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Date:   Tue, 31 Jul 2018 17:53:26 +0200
From:   Stefan Agner <stefan@agner.ch>
To:     Robin Murphy <robin.murphy@arm.com>
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>
Subject: Re: [BUG BISECT] Ethernet fail on VF50 (OF: Don't set default
 coherent DMA mask)
In-Reply-To: <0e893142-a5db-d119-6eb3-f849db6b5d04@arm.com>
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On 31.07.2018 14:32, Robin Murphy wrote:
> 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.

In the FEC driver case, there is an integrated DMA (uDMA). It has
alignment restrictions, but can otherwise address the full 32-bit range.

So something like this should do it right?

	if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))) {
		dev_warn(dev, "No suitable DMA available\n");
		return -ENODEV;
	}

However, that, as far as I understand, still requires that the bus set
up dma_mask properly.

Should I be using dma_coerce_mask_and_coherent?

--
Stefan

> 
> Robin.