linux-kernel - Re: [PATCH v3 3/3] portman2x4 - use new parport device model

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Date:	Fri, 05 Feb 2016 18:06:45 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	Sudip Mukherjee <sudipm.mukherjee@...il.com>
Cc:	Jaroslav Kysela <perex@...ex.cz>, alsa-devel@...a-project.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] portman2x4 - use new parport device model

On Fri, 05 Feb 2016 18:01:16 +0100,
Takashi Iwai wrote:
> 
> On Fri, 05 Feb 2016 17:50:51 +0100,
> Sudip Mukherjee wrote:
> > 
> > On Friday 05 February 2016 05:25 PM, Takashi Iwai wrote:
> > > On Fri, 05 Feb 2016 07:17:06 +0100,
> > > Sudip Mukherjee wrote:
> > >>
> > >> On Thu, Feb 04, 2016 at 05:51:07PM +0100, Takashi Iwai wrote:
> > >>> On Thu, 04 Feb 2016 17:38:23 +0100,
> > >>> Sudip Mukherjee wrote:
> > >>>>
> > >>>> Modify portman driver to use the new parallel port device model.
> > >>>> The advantage of using the device model is that the device gets binded
> > >>>> to the hardware, we get the feature of hotplug, we can bind/unbind
> > >>>> the driver at runtime.
> > >>>> The only change is in the way the driver gets registered with the
> > >>>> parallel port subsystem and so as a result there is no user visible
> > >>>> change or any chance of regression.
> > >>>>
> > >>>> Signed-off-by: Sudip Mukherjee <sudip@...torindia.org>
> > >>>> ---
> > >>>>
> > >>>> v3: changed commit message
> > >>>> v2:
> > >>>>   1. pardev_cb is initialized while declaring, thus removing the use of
> > >>>> memset.
> > >>>>   2. used pdev->id.
> > >>>>   3. v1 did not have the parport probe callback, but
> > >>>> we will need the probe callback for binding as the name of the driver
> > >>>> and the name of the device is different.
> > >>>>   4. in v1 I missed modifying snd_portman_probe_port().
> > >>>>
> > >>>>   sound/drivers/portman2x4.c | 53 ++++++++++++++++++++++++++++++++++------------
> > >>>>   1 file changed, 39 insertions(+), 14 deletions(-)
> > >>>>
> > >>>> diff --git a/sound/drivers/portman2x4.c b/sound/drivers/portman2x4.c
> > >>>> index 172685d..a22f56c 100644
> > >>>> --- a/sound/drivers/portman2x4.c
> > >>>> +++ b/sound/drivers/portman2x4.c
> > >>>> @@ -650,10 +650,21 @@ static int snd_portman_probe_port(struct parport *p)
> > >>>>   {
> > >>>>   	struct pardevice *pardev;
> > >>>>   	int res;
> > >>>> -
> > >>>> -	pardev = parport_register_device(p, DRIVER_NAME,
> > >>>> -					 NULL, NULL, NULL,
> > >>>> -					 0, NULL);
> > >>>> +	struct pardev_cb pdev_cb = {
> > >>>> +		.preempt = NULL,
> > >>>> +		.wakeup = NULL,
> > >>>> +		.private = NULL,
> > >>>> +		.irq_func = NULL,
> > >>>> +		.flags = 0,
> > >>>> +	};
> > >>>> +
> > >>>> +	/*
> > >>>> +	 * Specify the device number as SNDRV_CARDS + 1 so that the
> > >>>> +	 * device id alloted to this temporary device will never clash
> > >>>> +	 * with an actual device already registered.
> > >>>> +	 */
> > >>>> +	pardev = parport_register_dev_model(p, DRIVER_NAME, &pdev_cb,
> > >>>> +					    SNDRV_CARDS + 1);
> > >>>
> > >>> Hmm, doesn't this result in a device name like "xxx.33" ?
> > >>
> > >> yes, it will. But this is a temoporary device just to check if the
> > >> sound card is connected to that particular parallel port or not. After
> > >> checking this device is immediately unregistered. My idea here was to
> > >> have a device number which will never clash with another device number.
> > >> And we can never have a device like "xxx.33", so no conflict. :)
> > >
> > > Ah, this is the temporary one.  If so, does it make sense to convert
> > > this to dev_model one?  This means that the device will be notified to
> > > udev even though this is a temporary one to be removed immediately.
> > 
> > But since we are registering a device it should ideally follow the 
> > dev_model.
> 
> We shouldn't advertise the device that shouldn't be handled by the
> user-space.  The device you're trying to register there is the one
> that lives only shortly just for probing the address.
> 
> 
> > > It's what we'd want to avoid.  The function serves just as probing the
> > > availability of the given port, not really registering anything
> > > there.
> > 
> > To my understanding, it is probing for the availability of the port and 
> > it is also calling portman_probe() which is initializing hardware 
> > handshake lines to midi box and checking if the portman card is 
> > connected to that parallel port or not.
> > 
> > >
> > > That is, we need to change the registration flow itself if we really
> > > want to move dev_model for the whole.
> > 
> > Any hint, how to register then?
> > Without probing (reading and writing to that port) I can not know if 
> > that port is having the card and to use the port I need to register a 
> > device with that port.
> 
> Just returning the error at probe of the parport device itself instead
> of doing the probe twice?  The current way is racy in anyway.

... and the problem with that is, there is no way to check whether
your upcoming change works correctly without the hardware.  It would
be no longer a "cleanup", and it's risky to do that blindly.

I appreciate your work, but it doesn't look worthy enough.  If we're
trying to eliminate the all old-style parport code from the kernel
code, OK, it's an ambitious project and we may consider taking a risk
of breakage.  Is that the case?


Takashi