linux-kernel - Re: [patch 0/7] staging: speakup: introduce tty-based comms

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Date:   Tue, 14 Mar 2017 02:18:14 +0100
From:   Samuel Thibault <samuel.thibault@...-lyon.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        okash.khawaja@...il.com, Jiri Slaby <jslaby@...e.com>,
        linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
        linux-serial@...r.kernel.org, Rob Herring <robh@...nel.org>,
        Kirk Reiser <kirk@...sers.ca>,
        Chris Brannon <chris@...-brannons.com>,
        speakup@...ux-speakup.org
Subject: Re: [patch 0/7] staging: speakup: introduce tty-based comms

Samuel Thibault, on mar. 14 mars 2017 01:47:01 +0100, wrote:
> Greg KH, on mar. 14 mars 2017 06:14:04 +0800, wrote:
> > On Mon, Mar 13, 2017 at 10:05:51PM +0000, okash.khawaja@...il.com wrote:
> > > This patchset introduces a TTY-based way for the synths to communicate
> > > with devices as an alternate for direct serial comms used by the synths
> > > at the moment. It then migrates some of the synths to the TTY-based
> > > comms. Synths migrated in this patchset are dummy, acntsa, bns and
> > > txprt.
> > 
> > What about using the serbus code that is now in the tree?  That should
> > make this a lot easier than your patchset from what I can see.
> 
> Mmm... AIUI from reading tty_port_register_device_attr, one
> would have to have registered a speakup serdev device driver
> *before* tty_port_register_device_attr gets called, so that
> serdev_tty_port_register matches the driver in the loop of
> of_serdev_register_devices, and no TTY cdev is created?
> 
> That would mean that speakup can not be loaded as a module after ttyS0
> initialization, that won't fly for our use needs. The line discipline
> mechanism allows us to attach ourself to an existing tty.  Could we
> imagine a tty_port function which removes the cdev and tries to register
> the tty port again to serdev?
> 
> What we basically need to be able to say on speakup module load is
> e.g. "I'm now attaching a device to ttyS0, use this serdev_device_ops to
> discuss with it".

That for_each_available_child_of_node loop is really way more complex
than what we need.  And what's more, it's not working without CONFIG_OF
(!)

It would really make sense to me to have a

serdev_device *tty_port_register_serdev_device(tty, device)

which unregisters the character device of the tty, and creates instead a
controler with the given device plugged to it. Really much like a line
discipline, but way simpler :)

The issue that remains is the "tty" part: we'd need there the tty_port,
the parent device, the tty driver, the idx. All we know when we load
speakup is "ttyS0". That's where going through the cdev to attach a line
discipline was simpler :)

Samuel