Jungfraujoch/docs/FPGA_PCIE_DRIVER.md

# FPGA PCIe driver

## Compilation
To compile kernel module type:
```
make
```

## Installation
To install kernel module, you need to have root permissions and run:
```
sudo make install
```

## Loading driver into kernel
After installing the kernel driver, it should be possible to insert it into the kernel via:
```
modprobe jfjoch
```

## Ownership of the character devices
By default, character devices `/dev/jfjoch<device number>` are owned by root (user/group) and are not accessible by others. 
This means that `jfjoch_broker` must be running as superuser, which might not be optimal for security reasons in most cases.
The behavior can be changed by creating `udev` rules. Create a file called `/etc/udev/rules.d/99-jfjoch.rules` 
with the following content:
```
KERNEL=="jfjoch*" OWNER="<UNIX username>" GROUP="<UNIX group>"
```
It is OK to provide only group, for example to make the devices accessible by group `jungfrau`:
```
KERNEL=="jfjoch*" GROUP="jungfrau"
```

## DKMS
To avoid problems with updating the kernel, it is possible to use DKMS to autobuild Jungfraujoch kernel
module, when new kernel is installed. For RHEL 8 it is well tested to use the RPM module built automatically from Jungfraujoch source. 
For other systems, it is necessary to follow the procedure below, though it is not well tested.

This first requires to install DKMS - for RHEL it is available via EPEL repository:
```
sudo dnf install dkms
```
Then use script provided in the driver directory to copy driver code to DKMS directory:
```
./install_dkms.sh
```
If upgrading the driver, please first remove current driver from DKMS system:
```
dkms remove jfjoch -v <version> --all
```

## Driver parameters
Currently, there is one driver parameter `nbuffers`, that defines count of exchange buffers (see below). 
This can be adjusted in the modprobe operation, for example:
```
modprobe jfjoch nbuffers=1024
```

## Exchange buffers
The parameter defines number of buffers used to exchange data between card and host application.
Each buffer can hold one detector module (1024x512) in 16-bit or 32-bit mode + associated processing results and metadata.
These buffers are used by both card-to-host and host-to-card operations.

Buffers use special allocation, as they are continuous in physical address space, which helps the FPGA card to transfer all
data associated with detector module in two DMA transfers (one data, one metadata).
Useful buffer size is a bit more than 2 MiB, but given that kernel allocates physical memory in power of two, **4 MiB** is safe number for one buffer size.
Buffer can be mapped into user space, but performing `mmap` system call on the `/dev/jfjoch<number of device>` character device.

Buffer count can be adjusted by setting `nbuffers` parameter. There are two considerations for setting optimal value:
1. For card-to-host transfers, minimal value is roughly 
`<number of threads in receiver> * <number of modules processed by thread; usually equal to number of modules per card>`, 
this way each thread can have enough data for operation. Default thread count for Jungfraujoch receiver is 64.  
2. For host-to-card transfers, full detector calibration has to fit into memory and one buffer accommodates one calibration set for one module.
So minimal count is `<number of modules> * (3 + 3 * <number of storage cells>)`.

Based on both rules, optimal number is 512 buffers (2 GiB), though this can be adjusted for particular system and configuration.

## Known problems
To avoid inconsistent behavior, this driver won't load if release number differs between the kernel driver and FPGA card.

## CMake file
While CMake file is present in the driver directory, it is only for the purpose of proper detection of the files in CLion IDE. 
It is not made for actual compilation of the kernel driver and should not be used for that purpose.

## Character device access
For each FPGA device a character device is created called `/dev/jfjoch<number of device>`.
When device is opened two operations are possible:
mmap() to map exchange buffers
ioctl() to communicate with the cards
Interfacing should be done through the JungfraujochDevice class in `fpga/host_library` directory.

## Sysfs access
Certain performance counters can be read through sysfs mechanism in the kernel. 
One needs to `cat` files in `/sys/class/misc/jfjoch<number of device>/` directory.

## RHEL 9.5+ virtual memory flags
RedHat Enterprise Linux 9.5 backported the `vm_flags_set` interface from Linux kernel 6.3 while still reporting kernel version 5.14, so a plain kernel-version test picks the wrong branch and the build fails.
This is now detected automatically from `RHEL_RELEASE_CODE`, so the module builds unaided on RHEL 9.5 and later and on the CentOS Stream, Rocky and AlmaLinux equivalents, as well as on distributions that have not backported it.
**No user action is needed.** The `HAVE_VM_FLAGS_SET` environment variable that earlier releases required is obsolete; it is still honoured if set, but setting it is no longer necessary and the DKMS packaging never passed it anyway.

## Which kernel DKMS builds for
The DKMS package builds the module for the kernel it is being **installed for**, not the one currently running, so a module built while a kernel update is being applied loads correctly after the reboot.
Building by hand in `fpga/pcie_driver/` still defaults to the running kernel; pass `KDIR=/lib/modules/<version>/build` (or `KVER=<version>`) to target another one.