Hi Bryan, On Thursday 10 October 2013 17:07:22 Bryan Wu wrote: > On Tue, May 21, 2013 at 3:54 AM, Sakari Ailus <sakari.ailus@xxxxxx> wrote: > > On Tue, May 21, 2013 at 10:34:53AM +0200, Andrzej Hajda wrote: > >> On 12.05.2013 23:12, Sakari Ailus wrote: > >> > On Wed, May 08, 2013 at 09:32:17AM +0200, Andrzej Hajda wrote: > >> >> On 07.05.2013 17:07, Laurent Pinchart wrote: > >> >>> On Tuesday 07 May 2013 02:11:27 Kim, Milo wrote: > >> >>>> On Monday, May 06, 2013 6:34 PM Andrzej Hajda wrote: > >> >>>>> This RFC proposes generic API for exposing flash subdevices via LED > >> >>>>> framework. > >> >>>>> > >> >>>>> Rationale > >> >>>>> > >> >>>>> Currently there are two frameworks which are used for exposing LED > >> >>>>> flash to user space: > >> >>>>> - V4L2 flash controls, > >> >>>>> - LED framework(with custom sysfs attributes). > >> >>>>> > >> >>>>> The list below shows flash drivers in mainline kernel with initial > >> >>>>> commit date and typical chip application (according to producer): > >> >>>>> > >> >>>>> LED API: > >> >>>>> lm3642: 2012-09-12, Cameras > >> >>>>> lm355x: 2012-09-05, Cameras > >> >>>>> max8997: 2011-12-14, Cameras (?) > >> >>>>> lp3944: 2009-06-19, Cameras, Lights, Indicators, Toys > >> >>>>> pca955x: 2008-07-16, Cameras, Indicators (?) > >> >>>>> > >> >>>>> V4L2 API: > >> >>>>> as3645a: 2011-05-05, Cameras > >> >>>>> adp1653: 2011-05-05, Cameras > >> >>>>> > >> >>>>> V4L2 provides richest functionality, but there is often demand from > >> >>>>> application developers to provide already established LED API. We > >> >>>>> would like to have an unified user interface for flash devices. > >> >>>>> Some of devices already have the LED API driver exposing limited > >> >>>>> set of a Flash IC functionality. In order to support all required > >> >>>>> features the LED API would have to be extended or the V4L2 API > >> >>>>> would need to be used. However when switching from a LED to a V4L2 > >> >>>>> Flash driver existing LED API interface would need to be retained. > >> >>>>> > >> >>>>> Proposed solution > >> >>>>> > >> >>>>> This patch adds V4L2 helper functions to register existing V4L2 > >> >>>>> flash subdev as LED class device. After registration via > >> >>>>> v4l2_leddev_register appropriate entry in /sys/class/leds/ is > >> >>>>> created. During registration all V4L2 flash controls are enumerated > >> >>>>> and corresponding attributes are added. > >> >>>>> > >> >>>>> I have attached also patch with new max77693-led driver using > >> >>>>> v4l2_leddev. This patch requires presence of the patch "max77693: > >> >>>>> added device tree support": > >> >>>>> https://patchwork.kernel.org/patch/2414351/ . > >> >>>>> > >> >>>>> Additional features > >> >>>>> > >> >>>>> - simple API to access all V4L2 flash controls via sysfs, > >> >>>>> - V4L2 subdevice should not be registered by V4L2 device to use it, > >> >>>>> - LED triggers API can be used to control the device, > >> >>>>> - LED device is optional - it will be created only if V4L2_LEDDEV > >> >>>>> configuration option is enabled and the subdev driver calls > >> >>>>> v4l2_leddev_register. > >> >>>>> > >> >>>>> Doubts > >> >>>>> > >> >>>>> This RFC is a result of a uncertainty which API developers should > >> >>>>> expose by their flash drivers. It is a try to gluing together both > >> >>>>> APIs. I am not sure if it is the best solution, but I hope there > >> >>>>> will be some discussion and hopefully some decisions will be taken > >> >>>>> which way we should follow. > >> >>>> > >> >>>> The LED subsystem provides similar APIs for the Camera driver. > >> >>>> With LED trigger event, flash and torch are enabled/disabled. > >> >>>> I'm not sure this is applicable for you. > >> >>>> Could you take a look at LED camera trigger feature? > >> >>>> > >> >>>> For the camera LED trigger, > >> >>>> https://git.kernel.org/cgit/linux/kernel/git/cooloney/linux-leds.git > >> >>>> /commit/ ?h=f or-next&id=48a1d032c954b9b06c3adbf35ef4735dd70ab757 > >> >>>> > >> >>>> Example of camera flash driver, > >> >>>> https://git.kernel.org/cgit/linux/kernel/git/cooloney/linux-leds.git > >> >>>> /commit/ ?h=f