Hi Sakari,
Thanks for the RFC.
On Wednesday 06 April 2011 10:10:40 Sakari Ailus wrote:
> Hi,
>
> This is a second proposal for an interface for controlling flash devices
> on the V4L2/v4l2_subdev APIs. My plan is to use the interface in the
> ADP1653 driver, the flash controller used in the Nokia N900.
>
> Thanks to everyone who commented the previous version of this RFC! I
> hope I managed to factor in everyone's comments. Please bug me if you
> think I missed something. :-)
>
> Comments and questions are very, very welcome as always.
>
> Changes since v1 [7]
> ====================
>
> - V4L2_FLASH_STROBE_MODE_EXT_STROBE renamed to
> V4L2_FLASH_STROBE_MODE_EXTERNAL.
>
> - V4L2_CID_FLASH_STROBE control changed from button to bool.
>
> - Removed suggestion of adding V4L2_CID_FLASH_DURATION.
> V4L2_CID_FLASH_TIMEOUT is used as hardware timeout.
>
> - Added control access info (ro/rw).
>
> - V4L2_FLASH_MODE_NONE added, V4L2_FLASH_LED_MODE_FLASH no longer forced
> as 1 in enum.
>
> - Bits use (1 << x) instead of 0x00... format.
>
> - Added an open question on flash LED mode controls.
>
> - Added an open question on a new control:
> V4L2_CID_FLASH_EXTERNAL_STROBE_WHENCE.
>
> - Added an open question on control units.
>
>
> Scope
> =====
>
> This RFC is focused mostly on the ADP1653 [1] and similar chips [2, 3]
> which provides following functionality. [2, 3] mostly differ on the
> available faults --- for example, there are faults also for the
> indicator LED.
>
> - High power LED output (flash or torch modes)
> - Low power indicator LED output (a.k.a. privacy light)
> - Programmable flash timeout
> - Software and hardware strobe
> - Fault detection
> - Overvoltage
> - Overtemperature
> - Short circuit
> - Timeout
> - Programmable current (both high-power and indicator LEDs)
>
> If anyone else is aware of hardware which significantly differs from
> these and does not get served well under the proposed interface, please
> tell about it.
>
> This RFC does NOT address the synchronisation of the flash to a given
> frame since this task is typically performed by the sensor through a
> strobe signal. The host does not have enough information for this ---
> exact timing information on the exposure of the sensor pixel array. In
> this case the flash synchronisation is visible to the flash controller
> as the hardware strobe originating from the sensor.
>
> Flash synchronisation requires
>
> 1) flash control capability from the sensor including a strobe output,
> 2) strobe input in the flash controller,
> 3) (optionally) ability to program sensor parameters at given frame,
> such as flash strobe, and
> 4) ability to read back metadata produced by the sensor related to a
> given frame. This should include whether the frame is exposed with
> flash, i.e. the sensor's flash strobe output.
>
> Since we have little examples of both in terms of hardware support,
> which is in practice required, it was decided to postpone the interface
> specification for now. [6]
>
> Xenon flash controllers exist but I don't have a specific example of
> those. Typically the interface is quite simple. Gpio pins for charge and
> strobe. The length of the strobe signal determines the strength of the
> flash pulse. The strobe is controlled by the sensor as for LED flash if
> it is hardware based.
>
>
> Known use cases
> ===============
>
> The use case listed below concentrate on using a flash in a mobile
> device, for example in a mobile phone. The use cases could be somewhat
> different in devices the primary use of which is camera.
>
> Unsynchronised LED flash (software strobe)
> ------------------------------------------
>
> Unsynchronised LED flash is controlled directly by the host as the
> sensor. The flash must be enabled by the host before the exposure of the
> image starts and disabled once it ends. The host is fully responsible
> for the timing of the flash.
>
> Example of such device: Nokia N900.
>
>
> Synchronised LED flash (hardware strobe)
> ----------------------------------------
>
> The synchronised LED flash is pre-programmed by the host (power and
> timeout) but controlled by the sensor through a strobe signal from the
> sensor to the flash.
