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Paul Elder 879c551b0d media: ov5640: Fix analogue gain control 
[ Upstream commit afa4805799 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2023-03-17 08:58:02 +01:00
arch alpha: fix R_ALPHA_LITERAL reloc for large modules 2023-03-17 08:58:02 +01:00
block block: fix wrong mode for blkdev_put() from disk_scan_partitions() 2023-03-17 08:57:58 +01:00
certs certs: Fix build error when PKCS#11 URI contains semicolon 2023-01-31 17:53:01 +09:00
crypto crypto: rsa-pkcs1pad - Use akcipher_request_complete 2023-03-10 09:28:18 +01:00
Documentation usb: gadget: uvc: Make bSourceID read/write 2023-03-11 13:50:44 +01:00
drivers media: ov5640: Fix analogue gain control 2023-03-17 08:58:02 +01:00
fs ext4: Fix deadlock during directory rename 2023-03-17 08:58:00 +01:00
include PCI: Add SolidRun vendor ID 2023-03-17 08:58:02 +01:00
init Kbuild fixes for v6.2 (3rd) 2023-01-21 10:56:37 -08:00
io_uring io_uring/uring_cmd: ensure that device supports IOPOLL 2023-03-17 08:57:45 +01:00
ipc Non-MM patches for 6.2-rc1. 2022-12-12 17:28:58 -08:00
kernel watch_queue: fix IOC_WATCH_QUEUE_SET_SIZE alloc error paths 2023-03-17 08:58:00 +01:00
lib cpuidle: lib/bug: Disable rcu_is_watching() during WARN/BUG 2023-03-10 09:29:06 +01:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
mm memory tier: release the new_memtier in find_create_memory_tier() 2023-03-10 09:29:50 +01:00
net af_unix: fix struct pid leaks in OOB support 2023-03-17 08:57:59 +01:00
rust rust: print: avoid evaluating arguments in pr_* macros in unsafe blocks 2023-01-16 00:54:35 +01:00
samples ftrace: Export ftrace_free_filter() to modules 2023-01-24 11:20:58 -05:00
scripts scripts: handle BrokenPipeError for python scripts 2023-03-17 08:58:02 +01:00
security ima: Align ima_file_mmap() parameters with mmap_file LSM hook 2023-03-10 09:29:36 +01:00
sound ASoC: mediatek: mt8195: add missing initialization 2023-03-11 13:50:37 +01:00
tools bpf, test_run: fix &xdp_frame misplacement for LIVE_FRAMES 2023-03-17 08:57:56 +01:00
usr usr/gen_init_cpio.c: remove unnecessary -1 values from int file 2022-10-03 14:21:44 -07:00
virt KVM: Register /dev/kvm as the _very_ last thing during initialization 2023-03-10 09:29:33 +01:00
.clang-format iommufd for 6.2 2022-12-14 09:15:43 -08:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore .gitignore: ignore *.rpm 2022-12-30 17:22:14 +09:00
.mailmap 12 hotfixes, mostly against mm/. Five of these fixes are cc:stable. 2023-02-13 14:09:20 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: update SCTP maintainers 2023-02-02 11:35:33 -08:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS audit: update the mailing list in MAINTAINERS 2023-02-25 11:13:29 +01:00
Makefile Linux 6.2.6 2023-03-13 10:26:43 +01:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

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.