Hi Hans,
On Mon, Feb 05, 2018 at 04:19:28PM +0100, Hans Verkuil wrote:
> On 02/05/2018 03:30 PM, Sakari Ailus wrote:
> > Hi Hans,
> >
> > On Mon, Feb 05, 2018 at 01:30:04PM +0100, Hans Verkuil wrote:
> >> On 02/05/2018 12:59 PM, Sakari Ailus wrote:
> >>> Hi Hans,
> >>>
> >>> On Mon, Feb 05, 2018 at 11:26:47AM +0100, Hans Verkuil wrote:
> >>>> The function media_device_enum_entities() has this workaround:
> >>>>
> >>>> /*
> >>>> * Workaround for a bug at media-ctl <= v1.10 that makes it to
> >>>> * do the wrong thing if the entity function doesn't belong to
> >>>> * either MEDIA_ENT_F_OLD_BASE or MEDIA_ENT_F_OLD_SUBDEV_BASE
> >>>> * Ranges.
> >>>> *
> >>>> * Non-subdevices are expected to be at the MEDIA_ENT_F_OLD_BASE,
> >>>> * or, otherwise, will be silently ignored by media-ctl when
> >>>> * printing the graphviz diagram. So, map them into the devnode
> >>>> * old range.
> >>>> */
> >>>> if (ent->function < MEDIA_ENT_F_OLD_BASE ||
> >>>> ent->function > MEDIA_ENT_F_TUNER) {
> >>>> if (is_media_entity_v4l2_subdev(ent))
> >>>> entd->type = MEDIA_ENT_F_V4L2_SUBDEV_UNKNOWN;
> >>>> else if (ent->function != MEDIA_ENT_F_IO_V4L)
> >>>> entd->type = MEDIA_ENT_T_DEVNODE_UNKNOWN;
> >>>> }
> >>>>
> >>>> But this means that the entity type returned by ENUM_ENTITIES just overwrites
> >>>> perfectly fine types by bogus values and thus the returned value differs
> >>>> from that returned by G_TOPOLOGY.
> >>>>
> >>>> Can we please, please remove this workaround? I have no idea why a workaround
> >>>> for media-ctl of all things ever made it in the kernel.
> >>>
> >>> The entity types were replaced by entity functions back in 2015 with the
> >>> big Media controller reshuffle. While I agree functions are better for
> >>> describing entities than types (and those types had Linux specific
> >>> interfaces in them), the new function-based API still may support a single
> >>> value, i.e. a single function per device.
> >>>
> >>> This also created two different name spaces for describing entities: the
> >>> old types used by the MC API and the new functions used by MC v2 API.
> >>>
> >>> This doesn't go well with the fact that, as you noticed, the pad
> >>> information is not available through MC v2 API. The pad information is
> >>> required by the user space so software continues to use the original MC
> >>> API.
> >>>
> >>> I don't think there's a way to avoid maintaining two name spaces (types and
> >>> functions) without breaking at least one of the APIs.
> >>
> >> The comment specifically claims that this workaround is for media-ctl and
> >> it suggests that it is fixed after v1.10. Is that comment bogus? I can't
> >> really tell which commit fixed media-ctl. Does anyone know?
> >>
> >> As far as I can tell the function defines have been chosen in such a way that
> >> they will equally well work with the old name space. There should be no
> >> problem there whatsoever and media-ctl should switch to use the new defines.
> >
> > The old interface (type) was centered around the uAPI for the entity.
> > That's no longer the case for functions. The entity type
> > (MEDIA_ENT_TYPE_MASK) tells the uAPI which affects the interpretation of
> > the dev union in struct media_entity_struct as well as the interface that
> > device node implements. With the new function field that's no longer the
> > case.
> >
> > Also, the new MC v2 API makes a separation between the entity function and
> > the uAPI (interface) which was lacking in the old API.
> >
> >>
> >> We now have a broken ENUM_ENTITIES ioctl (it rudely overwrites VBI/DVB/etc types)
> >> and a broken G_TOPOLOGY ioctl (no pad index).
> >>
> >> This sucks. Let's fix both so that they at least report consistent information.
> >
> > The existing user space may assume that the type field of the entity
> > conveys that the entity does provide a V4L2 sub-device interface if that's
> > the case actually.
> >
> > This is what media-ctl does and I presume if existing user space checks for
> > the type field, it may well have similar checks: it was how the API was
> > defined. Therefore it's not entirely accurate to say that only media-ctl
> > has this "bug", I'd generally assume programs that use MC (v1) API do this.
> >
> > You could argue about the merits (or lack of them) of the old API, no
> > disagrement there.
>
> The old API is already broken. E.g. using MEDIA_ENT_F_PROC_VIDEO_SCALER in
> vimc/vimc-scaler.c (instead of the current - and bogus - MEDIA_ENT_F_ATV_DECODER)
> gives me this with media-ctl:
>
> - entity 21: Scaler (2 pads, 4 links)
> type V4L2 subdev subtype Unknown flags 0
> device node name /dev/v4l-subdev5
> pad0: Sink
> [fmt:RGB888_1X24/640x480 field:none]
> <- "Debayer A":1 [ENABLED]
> <- "Debayer B":1 []
> <- "RGB/YUV Input":0 []
> pad1: Source
> [fmt:RGB888_1X24/1920x1440 field:none]
> -> "RGB/YUV Capture":0 [ENABLED,IMMUTABLE]
>
> Useless. We now have an old API that gives us pad indices but not the
> function, and a new API that gives us the function but not the pad index.
Not really useless. What it tells is that the entity has a V4L2 sub-device
interface. If you remove the code snippet there, then the user space has no
knowledge of that anymore. I expect changing that to break stuff --- this
is how applications find their device nodes, after all.
>
> How about adding a 'function' field to struct media_entity_desc
> and fill that? Keep the type for backwards compatibility.
>
> Then have a define like this:
>
> #define MEDIA_ENT_HAS_FUNCTION(media_version) ((media_version) >= KERNEL_VERSION(a, b, c))
>
> that can be used to detect if the MC has function support.
I'm not really too worried that the exact device function isn't conveyed to
the user space. The interface the device provides is more important: when
the user space program is looking for a device with a particular name, it
already knows what kind of a device it is. But it might check that the
interface is what it expects, too.
--
Sakari Ailus
e-mail: sakari.ailus@xxxxxx