>
> The sensor controls the flash duration and timing. This control
> typically must be programmed to the sensor, and specifying an interface
> for this is out of scope of this RFC.
>
> The LED flash controllers we know of can function in both synchronised
> and unsynchronised modes.
What about synchronised LED flash where the hardware starts the flash, but
where the duration must be pre-programmed by software beforehand ?
> LED flash as torch
> ------------------
>
> LED flash may be used as torch in conjunction with another use case
> involving camera or individually. [4]
>
>
> Synchronised xenon flash
> ------------------------
>
> The synchronised xenon flash is controlled more closely by the sensor
> than the LED flash. There is no separate intensity control for the xenon
> flash as its intensity is determined by the length of the strobe pulse.
> Several consecutive strobe pluses are possible but this needs to be
> still controlled by the sensor.
>
>
> Proposed interface
> ==================
>
> The flash, either LED or xenon, does not require large amounts of data
> to control it. There are parameters to control it but they are
> independent and assumably some hardware would only support some subsets
> of the functionality available somewhere else. Thus V4L2 controls seem
> an ideal way to support flash controllers.
>
> A separate control class is reserved for the flash controls. It is
> called V4L2_CTRL_CLASS_FLASH.
>
> Type of the control; type of flash is in parentheses after the control.
>
>
> V4L2_CID_FLASH_STROBE (boolean; rw; LED)
>
> Strobe the flash using software strobe from the host, typically over I2C
> or a GPIO. The flash is NOT synchronised to sensor pixel are exposure
> since the command is given asynchronously. Alternatively, if the flash
> controller is a master in the system, the sensor exposure may be
> triggered based on software strobe.
>
> This control may also be used to shut down the strobe and query the
> state of the strobe (on/off).
Will the strobe control automatically go back to false when the timeout is
reached ? Drivers will need to use a work queue, as the control framework
doesn't support driver-initiated control changes in interrupt context.
> V4L2_CID_FLASH_STROBE_MODE (menu; rw; LED)
>
> Use hardware or software strobe. If hardware strobe is selected, the
> flash controller is a slave in the system where the sensor produces the
> strobe signal to the flash.
>
> In this case the flash controller setup is limited to programming strobe
> timeout and power (LED flash) and the sensor controls the timing and
> length of the strobe.
>
> enum v4l2_flash_strobe_mode {
> V4L2_FLASH_STROBE_MODE_SOFTWARE,
> V4L2_FLASH_STROBE_MODE_EXTERNAL,
> };
>
>
> V4L2_CID_FLASH_TIMEOUT (integer; rw; LED)
>
> The flash controller provides timeout functionality to shut down the led
> in case the host fails to do that. For hardware strobe, this is the
> maximum amount of time the flash should stay on, and the purpose of the
> setting is to prevent the LED from catching fire.
>
> For software strobe, the setting may be used to limit the length of the
> strobe using a hardware watchdog. The granularity of the timeout in [1,
> 2, 3] is very coarse.
>
> A standard unit such as ms or µs should be used.
If the only purpose of the timeout is to catch misbehaving applications (in
case of software strobe) or hardware (in case of hardware level-triggered
strobe), do applications need to configure the timeout ? Shouldn't it be a
fixed value that comes from platform data ?
Can the timeout limit also vary depending on the flash intensity ?
> V4L2_CID_FLASH_LED_MODE (menu; rw; LED)
>
> enum v4l2_flash_led_mode {
> V4L2_FLASH_LED_MODE_NONE,
> V4L2_FLASH_LED_MODE_FLASH,
> V4L2_FLASH_LED_MODE_TORCH,
> };
Could you define the semantics of V4L2_FLASH_LED_MODE_NONE ? How does interact
with V4L2_CID_FLASH_STROBE ?
> V4L2_CID_FLASH_INTENSITY (integer; rw; LED)
>
> Intensity of the flash in hardware specific units. The LED flash
> controller provides current to the LED but the actual luminous power is
> dictated by the LED connected to the controller.
