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Date:   Mon, 19 Sep 2022 10:31:02 +0000
From:   Sakari Ailus <sakari.ailus@linux.intel.com>
To:     Mikhail Rudenko <mike.rudenko@gmail.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
Message-ID: <YyhE5voxRz7gEYHY@paasikivi.fi.intel.com>
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Hi Mikhail,

On Mon, Sep 19, 2022 at 10:01:06AM +0300, Mikhail Rudenko wrote:
> 
> 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)?

Yes. This way the user space can somehow work without knowing this special
implementation, even though the granularity changes over the range. I guess
the granularity would need to be known in libcamera but that's a separate
issue.

-- 
Sakari Ailus