Re: [PATCH v2 0/2] Add Omnivision OV4689 image sensor driver

[Sakari Ailus]

Hi Mikhail,

On Sun, Sep 11, 2022 at 11:01:33PM +0300, Mikhail Rudenko 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:
> 
> (1) In the original driver, horizontal total size (HTS) was set to a
> value (2584) lower then the frame width (2688), resulting in negative
> hblank. In this driver, I increased HTS to 2688, but fps dropped from
> 29.88 to 28.73. What is the preferred way to handle this?

If horizontal total size is less than the frame width, something is
certainly wrong there. You can't have negative horizontal blanking. Neither
it can be zero.

> 
> (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.

-- 
Sakari Ailus