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From:   Robin Murphy <robin.murphy@arm.com>
Subject: Re: [BUG BISECT] Ethernet fail on VF50 (OF: Don't set default
 coherent DMA mask)
To:     Stefan Agner <stefan@agner.ch>
Cc:     Guenter Roeck <linux@roeck-us.net>, Christoph Hellwig <hch@lst.de>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Rob Herring <robh+dt@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        Fugang Duan <fugang.duan@nxp.com>
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Date:   Tue, 31 Jul 2018 13:32:27 +0100
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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@kernel.org> 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@arm.com>
>>>>>> 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.