[media] omap3isp: Add documentation

Add documentation on the OMAP 3 ISP driver. Document the subdevs, V4L2
events and private IOCTLs the driver implements
Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: David Cohen <dacohen@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

Documentation/video4linux/omap3isp.txt

+OMAP 3 Image Signal Processor (ISP) driver
+
+Copyright (C) 2010 Nokia Corporation
+Copyright (C) 2009 Texas Instruments, Inc.
+
+Contacts: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
+	  Sakari Ailus <sakari.ailus@iki.fi>
+	  David Cohen <dacohen@gmail.com>
+
+
+Introduction
+============
+
+This file documents the Texas Instruments OMAP 3 Image Signal Processor (ISP)
+driver located under drivers/media/video/omap3isp. The original driver was
+written by Texas Instruments but since that it has been rewritten (twice) at
+Nokia.
+
+The driver has been successfully used on the following versions of OMAP 3:
+
+	3430
+	3530
+	3630
+
+The driver implements V4L2, Media controller and v4l2_subdev interfaces.
+Sensor, lens and flash drivers using the v4l2_subdev interface in the kernel
+are supported.
+
+
+Split to subdevs
+================
+
+The OMAP 3 ISP is split into V4L2 subdevs, each of the blocks inside the ISP
+having one subdev to represent it. Each of the subdevs provide a V4L2 subdev
+interface to userspace.
+
+	OMAP3 ISP CCP2
+	OMAP3 ISP CSI2a
+	OMAP3 ISP CCDC
+	OMAP3 ISP preview
+	OMAP3 ISP resizer
+	OMAP3 ISP AEWB
+	OMAP3 ISP AF
+	OMAP3 ISP histogram
+
+Each possible link in the ISP is modelled by a link in the Media controller
+interface. For an example program see [2].
+
+
+Controlling the OMAP 3 ISP
+==========================
+
+In general, the settings given to the OMAP 3 ISP take effect at the beginning
+of the following frame. This is done when the module becomes idle during the
+vertical blanking period on the sensor. In memory-to-memory operation the pipe
+is run one frame at a time. Applying the settings is done between the frames.
+
+All the blocks in the ISP, excluding the CSI-2 and possibly the CCP2 receiver,
+insist on receiving complete frames. Sensors must thus never send the ISP
+partial frames.
+
+Autoidle does have issues with some ISP blocks on the 3430, at least.
+Autoidle is only enabled on 3630 when the omap3isp module parameter autoidle
+is non-zero.
+
+
+Events
+======
+
+The OMAP 3 ISP driver does support the V4L2 event interface on CCDC and
+statistics (AEWB, AF and histogram) subdevs.
+
+The CCDC subdev produces V4L2_EVENT_OMAP3ISP_HS_VS type event on HS_VS
+interrupt which is used to signal frame start. The event is triggered exactly
+when the reception of the first line of the frame starts in the CCDC module.
+The event can be subscribed on the CCDC subdev.
+
+(When using parallel interface one must pay account to correct configuration
+of the VS signal polarity. This is automatically correct when using the serial
+receivers.)
+
+Each of the statistics subdevs is able to produce events. An event is
+generated whenever a statistics buffer can be dequeued by a user space
+application using the VIDIOC_OMAP3ISP_STAT_REQ IOCTL. The events available
+are:
+
+	V4L2_EVENT_OMAP3ISP_AEWB
+	V4L2_EVENT_OMAP3ISP_AF
+	V4L2_EVENT_OMAP3ISP_HIST
+
+The type of the event data is struct omap3isp_stat_event_status for these
+ioctls. If there is an error calculating the statistics, there will be an
+event as usual, but no related statistics buffer. In this case
+omap3isp_stat_event_status.buf_err is set to non-zero.
+
+
+Private IOCTLs
+==============
+
+The OMAP 3 ISP driver supports standard V4L2 IOCTLs and controls where
+possible and practical. Much of the functions provided by the ISP, however,
+does not fall under the standard IOCTLs --- gamma tables and configuration of
+statistics collection are examples of such.
