From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from galahad.ideasonboard.com ([185.26.127.97]:49464 "EHLO galahad.ideasonboard.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753910AbbDQPHV (ORCPT ); Fri, 17 Apr 2015 11:07:21 -0400 From: Laurent Pinchart To: Hans Verkuil Cc: "linux-media@vger.kernel.org" Subject: Re: [RFC] Querycap for subdevs, finding MC from device nodes Date: Fri, 17 Apr 2015 18:07:22 +0300 Message-ID: <1768917.5uFxr7arIJ@avalon> In-Reply-To: <550D2EB9.5090806@xs4all.nl> References: <550D2EB9.5090806@xs4all.nl> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-media-owner@vger.kernel.org List-ID: Hi Hans, On Saturday 21 March 2015 09:41:29 Hans Verkuil wrote: > I've been thinking about extending v4l2-compliance with v4l-subdev tests. > > However, there are a few missing pieces that are needed before this can be > done. > > First of all is that there is no subdev equivalent to VIDIOC_QUERYCAP, i.e. > an ioctl that is guaranteed to always be available. > > So I propose the following ioctl: > > VIDIOC_SUBDEV_QUERYCAP(struct v4l2_subdev_capability); > > struct v4l2_subdev_capability { > char name[V4L2_SUBDEV_NAME_SIZE]; > __u32 version; // same as KERNEL_VERSION like QUERYCAP does > __u32 device_caps; > __u32 pads; > __u32 entity_id; > __u32 reserved[40]; > }; > > /* This v4l2_subdev is also a media entity and the entity_id field is valid > */ #define V4L2_SUBDEV_CAP_ENTITY (1 << 0) > > This will allow v4l2-compliance to discover that this is a bona fide v4l2 > subdevice. All the information in the struct above can be filled in by > v4l2-subdev.c, no need to change drivers. This looks quite good to me. There's a prerequisite though, we need to formally define the naming scheme for subdevs, otherwise we'll expose yet another ill-defined name that will cause issues later. > The reason I included 'pads' as well is that subdev drivers can have a > devnode without being an entity, but still support ioctls like > VIDIOC_SUBDEV_ENUM_MBUS_CODE. In that case v4l2-compliance needs to know > the number of pads in order to properly test. If it is an entity, then the > entity information can be obtained from the MC, see below how to find the > MC. Shouldn't we make it mandatory for subdevs to be entities if they want to expose a subdev node ? Otherwise applications won't be able to find out how the subdev relates to other subdevs and v4l2 devices, making the API quite shaky in my opinion. > Note: I think it makes sense to extend VIDIOC_QUERYCAP as well with a > CAP_ENTITY and an entity_id. > > The next step is to be able to associate a media device with a v4l2-subdev. > > Originally I though of adding the major and minor numbers of the media > device to the capability struct, but I'd like to do that for > VIDIOC_QUERYCAP as well, and there are only 3 reserved fields (2 after > taking one for the entity_id). > > Instead I think we can just implement the MEDIA_IOC_DEVICE_INFO ioctl: this > returns all info about the media_device, and it can easily be extended to > include the major and minor number of that device. Hmmm... I'm not too found of that approach. Can't userspace find the corresponding media device through sysfs ? I'd like to point that the common use case is to start from the media device and find the entities. Finding the media device from a subdev is useful mostly for test tools, but it's not the main use case. > Again, supporting this is easily done in the core, both for regular video > nodes and for subdev nodes, so drivers do not need to be changed. > > This way you can easily find whether a V4L2 device node is an entity and > what the associated media controller is. All the information is available, > we just need to expose it. > > Comments? Please see above :-) -- Regards, Laurent Pinchart