unless they are panel
connectors like DSI/DPI, which will be interpreted as USB (the protocol
does support multiple connectors, but only one can be used at a time).

So for the Pi4 as a display adapter, the host will see card1-HDMI-0 and
card1-HDMI-1, the same is true for the composite output (if enabled) it
shows up as card1-Composite-0 (can't be enabled together with HDMI).

If the Pi4 is used together with a DSI connected touchscreen, it makes
sense to disable the SoC HDMI outputs and the host only will see the
board as card1-USB-0 (I haven't done this exercise yet since there's
problems with getting the official Pi touchscreen to work with vc4 on Pi4).

The USB connector type is most important for tiny displays that is
microcontroller based without Linux running. There are lots of tiny SPI
displays and I expect this market to shift towards USB because it much
easier to connect and the display will be useable on a desktop/server
computer as status displays perhaps. But embedded will also benefit from
having these displays USB interfaced.

Another use case I see is repurposing old Android tablets as USB
displays that can be connected to any computer and become a touchscreen.
In this case I also want the connector to be called card1-USB-0 (I
haven't done any work in this area, old Android is fbdev so some work is
needed for this to happen).

Noralf.

>>
>> What's your opinion?
>>
>>>>
>>>> Ofc as Daniel mentions it's a downside that userspace doesn't know
>>>> about
>>>> the connector type, and who knows when it will updated (if I don't do
>>>> it).
>>>> Weston will name it: "UNNAMED-%d"
>>>> Mutter: "Unknown%d-%d"
>>>> X: "Unknown%d-%d"
>>>>
>>>> Sam and Laurent has discussed adding a PANEL connector type instead of
>>>> adding more connector types for panel connectors. I think that would
>>>> have been a better choice instead of the SPI connector type that I
>>>> added
>>>> in 2019. But I think PANEL was meant for panels connected to an
>>>> internal
>>>> connector.
>>>>
>>>> Here's my protocol connector types and how it's mapped to DRM:
>>>>
>>>> #define GUD_CONNECTOR_TYPE_PANEL        0
>>>> #define GUD_CONNECTOR_TYPE_VGA            1
>>>> #define GUD_CONNECTOR_TYPE_COMPOSITE        2
>>>> #define GUD_CONNECTOR_TYPE_SVIDEO        3
>>>> #define GUD_CONNECTOR_TYPE_COMPONENT        4
>>>> #define GUD_CONNECTOR_TYPE_DVI            5
>>>> #define GUD_CONNECTOR_TYPE_DISPLAYPORT        6
>>>> #define GUD_CONNECTOR_TYPE_HDMI            7
>>>>
>>>> static int gud_gadget_ctrl_get_connector(struct gud_gadget *gdg,
>>>> unsigned int index,
>>>>                       struct gud_connector_descriptor_req *desc)
>>>> {
>>>> ...
>>>>      gconn = &gdg->connectors[index];
>>>>
>>>>      switch (gconn->connector->connector_type) {
>>>>      case DRM_MODE_CONNECTOR_VGA:
>>>>          desc->connector_type = GUD_CONNECTOR_TYPE_VGA;
>>>>          break;
>>>>      case DRM_MODE_CONNECTOR_DVII:
>>>>          fallthrough;
>>>>      case DRM_MODE_CONNECTOR_DVID:
>>>>          fallthrough;
>>>>      case DRM_MODE_CONNECTOR_DVIA:
>>>>          desc->connector_type = GUD_CONNECTOR_TYPE_DVI;
>>>>          break;
>>>>      case DRM_MODE_CONNECTOR_Composite:
>>>>          desc->connector_type = GUD_CONNECTOR_TYPE_COMPOSITE;
>>>>          break;
>>>>      case DRM_MODE_CONNECTOR_SVIDEO:
>>>>          desc->connector_type = GUD_CONNECTOR_TYPE_SVIDEO;
>>>>          break;
>>>>      case DRM_MODE_CONNECTOR_Component:
>>>>          desc->connector_type = GUD_CONNECTOR_TYPE_COMPONENT;
>>>>          break;
>>>>      case DRM_MODE_CONNECTOR_DisplayPort:
