[PATCH] amba-pl011: support hardware flow control

[Jamie Lokier]

Russell King - ARM Linux wrote:
> This means that when the kernel's buffers fill up, the kernel calls
> down to the serial core layer to throttle the input.  This then
> calls into the set_mctrl function to de-assert RTS.  However, because
> the hardware ignores the requested software state, the RTS signal
> is not de-asserted, and the remote end continues sending data.
> 
> So, enabling hardware auto-RTS is bad news - you will lose data if
> the application stops reading data.

Surely the driver can just stop reading from the UART when the
kernel's buffers are full and hardware-RTS is enabled.  Then the
hardware will deassert RTS itself.

The problem with relying on the kernel to deassert RTS is that it's
too slow when something disables interrupts for a few ms, and you get
lost data that way instead.

The ideal combination may be a bit of both:

    1. When kernel deasserts RTS, do that and disable hardware-RTS
       so it really is deasserted.

    2. When kernel asserts RTS with mctrl, enable hardware-RTS
       so what's output depends on the receive FIFO state.

That should solve both types of overrun.

-- Jamie