The Patch
In this post, we will describe the end-to-end process of contributing to the Linux kernel, specifically focusing on the IIO (Industrial I/O) subsystem.
1. Cloning the Correct Tree
Since I was already working on tutorials using the IIO tree, it was not necessary to clone it again. However, as my previous work was done on a testing branch, the first step was to ensure a clean workspace.
I moved my previous experiments to a dedicated backup branch to preserve them, and then performed a checkout to the development branch: togreg. Authoritative information about kernel trees and branches are available at MAINTAINERS file.
Managing Branches
First, I created a backup of my current progress:
git branch tutoriais-backup
Then we created our new work branch based on togreg:
git checkout -b iio-deduplicate-code
2. Deduplicating Code
The code duplications targeted for refactoring were identified across several drivers within the IIO HID subsystem. Specifically, the following six functions—each consisting of 9 lines of identical logic were selected for deduplication:
Function find: accel/hid-sensor-accel-3d.c::accel_3d_adjust_channel_bit_mask, TOTAL NUMBER LINES IN FUNCTION: 9 Function find: pressure/hid-sensor-press.c::press_adjust_channel_bit_mask, TOTAL NUMBER LINES IN FUNCTION: 9 Function find: magnetometer/hid-sensor-magn-3d.c::magn_3d_adjust_channel_bit_mask, TOTAL NUMBER LINES IN FUNCTION: 9 Function find: light/hid-sensor-prox.c::prox_adjust_channel_bit_mask, TOTAL NUMBER LINES IN FUNCTION: 9 Function find: light/hid-sensor-als.c::als_adjust_channel_bit_mask, TOTAL NUMBER LINES IN FUNCTION: 9 Function find: gyro/hid-sensor-gyro-3d.c::gyro_3d_adjust_channel_bit_mask, TOTAL NUMBER LINES IN FUNCTION: 9
To begin the refactoring process, we checked the files:
code drivers/iio/accel/hid-sensor-accel-3d.c \
drivers/iio/pressure/hid-sensor-press.c \
drivers/iio/magnetometer/hid-sensor-magn-3d.c \
drivers/iio/light/hid-sensor-prox.c \
drivers/iio/light/hid-sensor-als.c \
drivers/iio/gyro/hid-sensor-gyro-3d.c
Aside from their names, the functions are identical:
static void magn_3d_adjust_channel_bit_mask(struct iio_chan_spec *channels,
int channel, int size)
{
channels[channel].scan_type.sign = 's';
/* Real storage bits will change based on the report desc. */
channels[channel].scan_type.realbits = size * 8;
/* Maximum size of a sample to capture is u32 */
channels[channel].scan_type.storagebits = sizeof(u32) * 8;
}
We decided to design a generic version of this function, since each file used a different naming convention
static void hid_sensor_adjust_channel_bit_mask(struct iio_chan_spec *channels,
int channel, int size)
{
channels[channel].scan_type.sign = 's';
/* Real storage bits will change based on the report desc. */
channels[channel].scan_type.realbits = size * 8;
/* Maximum size of a sample to capture is u32 */
channels[channel].scan_type.storagebits = sizeof(u32) * 8;
}
3. Choosing the location for our generic function
Since the code was identical across multiple drivers, we had to move it to a shared location. In the IIO HID subsystem, the most suitable place was drivers/iio/common/hid-sensors/hid-sensor-attributes.c, as it already manages shared attributes and formatting logic for these sensors.
We also exported the symbol using EXPORT_SYMBOL_NS to ensure that drivers compiled as loadable modules can access this function at runtime.
void hid_sensor_adjust_channel_bit_mask(struct iio_chan_spec *channels,
int channel, int size)
{
channels[channel].scan_type.sign = 's';
/* Real storage bits will change based on the report desc. */
channels[channel].scan_type.realbits = size * 8;
/* Maximum size of a sample to capture is u32 */
channels[channel].scan_type.storagebits = sizeof(u32) * 8;
}
EXPORT_SYMBOL_NS(hid_sensor_adjust_channel_bit_mask, "IIO_HID");
In addition to modifying this file, we also updated a header file: include/linux/hid-sensor-hub.h
void hid_sensor_adjust_channel_bit_mask(struct iio_chan_spec *channels,
int channel, int size);
We have removed the local functions from all six files and replaced their calls with our new generic function.