+
+In general, there is a private ioctl for configuring each of the blocks
+containing hardware-dependent functions.
+
+The following private IOCTLs are supported:
+
+	VIDIOC_OMAP3ISP_CCDC_CFG
+	VIDIOC_OMAP3ISP_PRV_CFG
+	VIDIOC_OMAP3ISP_AEWB_CFG
+	VIDIOC_OMAP3ISP_HIST_CFG
+	VIDIOC_OMAP3ISP_AF_CFG
+	VIDIOC_OMAP3ISP_STAT_REQ
+	VIDIOC_OMAP3ISP_STAT_EN
+
+The parameter structures used by these ioctls are described in
+include/linux/omap3isp.h. The detailed functions of the ISP itself related to
+a given ISP block is described in the Technical Reference Manuals (TRMs) ---
+see the end of the document for those.
+
+While it is possible to use the ISP driver without any use of these private
+IOCTLs it is not possible to obtain optimal image quality this way. The AEWB,
+AF and histogram modules cannot be used without configuring them using the
+appropriate private IOCTLs.
+
+
+CCDC and preview block IOCTLs
+=============================
+
+The VIDIOC_OMAP3ISP_CCDC_CFG and VIDIOC_OMAP3ISP_PRV_CFG IOCTLs are used to
+configure, enable and disable functions in the CCDC and preview blocks,
+respectively. Both IOCTLs control several functions in the blocks they
+control. VIDIOC_OMAP3ISP_CCDC_CFG IOCTL accepts a pointer to struct
+omap3isp_ccdc_update_config as its argument. Similarly VIDIOC_OMAP3ISP_PRV_CFG
+accepts a pointer to struct omap3isp_prev_update_config. The definition of
+both structures is available in [1].
+
+The update field in the structures tells whether to update the configuration
+for the specific function and the flag tells whether to enable or disable the
+function.
+
+The update and flag bit masks accept the following values. Each separate
+functions in the CCDC and preview blocks is associated with a flag (either
+disable or enable; part of the flag field in the structure) and a pointer to
+configuration data for the function.
+
+Valid values for the update and flag fields are listed here for
+VIDIOC_OMAP3ISP_CCDC_CFG. Values may be or'ed to configure more than one
+function in the same IOCTL call.
+
+        OMAP3ISP_CCDC_ALAW
+        OMAP3ISP_CCDC_LPF
+        OMAP3ISP_CCDC_BLCLAMP
+        OMAP3ISP_CCDC_BCOMP
+        OMAP3ISP_CCDC_FPC
+        OMAP3ISP_CCDC_CULL
+        OMAP3ISP_CCDC_CONFIG_LSC
+        OMAP3ISP_CCDC_TBL_LSC
+
+The corresponding values for the VIDIOC_OMAP3ISP_PRV_CFG are here:
+
+        OMAP3ISP_PREV_LUMAENH
+        OMAP3ISP_PREV_INVALAW
+        OMAP3ISP_PREV_HRZ_MED
+        OMAP3ISP_PREV_CFA
+        OMAP3ISP_PREV_CHROMA_SUPP
+        OMAP3ISP_PREV_WB
+        OMAP3ISP_PREV_BLKADJ
+        OMAP3ISP_PREV_RGB2RGB
+        OMAP3ISP_PREV_COLOR_CONV
+        OMAP3ISP_PREV_YC_LIMIT
+        OMAP3ISP_PREV_DEFECT_COR
+        OMAP3ISP_PREV_GAMMABYPASS
+        OMAP3ISP_PREV_DRK_FRM_CAPTURE
+        OMAP3ISP_PREV_DRK_FRM_SUBTRACT
+        OMAP3ISP_PREV_LENS_SHADING
+        OMAP3ISP_PREV_NF
+        OMAP3ISP_PREV_GAMMA
+
+The associated configuration pointer for the function may not be NULL when
+enabling the function. When disabling a function the configuration pointer is
+ignored.
