Date:	Thu, 25 Mar 2010 02:33:42 +0530 (IST)
From:	Pavan Savoy <pavan_savoy@...oo.co.in>
To:	Marcel Holtmann <marcel@...tmann.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, Greg KH <gregkh@...e.de>,
	PavanSavoy <pavan_savoy@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/6] drivers:misc: sources for Init manager module

Marcel,

--- On Thu, 25/3/10, Marcel Holtmann <marcel@...tmann.org> wrote:

> From: Marcel Holtmann <marcel@...tmann.org>
> Subject: Re: [PATCH 4/6] drivers:misc: sources for Init manager module
> To: "Pavan Savoy" <pavan_savoy@...oo.co.in>
> Cc: "Alan Cox" <alan@...rguk.ukuu.org.uk>, "Greg KH" <gregkh@...e.de>, "PavanSavoy" <pavan_savoy@...com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
> Date: Thursday, 25 March, 2010, 2:24 AM
> Hi Pavan,
> 
> > > > Which puts me back to square 1,
> > > the requirement is TTY device should be open,
> only when
> > > either BT, FM or GPS would want to use it.
> > > > [with your patch one of the steps of
> installing ldisc
> > > would be reduced upon opening].
> > > 
> > > Perhaps but its up to you how you write your low
> level
> > > driver and how the
> > > ldisc indicates it wishes to be active (eg speed
> B0 is used
> > > to indicate
> > > 'no carrier' in many cases)
> > > 
> > > > Actually why do I even need a TTY device in
> this case
> > > - right ?
> > > > ldisc driver can do the
> tty->ops->write and
> > > tty_read and put it up on different interfaces
> like
> > > eth0/hci0 or /dev/radio0 etc..
> > > 
> > > You don't need a tty if you are simply demuxing
> some kind
> > > of stream of
> > > bytes to/from the hardware and you gain nothing
> from the
> > > tty layer and
> > > user space interfaces such as control signal and
> speed
> > > setting.
> > > 
> > 
> > I still want to maintain this as a ldisc driver and
> the TTY layer is still required to be able to use this with
> different sort of serial drivers.
> > 
> > example:
> > We do a have a system here where both 8250/omap-serial
> co-exist creating their own device nodes (ttyS for 8250,
> ttyO for omap-serial) and I would want to be able to on boot
> suggest which serial driver to use, so I can open/install
> ldisc on ttyS or on ttyO.
> > 
> > 
> > > That may be even cleaner in your case as you can
> then then
> > > provide
> > > suitable links between your demux and the drivers
> attached
> > > to it
> > > indicating when they should be on or off.
> > 
> > And as Marcel suggested, can't really put in the
> PM/UART-clk shut off code in ldisc driver because few of
> these driver provide interfaces and few don't.
> > 
> > So is there no way this can be accepted with the sysfs
> entry way of communication ?
> 
> to be quite honest, I can't see how that can be accepted
> right now. It
> looks wrong on too many levels.

Sigh -
So to save power, and open/install tty-ldisc only when required, the sysfs entry was introduced.
So now I guess I have 2 options :-

1. Fix the UART/LDISC upon boot by the daemon, get away with the sysfs entry + signaling altogether.
2. find a better way of telling the user-space daemon to open/install the line discipline.

- So any suggestions on option 2 ? like some event mechanism which is simple and clean ?

While we are at it, I will send across the BT driver too which makes use of this LDISC.

> 
> Regards
> 
> Marcel
> 
> 
> 


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