or-next&id=313bf0b1a0eaeaac17ea8c4b748f16e28fce8b7a > >> >>> > >> >>> I think we should decide on one API. Implementing two APIs for a > >> >>> single device is usually messy, and will result in different feature > >> >>> sets (and different bugs) being implemented through each API, > >> >>> depending on the driver. Interactions between the APIs are also a > >> >>> pain point on the kernel side to properly synchronize calls. > >> > > >> > I don't like having two APIs either. Especially we shouldn't have > >> > multiple drivers implementing different APIs for the same device. > >> > > >> > That said, I wonder if it's possible to support camera-related use > >> > cases using the LED API: it's originally designed for quite different > >> > devices. Even if you could handle flash strobing using the LED API, the > >> > functionality provided by the Media controller and subdev APIs will > >> > always be missing: device enumeration and association with the right > >> > camera. > >> > >> Is there a generic way to associate flash and camera subdevs in > >> current V4L2 API? The only ways I see now are: > >> - both belongs to the same media controller, but this is not enough if > >> there is more than one camera subdev in that controller, > > > > Yes, there is. That's the group_id field in struct media_entity_desc. The > > lens subdev is associated to the rest of the devices the same way. > > > >> - using media links/pads - at first sight it seems to be > >> overkill/abuse... > > > > No. Links describe the flow of data, not relations between entities. > > > > ... > > > >> >>> The LED API is too limited for torch and flash usage, but I'm > >> >>> definitely open to moving flash devices to the LED API is we can > >> >>> extend it in a way that it covers all the use cases. > >> >> > >> >> Extending LED API IMHO seems to be quite straightforward - by adding > >> >> attributes for supported functionalities. We just need a specification > >> >> for standard flash/torch attributes. > >> >> I could prepare an RFC about it if there is a will to explore this > >> >> direction. > >> > > >> > I'm leaning towards providing a wrapper that provides torch > >> > functionality using V4L2 flash API unless it's really proven to be > >> > insane. ;-) The code supporting that in an individual flash driver > >> > should be minimal --- which is what the patchset essentially already > >> > does. > >> > >> Providing only torch functionality do not require adding new attributes > >> (besides the ones already present in the led_classdev), so the patch will > >> be much simpler. > > > > Yes. Attributes could be added later on to the LED API to support flash > > and the wrapper could be extended accordingly. My thinking is however that > > the main use case is torch, not strobing flash, so it would be fulfilled > > already without extensions to the LED API. > > Sorry for replying so late. > > I think Milo Kim did some work in our LED subsystem by add LED Flash > trigger for camera device. I agree it doesn't satisfy the usage of > V4L2 Flash API and what I'm thinking about is expanding the LED Flash > trigger driver to export a well defined sysfs interface, so user space > libv4l2 can wrap it for applications. I've replied to your comment in the media-summit agenda mail thread, but the discussion belongs here, so here's a repost. Let's try to keep replies in this mail thread. The biggest reason why we're not fond of sysfs-based APIs for media devices is that they can't provide atomicity. There's no way to set multiple parameters in a single operation. We can't get rid of the sysfs LEDs API, but maybe we could have a unified kernel LED/flash subsystem that would provide both a sysfs-based API to ensure compatibility with current userspace software and an ioctl-based API (possibly through V4L2 controls). That way LED/flash devices would be registered with a single subsystem, and the corresponding drivers won't have to care about the API exposed to userspace. That would require a major refactoring of the in- kernel APIs though. -- Regards, Laurent Pinchart -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
