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SubjectRe: [PATCH v3 1/9] staging: fsl-mc: move bus driver out of staging
On Fri, Dec 09, 2016 at 12:36:26AM +0000, Stuart Yoder wrote:
> > -----Original Message-----
> > From: Greg KH [mailto:gregkh@linuxfoundation.org]
> > Sent: Thursday, December 08, 2016 10:05 AM
> > To: Stuart Yoder <stuart.yoder@nxp.com>
> > Cc: devel@driverdev.osuosl.org; agraf@suse.de; arnd@arndb.de; linux-kernel@vger.kernel.org; Leo Li
> > <leoyang.li@nxp.com>; Catalin Horghidan <catalin.horghidan@nxp.com>; Ioana Ciornei
> > <ioana.ciornei@nxp.com>; Laurentiu Tudor <laurentiu.tudor@nxp.com>
> > Subject: Re: [PATCH v3 1/9] staging: fsl-mc: move bus driver out of staging
> >
> > On Wed, Dec 07, 2016 at 08:19:20PM +0000, Stuart Yoder wrote:
> > > > -----Original Message-----
> > > > From: Greg KH [mailto:gregkh@linuxfoundation.org]
> > > > Sent: Wednesday, December 07, 2016 9:53 AM
> > > > To: Stuart Yoder <stuart.yoder@nxp.com>
> > > > Cc: devel@driverdev.osuosl.org; linux-kernel@vger.kernel.org; agraf@suse.de; arnd@arndb.de; Leo Li
> > > > <leoyang.li@nxp.com>; Ioana Ciornei <ioana.ciornei@nxp.com>; Catalin Horghidan
> > > > <catalin.horghidan@nxp.com>; Laurentiu Tudor <laurentiu.tudor@nxp.com>; Ruxandra Ioana Radulescu
> > > > <ruxandra.radulescu@nxp.com>
> > > > Subject: Re: [PATCH v3 1/9] staging: fsl-mc: move bus driver out of staging
> > > >
> > > > On Thu, Dec 01, 2016 at 04:41:26PM -0600, Stuart Yoder wrote:
> > > > > Move the source files out of staging into their final locations:
> > > > > -include files in drivers/staging/fsl-mc/include go to include/linux/fsl
> > > > > -irq-gic-v3-its-fsl-mc-msi.c goes to drivers/irqchip
> > > > > -source in drivers/staging/fsl-mc/bus goes to drivers/bus/fsl-mc
> > > > > -README.txt, providing and overview of DPAA goes to
> > > > > Documentation/dpaa2/overview.txt
> > > > > -update MAINTAINERS with new location
> > > > >
> > > > > Delete other remaining staging files-- Makefile, Kconfig, TODO
> > > >
> > > > Ok, given that I haven't ever reviewed this code, I had a few questions
> > > > that I couldn't easily figure out by looking at your code:
> > > > - what is the lifecycle of your 'struct device' usage? Who
> > > > creates it, who frees it, and who accesses it?
> > >
> > > We embed a 'struct device' inside our bus specific device struct
> > > 'struct fsl_mc_device'. So, when a new fsl-mc object is discovered
> > > on the bus during initial enumeration or hotplug we create a new
> > > 'struct fsl_mc_device' and do a device_initialize()/device_add().
> > > (see fsl_mc_device_add() for where this is done)
> > >
> > > 'struct device' is freed when a device is removed-- the reverse
> > > of the above.
> >
> > Where is the device freed? I see you trying to do some "odd" stuff in
> > fsl_mc_device_remove() by deleting and then putting a device structure.
> > I can't find a "release()" callback anywhere for your bus, where is it?
> >
> > What happens when the reference count falls to 0 for your struct device?
>
> Hrm...something seems wrong in free path, and I think this needs to
> be refactored.
>
> IIRC, when German (former maintainer) wrote that code he loosely based
> it on the register/unregister platform bus code:
>
> int platform_device_register(struct platform_device *pdev)
> {
> device_initialize(&pdev->dev);
> arch_setup_pdev_archdata(pdev);
> return platform_device_add(pdev);
> }
> void platform_device_unregister(struct platform_device *pdev)
> {
> platform_device_del(pdev);
> platform_device_put(pdev);
> }
>
> ...I'm puzzling over how that code handles a refcount of zero
> as I see no 'release' callback anywhere, but I must be missing
> something.
>
> In any case, we'll get this refactored.

Have you tried removing a device? The kernel should complain loudly
about there not being a release function for your device.

> > > > - root_dprc_count, why are you using an atomic variable for
> > > > this? What is it for other than "look, I'm running!"?
> > >
> > > There can be multiple root buses, and this variable simply tracks the count
> > > of them.
> >
> > Why does it matter?
> >
> > > It's is atomic there might be a theoretical race condition where 2
> > > buses might be added at the same time. The root buses are found in
> > > the device tree and so if there is no chance that device tree
> > > processing happens in parallel on multiple cores then we could remove
> > > the atomic.
> >
> > Why not just use a lock, or better yet, not care about a "count" at all?
> > I don't see you doing anything with the count, other than emitting a
> > WARN() if it drops down below 0 for some reason, or when you call
> > fsl_mc_bus_exists() which for some reason is exported yet no one uses
> > it...
>
> We can drop this count. At one time I think there was envisioned an
> external user who needed it, but it's no longer the case.

Please do, we are trying to get rid of atomic_t abuse on other mailing
lists, and this one fits the pattern of "no real need for it" :)

> Given the additional refactoring, I think the fsl-mc bus driver needs
> to stay in staging for a bit. In order to facilitate further review
> I'm going to refactor the patch series:
> staging: fsl-mc: move bus driver out of staging, add dpio
>
> ...to just add dpio (into staging). This will allow the Eth driver
> series sent earlier this week to go into staging:
> staging: Introduce Freescale DPAA2 Ethernet driver
>
> With all that in staging we'll have a fully functional Ethernet
> driver.

Ok, that sounds reasonable.

thanks,

greg k-h

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