Re: [PATCH 1/5] spi: Fix controller unregister order

[Lukas Wunner <lukas@xxxxxxxxx>]

On Sat, May 16, 2020 at 12:37:17AM +0300, Andy Shevchenko wrote:
> On Fri, May 15, 2020 at 7:41 PM Lukas Wunner <lukas@xxxxxxxxx> wrote:
> > On Fri, May 15, 2020 at 05:27:25PM +0100, Mark Brown wrote:
> > > On Fri, May 15, 2020 at 05:58:01PM +0200, Lukas Wunner wrote:
> > > > However since commit ffbbdd21329f ("spi: create a message queueing
> > > > infrastructure"), spi_destroy_queue() is executed before unbinding the
> > > > slaves.  It sets ctlr->running = false, thereby preventing SPI bus
> > > > access and causing unbinding of slave devices to fail.
> > >
> > > Devices should basically never fail an unbind operation - if something
> > > went seriously wrong there's basically nothing that can be done in terms
> > > of error handling and keeping the device around isn't going to help.
> >
> > I guess the word "fail" in the commit message invites misinterpretations.
> > The driver does unbind from the slave device, but the physical device is
> > not left in a proper state.  E.g. interrupts may still be generated by
> > the device because writing a register to disable them failed.
> 
> I didn't check a patch, but I see a bug on kernel bugzilla against
> spi-pxa2xx because of this. It requires quite untrivial ->remove() in
> order to quiescent the DMA and other activities.

Yes from a quick look at spi-pxa2xx.c it's immediately obvious that
the use of devm_spi_register_controller() is likewise completely wrong.

The crucial thing to understand is that the SPI driver's ->remove()
hook is executed *before* any device-managed resources are released.
pxa2xx_spi_remove() disables the clock, frees the IRQ, releases DMA,
so the SPI controller is no longer usable even though it's still
registered!  Somehow this incorrect order got cargo-culted to dozens
of drivers over the years.

We use SPI-attached Ethernet chips and when the SPI driver's module
is unloaded, the Ethernet driver's ->ndo_stop() hook is executed to
bring down the interface.  For this it needs to communicate with the
Ethernet chip, but it can't because the ->remove() hook has already been
executed and unbinding of the SPI slave happens afterwards, when the
SPI controller is unregistered via devres_release_all().

There's another issue in spi-pxa2xx.c:  It acquires a runtime PM ref
even though the driver core already does that.

Do you have a link to the spi-pxa2xx.c bugzilla?  Are you able to
test patches?  I can submit a patch but I can only compile-test it,
I don't have that hardware.

Thanks,

Lukas