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

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Date:   Wed, 7 Sep 2016 17:29:01 +0800
From:   Peter Chen <hzpeterchen@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Felipe Balbi <balbi@...nel.org>, Leo Li <pku.leo@...il.com>,
        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@...r.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@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] usb: dwc3: host: inherit dma configuration from parent
 dev

On Wed, Sep 07, 2016 at 10:52:46AM +0200, Arnd Bergmann wrote:
> On Wednesday, September 7, 2016 3:44:28 PM CEST Peter Chen wrote:
> > 
> > The pre-condition of DT function at USB HCD core works is the host
> > controller device has of_node, since it is the root node for USB tree
> > described at DT. If the host controller device is not at DT, it needs
> > to try to get its of_node, the chipidea driver gets it through its
> > parent node [1]
> 
> > 
> > [1] https://lkml.org/lkml/2016/8/8/119
> > 
> 
> Ah, this is what I was referring to in the other mail.
> 
> However, the way you set the of_node might be dangerous too:
> We should generally not have two platform_device structures with
> the same of_node pointer, most importantly it may cause the
> child device to be bound to the same driver as the parent
> device since the probing is done by compatible string.
> 
> As you tested it successfully, it must work at the moment on your
> machine, but it could easily break depending on deferred probing
> or module load order.
> 

Currently, I work around above problems by setting core device of_node
as NULL at both probe error path and platform driver .remove routine.

I admit it is not a good way, but if we only have of_node at device's
life periods after probe, it seems ok currently. It is hard to create
of_node dynamically when create device, and keep some contents
of parent's of_node, and some are not.

-- 

Best Regards,
Peter Chen