linux-kernel - Re: RFC: out-of-tree tty driver breakage (changing ASYNC

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Date:	Sun, 10 Jan 2016 20:42:04 -0800
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	Linux kernel <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jslaby@...e.cz>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Subject: Re: RFC: out-of-tree tty driver breakage (changing ASYNC_ bits)

On Sun, Jan 10, 2016 at 01:42:44PM -0800, Peter Hurley wrote:
> The tty/serial core uses 5 bits in the tty_port.flags field to
> manage state. They are:
> 
> ASYNCB_INITIALIZED
> ASYNCB_SUSPENDED
> ASYNCB_NORMAL_ACTIVE
> - and -
> ASYNCB_CTS_FLOW
> ASYNCB_CHECK_CD
> 
> Unfortunately, updates to this field (tty_port.flags) are often
> non-atomic. Additionally, the field is visible to/modifiable by
> userspace (the first 3 bits above are not modifiable by userspace
> though). Lastly, the multi-bit ASYNC_SPD_ bitfield is in this
> tty_port.flags field as well.
> 
> What needs to happen is the tty/serial core needs to update its
> state transitions atomically. I want to re-define the above 5 flags
> into a separate field in the tty_port structure and designate new
> symbols for these bits. The base patch that does this is inlined
> below.
> 
> This will break out-of-tree drivers but I don't really see a
> realistic alternative. Also, I think the new symbol prefix ASY_ isn't
> great and I'd like to get some suggestions.

Don't worry about breaking out-of-tree drivers, that's fine.

And try "hiding" the symbol prefix behind inline functions
(tty_port_initialized(port) and the like) that way the prefix of the
symbol, or even how you do this with locking or bits or atomics will all
not matter at all.

thanks,

greg k-h