Re: [PATCH v2] dma-mapping: skip USB devices when configuring DMA during probe

[Robin Murphy <robin.murphy@arm.com>]

On 03/08/17 15:23, Johan Hovold wrote:
> On Thu, Aug 03, 2017 at 10:04:21AM -0400, Alan Stern wrote:
>> On Thu, 3 Aug 2017, Johan Hovold wrote:
>>
>>> USB devices use the DMA mask and offset of the controller, which have
>>> already been setup when a device is probed. Note that modifying the
>>> DMA mask of a USB device would change the mask for the controller (and
>>> all devices on the bus) as the mask is literally shared.
>>>
>>> Since commit 2bf698671205 ("USB: of: fix root-hub device-tree node
>>> handling"), of_dma_configure() would be called also for root hubs, which
>>> use the device node of the controller. A separate, long-standing bug
>>> that makes of_dma_configure() generate a 30-bit DMA mask from the RPI3's
>>> "dma-ranges" would thus set a broken mask also for the controller. This
>>> in turn prevents USB devices from enumerating when control transfers
>>> fail:
>>>
>>> 	dwc2 3f980000.usb: Cannot do DMA to address 0x000000003a166a00
>>>
>>> Note that the aforementioned DMA-mask bug was benign for the HCD itself
>>> as the dwc2 driver overwrites the mask previously set by
>>> of_dma_configure() for the platform device in its probe callback. The
>>> mask would only later get corrupted when the root-hub child device was
>>> probed.
>>>
>>> Fix this, and similar future problems, by simply skipping USB devices
>>> when dma_configure() is called during probe.
>>>
>>> Fixes: 09515ef5ddad ("of/acpi: Configure dma operations at probe time for platform/amba/pci bus devices")
>>> Cc: stable <stable@vger.kernel.org>	# 4.12
>>> Cc: Robin Murphy <robin.murphy@arm.com>
>>> Cc: Sricharan R <sricharan@codeaurora.org>
>>> Cc: Stefan Wahren <stefan.wahren@i2se.com>
>>> Reported-by: Hans Verkuil <hverkuil@xs4all.nl>
>>> Signed-off-by: Johan Hovold <johan@kernel.org>
>>> ---
> 
>>> diff --git a/include/linux/usb.h b/include/linux/usb.h
>>> index cb9fbd54386e..f86ad9d8c756 100644
>>> --- a/include/linux/usb.h
>>> +++ b/include/linux/usb.h
>>> @@ -1222,6 +1222,11 @@ struct usb_device_driver {
>>>  
>>>  extern struct bus_type usb_bus_type;
>>>  
>>> +static inline bool dev_is_usb(struct device *dev)
>>> +{
>>> +	return dev->bus == &usb_bus_type;
>>> +}
>>> +
>>
>> Will this work if the USB subsystem is built as a module?
> 
> Nope. :-/

Oh bum, I hadn't even realised usb_bus_type could be modular.

> Add another flag (e.g. skip_dma_configure) to struct device for now?

Would it be sufficient to look for dev->of_node_reused, or is it also
possible for USB devices to have OF nodes of their own such that
of_dma_configure() would still blat the mask with a 32-bit default?
(Although that would still un-break Rpi3, even if strictly wrong)

Robin.