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

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Date:   Fri, 02 Sep 2016 15:56:53 +0300
From:   Felipe Balbi <balbi@...nel.org>
To:     Robin Murphy <robin.murphy@....com>, Arnd Bergmann <arnd@...db.de>,
        Leo Li <pku.leo@...il.com>
Cc:     Grygorii Strashko <grygorii.strashko@...com>,
        Russell King - ARM Linux <linux@....linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        "linux-usb\@vger.kernel.org" <linux-usb@...r.kernel.org>,
        Sekhar Nori <nsekhar@...com>,
        lkml <linux-kernel@...r.kernel.org>,
        Stuart Yoder <stuart.yoder@....com>,
        Scott Wood <oss@...error.net>,
        David Fisher <david.fisher1@...opsys.com>,
        "Thang Q. Nguyen" <tqnguyen@....com>,
        Alan Stern <stern@...land.harvard.edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-arm-kernel\@lists.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] usb: dwc3: host: inherit dma configuration from parent dev


Hi,

Robin Murphy <robin.murphy@....com> writes:
>>>> 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.
>
> Huh? There's only no DMA description in DT if the device can be assumed
> to be happy with the defaults. Anything else should be using
> "dma-ranges", "dma-coherent", etc. to describe non-default integration

heh, guilty as charged. I never noticed we had dma-ranges or
dma-coherent.

-- 
balbi

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