>
>
> V4L2_CID_FLASH_TORCH_INTENSITY (integer; rw; LED)
>
> Intensity of the flash in hardware specific units.
>
>
> V4L2_CID_FLASH_INDICATOR_INTENSITY (integer; rw; LED)
>
> Intensity of the indicator light in hardware specific units.
>
>
> V4L2_CID_FLASH_FAULT (bit field; ro; LED)
>
> This is a bitmask containing the fault information for the flash. This
> assumes the proposed V4L2 bit mask controls [5]; otherwise this would
> likely need to be a set of controls.
>
> #define V4L2_FLASH_FAULT_OVER_VOLTAGE (1 << 0)
> #define V4L2_FLASH_FAULT_TIMEOUT (1 << 1)
Is timeout always considered as a fault ?
> #define V4L2_FLASH_FAULT_OVER_TEMPERATURE (1 << 2)
> #define V4L2_FLASH_FAULT_SHORT_CIRCUIT (1 << 3)
>
> Several faults may occur at single occasion. The ADP1653 is able to
> inform the user a fault has occurred, so a V4L2 control event (proposed
> earlier) could be used for that.
>
> These faults are supported by the ADP1653. More faults may be added as
> support for more chips require that. In some other hardware faults are
> available for indicator led as well.
>
> Question: should indicator faults be part of the same control, or a
> different control, e.g. V4L2_CID_FLASH_INDICATOR_FAULT?
I would go for a separate control, but I'm still not too sure about it.
> V4L2_CID_FLASH_CHARGE (bool; rw; xenon)
>
> Charge control for the xenon flash. Enable or disable charging.
>
>
> V4L2_CID_FLASH_READY (bool; ro; xenon, LED)
>
> Flash is ready to strobe. On xenon flash this tells the capacitor has
> been charged, on LED flash it's that the LED is no longer too hot.
>
> The implementation on LED flash may be modelling the temperature
> behaviour of the LED in the driver (or elsewhere, e.g. library or board
> code) if the hardware does not provide direct temperature information
> from the LED.
>
> A V4L2 control event should be produced whenever the flash becomes ready.
We should define a list of events in addition to the controls.
> Open questions
> ==============
>
> 1. Flash LED mode control
> -------------------------
>
> Should the flash led mode control be a single control or a set of
> controls? A single control would make it easier for application to
> choose between different modes, but on the other hand limits future
> extensibility if there would have to be further splitting of the modes. [8]
As we can't select more than one mode at the same time, I think we should keep
the single flash led mode control and not split it.
> 2. External strobe edge / level
> -------------------------------
>
> No use is seen currently for this, but it may well appear, and the
> hardware supports this. Level based trigger should be used since it is
> more precise.
>
> V4L2_CID_FLASH_EXTERNAL_STROBE_WHENCE
>
> Whether the flash controller considers external strobe as edge, when the
> only limit of the strobe is the timeout on flash controller, or level,
> when the flash strobe will last as long as the strobe signal, or as long
> until the timeout expires.
>
> enum v4l2_flash_external_strobe_whence {
> V4L2_CID_FLASH_EXTERNAL_STROBE_LEVEL,
> V4L2_CID_FLASH_EXTERNAL_STROBE_EDGE,
> };
As I've already mentioned in another e-mail, shouldn't this be static
information provided through platform data ? Or do we have use cases where the
user will want to control the edge explicitly ?
> 3. Units
> --------
>
> Which units should e.g. V4L2_CID_FLASH_TIMEOUT be using? It'd be very
> useful to have standard unit on this control, like ms or µs.
µs sounds good to me. I don't think we will ever need something more precise
than that, and it would allow timeouts up to a bit more than 35 minutes, which
should be long enough.
> Some controls, like V4L2_CID_FLASH_INTENSITY, can't easily use a
> standard unit. The luminous output depends on the LED connected (you
> could use an incandescent lightbulb too, I suppose?) to the flash
> controller. These controls typically control the current supplied by the
> flash controller.
I don't think we should define a standard unit in that case.
--
Regards,
Laurent Pinchart
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