Message-ID: <s5htwln0y7n.wl-tiwai@suse.de>
Date:	Fri, 05 Feb 2016 20:11:08 +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 18:21:46 +0100,
Sudip Mukherjee wrote:
> 
> On Friday 05 February 2016 10:36 PM, Takashi Iwai wrote:
> > 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.
> 
> Yes. That is why I try to change the driver with the minimum possible 
> change.

But it's no 100% compatible transition.  That's the first problem.

> > 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?
> 
> Yes, the old api is supposed to be removed and we should only have the 
> device model api. I was expecting to remove the old API by 4.7.
> Is there any way to get the hardware?

No, unfortunately.  It's an old hardware, after all.  It's difficult
to find even a decent machine with a parallel port...


Takashi

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