4. Checks before submiting the patch
An important step before submitting a patch is to verify that it complies with the style guidelines. We check this using the following command:
./scripts/checkpatch.pl --terse --file \
drivers/iio/accel/hid-sensor-accel-3d.c \
drivers/iio/common/hid-sensors/hid-sensor-attributes.c \
drivers/iio/gyro/hid-sensor-gyro-3d.c \
drivers/iio/light/hid-sensor-als.c \
drivers/iio/light/hid-sensor-prox.c \
drivers/iio/magnetometer/hid-sensor-magn-3d.c \
drivers/iio/pressure/hid-sensor-press.c \
include/linux/hid-sensor-hub.h
Since we had already set the kw configurations, they remain valid ans no changes were necessary.
cd "$IIO_TREE"
cat .kw/build.config
We compile our code to ensure everything is working correctly, following the process of the tutorial number 4.
We looked in the same directory of the files modified for a Kconfig, searched a config name and description that relates to where we were working. Then, opened the .config file for editing with make nconfig or kw build –menu. We also checked the Depends on: attribute with the dependencies needed to be enabled before enable the module. There was also a Location: with a series of indentations that show the menus sequence to activate the config you’ve entered;
We enabled: HID_SENSOR_GYRO_3D, HID_SENSOR_ALS, HID_SENSOR_PROX, HID_SENSOR_MAGNETOMETER_3D, HID_SENSOR_PRESS, HID_SENSOR_ACCEL_3D, HID_SENSOR_HUB
After testing that our changes compile, we committed our work:
git commit --signoff \
--trailer='Co-developed-by: Pietro Di Consolo Gregorio <pietro.gregorio@usp.br>' \
--trailer='Signed-off-by: Pietro Di Consolo Gregorio <pietro.gregorio@usp.br>' \
-m "iio: HID: Add helper method hid_sensor_adjust_channel_bit_mask()" \
-m "Add helper method to deduplicate code in HID sensors."
At total we made 7 atomic commits.
The last check was sending the patch to the CI. To send patch using the usp email we also did the steps in tutorial 6.
kw send-patch --send -7 --private --to=kernel@lists.ime.usp.br --cc=natalia.andre@ime.usp.br,pietro.gregorio@usp.br
5. Sending the patch
Since no errors were found, we sent the patch to the correct list of mantainers
kw send-patch --send -7
We edited the email subject and blurb before sending too.
6. The first review
We received the first review two days later.
During the review, David Lechner provided specific feedback to improve code consistency and modern standards. He requested adjustments to the indentation logic—specifically ensuring alignment with opening parentheses or using an extra tab for long lines—and recommended replacing hardcoded values with the BITS_PER_BYTE macro. Additionally, he suggested updating the sign field to the newly adopted format name, advising that a precursor patch be created to rename existing instances within the file to maintain uniformity.
The new version of the code he talked about was actually in testing branch, so we did a rebase to have the correct work in our branch.
Other maintainers also reviewed the patch series but offered no additional suggestions.
We addressed the requested fixes and used git commit --amend to update our previous work without introducing new commits.
git rebase -i HEAD~7
git commit --amend
git rebase --continue
git format-patch -7 --stdout | ./scripts/checkpatch.pl --
Then, we made the checks again and since no errors were found we submitted the patch, but with version 2 flag:
kw send-patch --send -7 -v2
7. The second review
We received a second round of reviews from multiple maintainers. They highlighted that Some lines in the commit messages exceeded the 72-character limit. One of the maintainers pointed out that creating a new “helper” function might not be the most efficient approach for this specific case. Instead, they suggested using a compound literal struct initializer. This makes the code more explicit and follows modern C standards (C99) used in the kernel.
We created a backup branch to ensure I could revert to my previous work if needed.
We replaced the external function call with a direct struct iio_scan_type initialization within the loop. (accel 3d)
Loop Optimization: we simplified the loop logic by using the channel index as the primary iterator, reducing redundant calculations.
Header Updates: To support the new logic, we had to include <linux/bitops.h> and <linux/bits.h>. These headers provide essential macros like BYTES_TO_BITS and BITS_PER_TYPE.