Re: [BUG BISECT] Ethernet fail on VF50 (OF: Don't set default coherent DMA mask)

[Robin Murphy]

On 31/07/18 09:19, Stefan Agner wrote:
> On 30.07.2018 16:38, Robin Murphy wrote:
> > On 28/07/18 17:58, Guenter Roeck wrote:
> > > On Fri, Jul 27, 2018 at 04:04:48PM +0200, Christoph Hellwig wrote:
> > > > On Fri, Jul 27, 2018 at 03:18:14PM +0200, Krzysztof Kozlowski wrote:
> > > > > On 27 July 2018 at 15:11, Krzysztof Kozlowski <krzk@xxxxxxxxxx> wrote:
> > > > > > Hi,
> > > > > >
> > > > > > On today's next, the bisect pointed commit
> > > > > > ff33d1030a6ca87cea9a41e1a2ea7750a781ab3d as fault for my boot failures
> > > > > > with NFSv4 root on Toradex Colibri VF50 (Iris carrier board).
> > > > > >
> > > > > > Author: Robin Murphy <robin.murphy@xxxxxxx>
> > > > > > Date:   Mon Jul 23 23:16:12 2018 +0100
> > > > > >       OF: Don't set default coherent DMA mask
> > > > > >
> > > > > > Board: Toradex Colibri VF50 (NXP VF500, Cortex A5, serial configured
> > > > > > with DMA) on Iris Carrier.
> > > > > >
> > > > > > It looks like problem with Freescale Ethernet driver:
> > > > > > [   15.458477] fsl-edma 40018000.dma-controller: coherent DMA mask is unset
> > > > > > [   15.465284] fsl-lpuart 40027000.serial: Cannot prepare cyclic DMA
> > > > > > [   15.472086] Root-NFS: no NFS server address
> > > > > > [   15.476359] VFS: Unable to mount root fs via NFS, trying floppy.
> > > > > > [   15.484228] VFS: Cannot open root device "nfs" or
> > > > > > unknown-block(2,0): error -6
> > > > > > [   15.491664] Please append a correct "root=" boot option; here are
> > > > > > the available partitions:
> > > > > > [   15.500188] 0100           16384 ram0
> > > > > > [   15.500200]  (driver?)
> > > > > > [   15.506406] Kernel panic - not syncing: VFS: Unable to mount root
> > > > > > fs on unknown-block(2,0)
> > > > > > [   15.514747] ---[ end Kernel panic - not syncing: VFS: Unable to
> > > > > > mount root fs on unknown-block(2,0) ]---
> > > > > >
> > > > > > Attached - defconfig and full boot log.
> > > > > >
> > > > > > Any hints?
> > > > > > Let me know if you need any more information.
> > > > >
> > > > > My Exynos boards also fail to boot on missing network:
> > > > > https://krzk.eu/#/builders/21/builds/799/steps/10/logs/serial0
> > > > >
> > > > > As expected there are plenty of "DMA mask not set" warnings... and
> > > > > later dwc3 driver fails with:
> > > > >       dwc3: probe of 12400000.dwc3 failed with error -12
> > > > > which is probably the answer why LAN attached to USB is not present.
> > > >
> > > > Looks like all the drivers failed to set a dma mask and were lucky.
> > >
> > > I would call it a serious regression. Also, no longer setting a default
> > > coherent DMA mask is a quite substantial behavioral change, especially
> > > if and since the code worked just fine up to now.
> >
> > To reiterate, that particular side-effect was an unintentional
> > oversight, and I was simply (un)lucky enough that none of the drivers
> > I did test depended on that default mask. Sorry for the blip; please
> > check whether it's now fixed in next-20180730 as it should be.
>
> Just for my understanding:
>
> Your first patch ("OF: Don't set default coherent DMA mask") sounded
> like that *not* setting default coherent DMA mask was intentionally.
> Since the commit message reads: "...the bus code has not initialised any
> default value" that was assuming that all bus code sets a default DMA
> mask which wasn't the case for "simple-bus".

Yes, reading the patches in the order they were written is perhaps a 
little unclear, but hopefully the order in which they are now applied 
makes more sense.

> So I guess that is what ("of/platform: Initialise default DMA masks")
> makes up for in the typical device tree case ("simple-bus")?

Indeed, I'd missed the fact that the now-out-of-place-looking 
initialisation in of_dma_configure() still actually belonged to 
of_platform_device_create_pdata() - that patch should make the 
assumptions of "OF: Don't set default coherent DMA mask" true again, 
even for OF-platform devices.

> Now, since almost all drivers are inside a soc "simple-bus" and DMA mask
> is set again, can/should we rely on the coherent DMA mask set?
>
> Or is the expectation still that this is set on driver level too?

Ideally, we'd like all drivers to explicitly request their masks as the 
documentation in DMA-API-HOWTO.txt recommends, if only to ensure DMA is 
actually possible - there can be systems where even the default 32-bit 
mask is no good - but clearly we're a little way off trying to enforce 
that just yet.

Robin.