linux-kernel - Re: [PATCH 7/9] tty: Add MIPS EJTAG Fast Debug Channel TTY driver

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Date:	Thu, 26 Mar 2015 16:09:01 +0100
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	James Hogan <james.hogan@...tec.com>
Cc:	Ralf Baechle <ralf@...ux-mips.org>, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org, Jiri Slaby <jslaby@...e.cz>
Subject: Re: [PATCH 7/9] tty: Add MIPS EJTAG Fast Debug Channel TTY driver

On Thu, Jan 29, 2015 at 11:14:12AM +0000, James Hogan wrote:
> Add TTY driver and consoles for the MIPS EJTAG Fast Debug Channel (FDC),
> which is found on the per-CPU MIPS Common Device Mapped Memory (CDMM)
> bus.
> 
> The FDC is a per-CPU device which is used to communicate with an EJTAG
> probe. RX and TX FIFOs exist, containing 32-bits of data and 4-bit
> channel numbers. 16 general data streams are implemented on this for TTY
> and console use by encoding up to 4 bytes on each 32-bit FDC word.
> 
> The TTY devices are named e.g. /dev/ttyFDC3c2 for channel 2 of the FDC
> attached to logical CPU 3.
> 
> These can be used for getting the kernel log, a login prompt, or as a
> GDB remote transport, all over EJTAG and without needing a serial port.
> 
> It can have an interrupt to notify of when incoming data is available in
> the RX FIFO or when the TX FIFO is no longer full. The detection of this
> interrupt occurs in architecture / platform code, but it may be shared
> with the timer and/or performance counter interrupt.
> 
> Due to the per-CPU nature of the hardware, all outgoing TTY data is
> written out from a kthread which is pinned to the appropriate CPU.
> 
> The console is not bound to a specific CPU, so output will appear on the
> chosen channel on whichever CPU the code is executing on. Enable with
> e.g. console=fdc1 in kernel arguments. /dev/console is bound to the same
> channel on the boot CPU's FDC if it exists.
> 
> Signed-off-by: James Hogan <james.hogan@...tec.com>
> Cc: Ralf Baechle <ralf@...ux-mips.org>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Jiri Slaby <jslaby@...e.cz>
> Cc: linux-mips@...ux-mips.org

Acked-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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