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From:   Mikhail Rudenko <mike.rudenko@gmail.com>
To:     Sakari Ailus <sakari.ailus@linux.intel.com>
Cc:     Dave Stevenson <dave.stevenson@raspberrypi.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Hans Verkuil <hverkuil-cisco@xs4all.nl>,
        Jacopo Mondi <jacopo@jmondi.org>,
        Shawn Tu <shawnx.tu@intel.com>, Jimmy Su <jimmy.su@intel.com>,
        Arnd Bergmann <arnd@arndb.de>, Arec Kao <arec.kao@intel.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Marek Vasut <marex@denx.de>, linux-media@vger.kernel.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 0/2] Add Omnivision OV4689 image sensor driver
Date:   Mon, 19 Sep 2022 10:01:06 +0300
In-reply-to: <YygOzWAHyoP+KwTv@paasikivi.fi.intel.com>
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Hi Sakari,

On 2022-09-19 at 06:40 GMT, Sakari Ailus <sakari.ailus@linux.intel.com> wrote:

> Hi Mikhail,
>
> On Fri, Sep 16, 2022 at 12:27:42AM +0300, Mikhail Rudenko wrote:
>>
>> Hi Dave,
>>
>> On 2022-09-14 at 10:58 +01, Dave Stevenson <dave.stevenson@raspberrypi.com> wrote:
>> > Hi Mikhail
>> >
>> > On Sun, 11 Sept 2022 at 21:02, Mikhail Rudenko <mike.rudenko@gmail.com> wrote:
>> >>
>> >> Hello,
>> >>
>> >> this series implements support for Omnivision OV4689 image
>> >> sensor. The Omnivision OV4689 is a high performance, 1/3-inch, 4
>> >> megapixel image sensor. Ihis chip supports high frame rate speeds up
>> >> to 90 fps at 2688x1520 resolution. It is programmable through an I2C
>> >> interface, and sensor output is sent via 1/2/4 lane MIPI CSI-2
>> >> connection.
>> >>
>> >> The driver is based on Rockchip BSP kernel [1]. It implements 4-lane CSI-2
>> >> and single 2688x1520 @ 30 fps mode. The driver was tested on Rockchip
>> >> 3399-based FriendlyElec NanoPi M4 board with MCAM400 camera
>> >> module.
>> >> While porting the driver, I stumbled upon two issues:
[snip]
>> >> (2) The original driver exposes analog gain range 0x0 - 0x7ff, but the
>> >> gain is not linear across that range. Instead, it is piecewise linear
>> >> (and discontinuous). 0x0-0xff register values result in 0x-2x gain,
>> >> 0x100-0x1ff to 0x-4x, 0x300-0x3ff to 0x-8x, and 0x700-0x7ff to 0x-16x,
>> >> with more linear segments in between. Rockchip's camera engine code
>> >> chooses one of the above segments depenging on the desired gain
>> >> value. The question is, how should we proceed keeping in mind
>> >> libcamera use case? Should the whole 0x0-0x7ff be exposed as-is and
>> >> libcamera will do the mapping, or the driver will do the mapping
>> >> itself and expose some logical gain units not tied to the actual gain
>> >> register value? Meanwhile, this driver conservatively exposes only
>> >> 0x0-0xf8 gain register range.
>> >
>> > The datasheet linked above says "for the gain formula, please contact
>> > your local OmniVision FAE" :-(
>> > I would assume that the range is from 1x rather than 0x - people
>> > rarely want a totally black image that 0x would give. Or is it ranges
>> > of 1x - 2x, 2x - 4x, 4x - 8x, and 8x - 16x?
>>
>> A picture is worth a thousand words, so I've attached the results of my
>> experimentation with the gain register. They were obtained with Rockchip
>> 3399, with AEC, AGC and black level subtraction disabled. The image was
>> converted from 10-bit RGGB to 8-bit YUV 4:2:0 by the Rockchip ISP.
>
> Based on that it looks like their medication may have been a little too
> strong.
>
> Could this be implemented so that the control value would be linear linear
> but its range would correspond 1x--16x values?
>
> libcamera will be able to cope with that.
>

According to the following fragment of the Rockchip camera engine sensor
configuration file for ov4689 [1]

    <Linear index="1" type="double" size="[4 7]">
       [1 2 128 0 1 128 255
        2 4 64 -248 1 376 504
        4 8 32 -756 1 884 1012
        8 16 16 -1784 1 1912 2040]
    </Linear>,

it uses gain register value range 128-255 for gain 1x-2x, 376-504 for
gain 2x-4x, 884-1024 for 4x-8x, and 1912-2040 for 8x-16x. Do you suggest
to implement this calculation in the sensor driver and expose some
linear "logical" gain to userspace (ranging, e.g., 128-2048 for gains
1x-16x)?


[1] https://github.com/aosp-rockchip/android_external_camera_engine_rkaiq/blob/quartz64/iqfiles/ov4689_JSD3425-C1_JSD3425-C1-36IRC-4M-F20.xml

--
Best regards,
Mikhail