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

[Felipe Balbi]

Hi,

Arnd Bergmann <arnd@xxxxxxxx> writes:
> On Thursday, September 1, 2016 5:14:28 PM CEST Leo Li wrote:
>> 
>> Hi Felipe and Arnd,
>> 
>> It has been a while since the last response to this discussion, but we
>> haven't reached an agreement yet!  Can we get to a conclusion on if it
>> is valid to create child platform device for abstraction purpose?  If
>> yes, can this child device do DMA by itself?
>
> I'd say it's no problem for a driver to create child devices in order
> to represent different aspects of a device, but you should not rely on
> those devices working when used with the dma-mapping interfaces.

heh, that looks like an excuse to me :-)

This will always be a problem for e.g. MFD, for example. Are you saying
MFD child-devices shouldn't be allowed to do DMA? It becomes silly when
you read it that way, right?

> This used to be simpler back when we could configure the kernel for
> only one SoC platform at a time, and the platforms could provide their
> own overrides for the dma-mapping interfaces. These days, we rely on

right, so we have a very old regression that just took a complex driver
such as dwc3 to trigger ;-)

> firmware or bootloader to describe various aspects of how DMA is done,

there's no DMA description in DT. Every OF device gets the same 32-bit
DMA mask and that is, itself, wrong for several devices.

> so you can't assume that passing a device without an of_node pointer
> or ACPI data into those functions will do the right thing.

That's not the problem, however. We can very easily pass along
ACPI_COMPANION() to any platform_device we want, but that's not enough
because DMA-related bits are passed along with archdata; but archdata
isn't generic in any way. Some arches (like x86) _do_ use it for DMA,
but some don't.

-- 
balbi

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