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Gain control is badly documented in publicly available (including leaked) documentation. There is an AGC pre-gain in register 0x3a13, expressed as a 6-bit value (plus an enable bit in bit 6). The driver hardcodes it to 0x43, which one application note states is equal to x1.047. The documentation also states that 0x40 is equel to x1.000. The pre-gain thus seems to be expressed as in 1/64 increments, and thus ranges from x1.00 to x1.984. What the pre-gain does is however unspecified. There is then an AGC gain limit, in registers 0x3a18 and 0x3a19, expressed as a 10-bit "real gain format" value. One application note sets it to 0x00f8 and states it is equal to x15.5, so it appears to be expressed in 1/16 increments, up to x63.9375. The manual gain is stored in registers 0x350a and 0x350b, also as a 10-bit "real gain format" value. It is documented in the application note as a Q6.4 values, up to x63.9375. One version of the datasheet indicates that the sensor supports a digital gain: The OV5640 supports 1/2/4 digital gain. Normally, the gain is controlled automatically by the automatic gain control (AGC) block. It isn't clear how that would be controlled manually. There appears to be no indication regarding whether the gain controlled through registers 0x350a and 0x350b is an analogue gain only or also includes digital gain. The words "real gain" don't necessarily mean "combined analogue and digital gains". Some OmniVision sensors (such as the OV8858) are documented as supoprting different formats for the gain values, selectable through a register bit, and they are called "real gain format" and "sensor gain format". For that sensor, we have (one of) the gain registers documented as 0x3503[2]=0, gain[7:0] is real gain format, where low 4 bits are fraction bits, for example, 0x10 is 1x gain, 0x28 is 2.5x gain If 0x3503[2]=1, gain[7:0] is sensor gain format, gain[7:4] is coarse gain, 00000: 1x, 00001: 2x, 00011: 4x, 00111: 8x, gain[7] is 1, gain[3:0] is fine gain. For example, 0x10 is 1x gain, 0x30 is 2x gain, 0x70 is 4x gain (The second part of the text makes little sense) "Real gain" may thus refer to the combination of the coarse and fine analogue gains as a single value. The OV5640 0x350a and 0x350b registers thus appear to control analogue gain. The driver incorrectly uses V4L2_CID_GAIN as V4L2 has a specific control for analogue gain, V4L2_CID_ANALOGUE_GAIN. Use it. If registers 0x350a and 0x350b are later found to control digital gain as well, the driver could then restrict the range of the analogue gain control value to lower than x64 and add a separate digital gain control. Signed-off-by: Paul Elder <paul.elder@ideasonboard.com> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com> Reviewed-by: Jai Luthra <j-luthra@ti.com> Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com> Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.