linux-kernel - Re: Writing a console/tty driver -- how to use tty

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Message-ID: <4CC9E646.6070405@freescale.com>
Date:	Thu, 28 Oct 2010 16:08:22 -0500
From:	Timur Tabi <timur@...escale.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	gregkh <gregkh@...e.de>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: Writing a console/tty driver -- how to use tty_port?

Arnd Bergmann wrote:
> If the device is not a UART, the best option may be to make the driver
> a backend to the hvc driver, like e.g. drivers/char/hvc_tile.c.

The current version *is* a backend to hvc, but I have to abandon that because of
this thread:

http://lists.ozlabs.org/pipermail/linuxppc-dev/2010-September/085664.html

I posted a patch that causes hvc to spin if the backend driver returns -EAGAIN,
which is my driver does when the output buffer is full.

In other words, hvc does not support backend drivers that can detect full output
buffers.

> This works for all devices with or without interrupts that don't need
> to set up the communication parameters but simply provide a read/write
> character interface.

If my patch were accepted, I wouldn't need to do the rewrite.  Dropping
characters during a printk() is not acceptable for us.

-- 
Timur Tabi
Linux kernel developer at Freescale

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