linux-kernel - Re: [PATCH 1/5] iTCO_wdt: Expose watchdog properties using platform data

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Date:	Wed, 29 Jul 2015 11:20:05 -0500 (CDT)
From:	Aaron Sierra <asierra@...-inc.com>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	Matt Fleming <matt@...eblueprint.co.uk>,
	Wim Van Sebroeck <wim@...ana.be>,
	linux-kernel@...r.kernel.org, linux-watchdog@...r.kernel.org,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Jean Delvare <jdelvare@...e.com>,
	Wolfram Sang <wsa@...-dreams.de>,
	Matt Fleming <matt.fleming@...el.com>,
	Peter Tyser <ptyser@...-inc.com>,
	Samuel Ortiz <sameo@...ux.intel.com>
Subject: Re: [PATCH 1/5] iTCO_wdt: Expose watchdog properties using platform
 data

> From: "Lee Jones" <lee.jones@...aro.org>
> Sent: Wednesday, July 29, 2015 10:32:26 AM
> 
> On Wed, 29 Jul 2015, Aaron Sierra wrote:
> 
> > > From: "Lee Jones" <lee.jones@...aro.org>
> > > Sent: Wednesday, July 29, 2015 2:38:41 AM
> > > 
> > > On Tue, 28 Jul 2015, Aaron Sierra wrote:
> > > 
> > > > > > > > @@ -933,7 +956,7 @@ gpe0_done:
> > > > > > > >  	lpc_chipset_info[priv->chipset].use_gpio = ret;
> > > > > > > >  	lpc_ich_enable_gpio_space(dev);
> > > > > > > >  
> > > > > > > > -	lpc_ich_finalize_cell(dev, &lpc_ich_cells[LPC_GPIO]);
> > > > > > > > +	lpc_ich_finalize_gpio_cell(dev);
> > > > > > > >  	ret = mfd_add_devices(&dev->dev, PLATFORM_DEVID_AUTO,
> > > > > > > >  			      &lpc_ich_cells[LPC_GPIO], 1, NULL, 0, NULL);
> > > > > > > >  
> > > > > > > > @@ -1007,7 +1030,10 @@ static int lpc_ich_init_wdt(struct
> > > > > > > > pci_dev
> > > > > > > > *dev)
> > > > > > > >  		res->end = base_addr + ACPIBASE_PMC_END;
> > > > > > > >  	}
> > > > > > > >  
> > > > > > > > -	lpc_ich_finalize_cell(dev, &lpc_ich_cells[LPC_WDT]);
> > > > > > > > +	ret = lpc_ich_finalize_wdt_cell(dev);
> > > > > > > > +	if (ret)
> > > > > > > > +		goto wdt_done;
> > > > > > > > +
> > > > > > > >  	ret = mfd_add_devices(&dev->dev, PLATFORM_DEVID_AUTO,
> > > > > > > >  			      &lpc_ich_cells[LPC_WDT], 1, NULL, 0, NULL);
> > > > > > > 
> > > > > > > Why do you have an mfd_add_devices() call for each device?
> > > > > >  
> > > > > > Good question. This call has been present since March 2012 when
> > > > > > support
> > > > > > was first added for iTCO_wdt in commit 887c8ec7219f ("watchdog:
> > > > > > Convert
> > > > > > iTCO_wdt driver to mfd model").
> > > > > > 
> > > > > > There's no good reason that I can see. Aaron?
> > > > 
> > > > I chose to call mfd_add_devices() in each device init function
> > > > because I thought it was the easiest way to avoid registering an
> > > > incomplete/invalid MFD cell should an error occur during init.
> > > > 
> > > > That way device registration wouldn't be an all-or-nothing affair.
> > > > 
> > > > Doesn't mfd_add_devices() bail out after the first unsuccessful
> > > > mfd to platform device translation?
> > > 
> > > Right, as it should.
> > > 
> > > Under what circumstance would an error occur and you'd wish to carry
> > > on registering devices?
> > 
> > Lee,
> > 
> > The two devices that this driver is responsible for are conceptually
> > independent; they simply are lumped together in one PCI device. No
> > failure while preparing resources for the watchdog device should
> > prevent the GPIO device from being registered.
> 
> This makes me think that perhaps this isn't an MFD at all then?
> 
> Perhaps I should invest some time to looking into that.
> 
> > The most common real world circumstance that I experience is when a
> > BIOS reserves resources associated with the GPIO device, thus
> > preventing the GPIO resources (ICH_RES_GPE0 and/or ICH_RES_GPIO) from
> > being fully prepared.
> > 
> > I have not experienced issues with the watchdog device, but a similar
> > issue would exist if the RCBA were disabled in a "v2" device.
> > 
> > It seems like a dangerous change to simply attempt to register both
> > of these devices with a single call, when one or both of them could
> > be incomplete.
> > 
> > Perhaps your real issue with this driver structure is that these
> > cells are elements of a single lpc_ich_cells array for no clear
> > reason. If each had a dedicated mfd_cell variable, would that be
> > more acceptable to you?
> > 
> > -static struct mfd_cell lpc_ich_cells[] = {
> > +static struct mfd_cell lpc_ich_wdt_cell = {
> > ...
> > +static struct mfd_cell lpc_ich_gpio_cell = {
> > 
> > That would eliminate the need for the lpc_cells enum, too.
> 
> Yes, that would make more sense.  Also consider using mfd_add_device()
> instead of mfd_add_devices(), as you are only attempting registration
> for a single device.
> 

I can submit a patch the splits up the array elements, but I
only see mfd_add_device() as a static function in mfd-core.c;
Is that being exported in a development branch somewhere?

-Aaron S.
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