Re: [PATCH] usb: dwc3: host: inherit dma configuration from parent dev

[Felipe Balbi]

Hi,

Arnd Bergmann <arnd@xxxxxxxx> writes:
> On Wednesday 27 April 2016 19:53:51 Felipe Balbi wrote:
>> Arnd Bergmann <arnd@xxxxxxxx> writes:
>> > On Wednesday 27 April 2016 16:50:19 Catalin Marinas wrote:
>> >> On Wed, Apr 27, 2016 at 04:11:17PM +0200, Arnd Bergmann wrote:
>> >> > On Wednesday 27 April 2016 14:59:00 Catalin Marinas wrote:
>> >> > > 
>> >> > > I would be in favour of a dma_inherit() function as well. We could hack
>> >> > > something up in the arch code (like below) but I would rather prefer an
>> >> > > explicit dma_inherit() call by drivers creating such devices.
>> >> > > 
>> >> > > diff --git a/arch/arm64/include/asm/dma-mapping.h b/arch/arm64/include/asm/dma-mapping.h
>> >> > > index ba437f090a74..ea6fb9b0e8fa 100644
>> >> > > --- a/arch/arm64/include/asm/dma-mapping.h
>> >> > > +++ b/arch/arm64/include/asm/dma-mapping.h
>> >> > > @@ -29,8 +29,11 @@ extern struct dma_map_ops dummy_dma_ops;
>> >> > >  
>> >> > >  static inline struct dma_map_ops *__generic_dma_ops(struct device *dev)
>> >> > >  {
>> >> > > -       if (dev && dev->archdata.dma_ops)
>> >> > > -               return dev->archdata.dma_ops;
>> >> > > +       while (dev) {
>> >> > > +               if (dev->archdata.dma_ops)
>> >> > > +                       return dev->archdata.dma_ops;
>> >> > > +               dev = dev->parent;
>> >> > > +       }
>> >> > 
>> >> > I think this would be a very bad idea: we don't want to have random
>> >> > devices be able to perform DMA just because their parent devices
>> >> > have been set up that way.
>> >> 
>> >> I agree, it's a big hack. It would be nice to have a simpler way to do
>> >> this in driver code rather than explicitly calling
>> >> of_dma_configure/arch_setup_dma_ops as per the original patch in this
>> >> thread.
>> >
>> > I haven't followed the entire discussion, but what's wrong with passing
>> > around a pointer to a 'struct device *hwdev' that represents the physical
>> > device that does the DMA?
>> 
>> that will likely create duplicated solutions in several drivers and
>> it'll be a pain to maintain. There's another complication, dwc3 can be
>> integrated in many different ways. See the device child-parent tree
>> representations below:
>> 
>> a) with a parent PCI device:
>> 
>> pci_bus_type
>>  - dwc3-pci
>>    - dwc3
>>      - xhci-plat
>> 
>> b) with a parent platform_device (OF):
>> 
>> platform_bus_type
>>  - dwc3-${omap,st,of-simple,exynos,keystone}
>>    - dwc3
>>      - xhci-plat
>> 
>> c) without a parent at all (thanks Grygorii):
>> 
>> platform_bus_type
>>  - dwc3
>>    - xhci-plat
>> 
>> (a) and (b) above are the common cases. The DMA-capable device is
>> clearly dwc3-${pci,omap,st,of-simple,exynos,keystone} with dwc3 only
>> having proper DMA configuration in OF platforms (because of the
>> unconditional of_dma_configure() during OF device creation) and
>> xhci-plat not knowing about DMA at all and hardcoding some crappy
>> defaults.
>> 
>> (c) is the uncommon case which creates some problems. In this case, dwc3
>> itself is the DMA-capable device and dwc3->dev->parent is the
>> platform_bus_type itself. Now consider the problem this creates:
>> 
>> i. the patch that I wrote [1] becomes invalid for (c), thanks to
>> Grygorii for pointing this out before it was too late.
>> 
>> ii. xhci-plat can also be described directly in DT (and is in some
>> cases). This means that assuming xhci-plat's parent's parent to be the
>> DMA-capable device is also an invalid assumption.
>> 
>> iii. one might argue that for DT-based platforms *with* a glue layer
>> ((b) above), OF already "copies" some sensible DMA defaults during
>> device creation.
>
> But that is exactly the point I was trying to make: instead of copying
> all the data into the xhci-plat device, just assign one pointer
> inside the xhci-plat device from whoever creates it.

and how do you pass that pointer to xhci-plat from another driver ?
No, platform_data is not an option ;-)

>> The only clean way to fix this up is with a dma_inherit()-like API which
>> would allow dwc3's glue layers ((a) and (b) above) to initialize child's
>> (dwc3) DMA configuration during child's creation. Something like below:
>> 
>> diff --git a/drivers/usb/dwc3/dwc3-pci.c b/drivers/usb/dwc3/dwc3-pci.c
>> index adc1e8a624cb..74b599269e2c 100644
>> --- a/drivers/usb/dwc3/dwc3-pci.c
>> +++ b/drivers/usb/dwc3/dwc3-pci.c
>> @@ -152,6 +152,8 @@ static int dwc3_pci_probe(struct pci_dev *pci,
>>  		return -ENOMEM;
>>  	}
>>  
>> +	dma_inherit(&dwc->dev, dev);
>> +
>>  	memset(res, 0x00, sizeof(struct resource) * ARRAY_SIZE(res));
>>  
>>  	res[0].start	= pci_resource_start(pci, 0);
>> 
>> that's all I'm asking for :-) dma_inherit() should, probably, be
>> arch-specific to handle details like IOMMU (which today sits under
>> archdata).
>
> It's still wrong: the archdata in the device can contain all sorts of
> additional information about how to do DMA, and some of that information

yes, inherit all of that ;-)

> is bus specific, e.g. when your dma_map_ops look like the on sparc:

okay, let me be clear. The underlying bus is still pci_bus_type or
platform_bus_type; that won't change.

> static inline struct dma_map_ops *get_dma_ops(struct device *dev)
> {
> #ifdef CONFIG_SPARC_LEON
>         if (sparc_cpu_model == sparc_leon)
>                 return leon_dma_ops;
> #endif
> #if defined(CONFIG_SPARC32) && defined(CONFIG_PCI)
>         if (dev->bus == &pci_bus_type)
>                 return &pci32_dma_ops;
> #endif
>         return dma_ops;
> }

this seems to be a rather fragile assumption, no ? Only works because
*_bus_type declarations are exported. I wonder if this is the only
reason *why* they are exported, probably not...

> There is no way for a device to "inherit" the bus_type of its parent,

I don't want to inherit bus_type, I just want DMA configuration to not
depend on bus_type directly ;-)

-- 
balbi

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