Question

I’m customising Linux for an ARM9 Atmel AT91SAM960 board.

In the device file Atmel named all the USART the same atmel_usart. Of course with id enumeration:

static struct platform_device at91sam9260_uart0_device = {
        .name           = "atmel_usart",
        .id             = 1,
        .dev            = { ...}
}

According to the Linux Device model, all these devices (5 UARTS on a SAM9260) would be bind to the driver named atmel_usart.

I don’t want to set a TTYS driver on all UARTS which will be registerd. I have several own drivers which serve for different specialised purposes (LON, RS-485 etc.) I want the control which driver does serve a certain USART. So what could I do:

• The Atmel device files are unsatisfiable and I can do it better. So I rename (patch) the devices in the device file. However, in case I want a TTYS driver on UART4 I would be in trouble.

• I manipulate (patch) the device file,
  so that I’m able the access the
  structures platform_device. I could
  change their names before I would
  register them. But as far as I
  understood the idea of the Linux Driver Model,
  devices should be
  registered early during boot-up but the binding to a driver follows …. later.

• I could write a driver, which has an
  alias name and which would be binded
  to a specific bus_Id ->
  atmel_usart.4. Can I really?

What solutions else exist. I want to touch a minimal set of Kernel files but I want all the freedom possible?

Addendum what freedom means to me: I can specify at runtime how the UARTS can be used

• with the Atmel-Serial driver (ttyS)
• with my own drivers

It means also, that changes to the kernel source are minimal.

Answer 1

I built my own line discipline drivers. You can build them as kernel modules and attach them to the UARTs at runtime. No changes to the Linux source are necessary.

Any funny timing or control stuff can be done through ioctl(). Specifically, I implemented a timing-sensitive RS-485 protocol in this way.

When I did this (Linux 2.6.17) there was no dynamic registration mechanism, so I overwrote the existing line disciplines. The Linux code is (was) pretty straightforward, and I was satisfied that this would be a safe thing to do.