16C950 UART 9-bit and custom baud rates support for Linux
Last friday I found myself patching the Linux kernel sources to be able to add custom baud rates support for 16C950 UART cards. I needed to communicate, via a serial port, with one of the devices built by the hardware guys at work. Unfortunately, a non-standard baud rate and 9-bits were needed.
The Sealevel Ultra COMM+422 PCI card we are using is already provided with a patch that adds 9-bit support. However, I was not able to found how to change the baud rate to a non-standard one.
Following the 9-bit patch approach, I added extra requests to ioctl in order to modify the 16C950 registers needed to achieve custom baud rates. Here is the list of added extra ioctl requests (also the 9-bit ones) with the necessary parameters and updated/accessed 16C950 registers:
| Request | Parameters | 16C950 registers | Description |
| TIOC9BITGET | out: integer | NMR | Get current 9-bit status |
| TIOC9BITSET | NMR | Enable 9-bit support | |
| TIOC9BITCLR | NMR | Disable 9-bit support | |
| TIOCPRESCALERGET | out: integer | EFR, MCR | Get prescaler status |
| TIOCPRESCALERSET | EFR, MCR | Enable prescaler | |
| TIOCPRESCALERCLR | EFR, MCR | Disable prescaler | |
| TIOCDIVLATCHGET | out: integer | LCR, DLL/DLM | Get divisor latch register |
| TIOCDIVLATCHSET | in: integer | LCR, DLL/DLM | Set divisor latch register |
| TIOCSAMPLINGCLKGET | out: integer | TCR | Get sampling clock |
| TIOCSAMPLINGCLKSET | in: integer | TCR | Set sampling clock |
| TIOCPRESCALERCLKGET | out: integer | CPR | Get prescaler clock |
| TIOCPRESCALERCLKSET | in: integer | CPR | Set prescaler clock |
I’m not sure if this is the best way to do it, but it works. So, if you need 9-bit and custom baud rates support, apply one the following patches to the kernel (I have aslo updated the Sealevel patch for older and newer kernels than the one provided):
| 9-bit | 2.6.24 | 2.6.26 | 2.6.31 | 2.6.32 |
| 9-bit and baud rate | 2.6.24 | 2.6.26 | 2.6.31 | 2.6.32 |
I am not responsible for any damage that these patches can cause to your hardware. No warranty is provided, so use them at your own risk.