>>>>          desc->connector_type = GUD_CONNECTOR_TYPE_DISPLAYPORT;
>>>>          break;
>>>>      case DRM_MODE_CONNECTOR_HDMIA:
>>>>          fallthrough;
>>>>      case DRM_MODE_CONNECTOR_HDMIB:
>>>>          desc->connector_type = GUD_CONNECTOR_TYPE_HDMI;
>>>>          break;
>>>>      default:
>>>>          desc->connector_type = GUD_CONNECTOR_TYPE_PANEL;
>>>>          break;
>>>>      };
>>>>
>>>>
>>>> int gud_connector_create(struct gud_device *gdrm, unsigned int index)
>>>> {
>>>> ...
>>>>      switch (desc.connector_type) {
>>>>      case GUD_CONNECTOR_TYPE_PANEL:
>>>>          connector_type = DRM_MODE_CONNECTOR_USB;
>>>>          break;
>>>>      case GUD_CONNECTOR_TYPE_VGA:
>>>>          connector_type = DRM_MODE_CONNECTOR_VGA;
>>>>          break;
>>>>      case GUD_CONNECTOR_TYPE_DVI:
>>>>          connector_type = DRM_MODE_CONNECTOR_DVID;
>>>>          break;
>>>>      case GUD_CONNECTOR_TYPE_COMPOSITE:
>>>>          connector_type = DRM_MODE_CONNECTOR_Composite;
>>>>          break;
>>>>      case GUD_CONNECTOR_TYPE_SVIDEO:
>>>>          connector_type = DRM_MODE_CONNECTOR_SVIDEO;
>>>>          break;
>>>>      case GUD_CONNECTOR_TYPE_COMPONENT:
>>>>          connector_type = DRM_MODE_CONNECTOR_Component;
>>>>          break;
>>>>      case GUD_CONNECTOR_TYPE_DISPLAYPORT:
>>>>          connector_type = DRM_MODE_CONNECTOR_DisplayPort;
>>>>          break;
>>>>      case GUD_CONNECTOR_TYPE_HDMI:
>>>>          connector_type = DRM_MODE_CONNECTOR_HDMIA;
>>>>          break;
>>>>      default: /* future types */
>>>>          connector_type = DRM_MODE_CONNECTOR_USB;
>>>
>>> The more I look at it the more I think it should be 'Unknown' here.
>>>
>>
>> I don't understand this, how will that be better for the user?
> 
> As I said before, the display is not connected via USB. The RPi (i.e.,
> graphics card) is. The naming would be off.
> 
> Best regards
> Thomas
> 
>>
>>> BTW, can I try this out somehow? I do have an RPi3. Do I need a special
>>> disk image?
>>
>> The Pi3 doesn'have a USB device/otg connector so I haven't made an image
>> for that one. Only the Pi Zero, model A and Pi 4 have that.
>>
>> The Pi2 and Pi3 have a USB hub on the soc's single USB port.
>>
>> Noralf.
>>
>>>
>>> Best regards
>>> Thomas
>>>
>>>>          break;
>>>>      };
>>>>
>>>> Noralf.
>>>>
>>>>> Best regards
>>>>> Thomas
>>>>>
>>>>>> -Daniel
>>>>>>
>>>>>>>
>>>>>>> Best regards
>>>>>>> Thomas
>>>>>>>
>>>>>>>>
>>>>>>>> Beware, new connector types have in the past resulted in userspace
>>>>>>>> burning&crashing. Maybe it's become better ...
>>>>>>>>
>>>>>>>> Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>>>>>>>>>
>>>>>>>>>      /**
>>>>>>>>>       * struct drm_mode_get_connector - Get connector metadata.
>>>>>>>>> -- 
>>>>>>>>> 2.23.0
>>>>>>>>>
>>>>>>>>> _______________________________________________
>>>>>>>>> dri-devel mailing list
>>>>>>>>> dri-devel@lists.freedesktop.org
>>>>>>>>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>> -- 
>>>>>>> Thomas Zimmermann
>>>>>>> Graphics Driver Developer
>>>>>>> SUSE Software Solutions Germany GmbH
>>>>>>> Maxfeldstr. 5, 90409 Nürnberg, Germany
>>>>>>> (HRB 36809, AG Nürnberg)
>>>>>>> Geschäftsführer: Felix Imendörffer
>>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> dri-devel mailing list
>>>>> dri-devel@lists.freedesktop.org
>>>>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>>>>>
>>>> _______________________________________________
>>>> dri-devel mailing list
>>>> dri-devel@lists.freedesktop.org
>>>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>>>>
>>>
>