+
+
+Statistic blocks IOCTLs
+=======================
+
+The statistics subdevs do offer more dynamic configuration options than the
+other subdevs. They can be enabled, disable and reconfigured when the pipeline
+is in streaming state.
+
+The statistics blocks always get the input image data from the CCDC (as the
+histogram memory read isn't implemented). The statistics are dequeueable by
+the user from the statistics subdev nodes using private IOCTLs.
+
+The private IOCTLs offered by the AEWB, AF and histogram subdevs are heavily
+reflected by the register level interface offered by the ISP hardware. There
+are aspects that are purely related to the driver implementation and these are
+discussed next.
+
+VIDIOC_OMAP3ISP_STAT_EN
+-----------------------
+
+This private IOCTL enables/disables a statistic module. If this request is
+done before streaming, it will take effect as soon as the pipeline starts to
+stream.  If the pipeline is already streaming, it will take effect as soon as
+the CCDC becomes idle.
+
+VIDIOC_OMAP3ISP_AEWB_CFG, VIDIOC_OMAP3ISP_HIST_CFG and VIDIOC_OMAP3ISP_AF_CFG
+-----------------------------------------------------------------------------
+
+Those IOCTLs are used to configure the modules. They require user applications
+to have an in-depth knowledge of the hardware. Most of the fields explanation
+can be found on OMAP's TRMs. The two following fields common to all the above
+configure private IOCTLs require explanation for better understanding as they
+are not part of the TRM.
+
+omap3isp_[h3a_af/h3a_aewb/hist]_config.buf_size:
+
+The modules handle their buffers internally. The necessary buffer size for the
+module's data output depends on the requested configuration. Although the
+driver supports reconfiguration while streaming, it does not support a
+reconfiguration which requires bigger buffer size than what is already
+internally allocated if the module is enabled. It will return -EBUSY on this
+case. In order to avoid such condition, either disable/reconfigure/enable the
+module or request the necessary buffer size during the first configuration
+while the module is disabled.
+
+The internal buffer size allocation considers the requested configuration's
+minimum buffer size and the value set on buf_size field. If buf_size field is
+out of [minimum, maximum] buffer size range, it's clamped to fit in there.
+The driver then selects the biggest value. The corrected buf_size value is
+written back to user application.
+
+omap3isp_[h3a_af/h3a_aewb/hist]_config.config_counter:
+
+As the configuration doesn't take effect synchronously to the request, the
+driver must provide a way to track this information to provide more accurate
+data. After a configuration is requested, the config_counter returned to user
+space application will be an unique value associated to that request. When
+user application receives an event for buffer availability or when a new
+buffer is requested, this config_counter is used to match a buffer data and a
+configuration.
+
+VIDIOC_OMAP3ISP_STAT_REQ
+------------------------
+
+Send to user space the oldest data available in the internal buffer queue and
+discards such buffer afterwards. The field omap3isp_stat_data.frame_number
+matches with the video buffer's field_count.
+
+
+Technical reference manuals (TRMs) and other documentation
+==========================================================
+
+OMAP 3430 TRM:
+<URL:http://focus.ti.com/pdfs/wtbu/OMAP34xx_ES3.1.x_PUBLIC_TRM_vZM.zip>
+Referenced 2011-03-05.
+
+OMAP 35xx TRM:
+<URL:http://www.ti.com/litv/pdf/spruf98o> Referenced 2011-03-05.
+
+OMAP 3630 TRM:
+<URL:http://focus.ti.com/pdfs/wtbu/OMAP36xx_ES1.x_PUBLIC_TRM_vQ.zip>
+Referenced 2011-03-05.
+
+DM 3730 TRM:
+<URL:http://www.ti.com/litv/pdf/sprugn4h> Referenced 2011-03-06.
+
+
+References
+==========
+
+[1] include/linux/omap3isp.h
+
+[2] http://git.ideasonboard.org/?p=media-ctl.git;a=summary