From: Arnd Bergmann <arnd@arndb.de>
To: linux-arm-kernel@lists.infradead.org
Cc: Duc Dang <dhdang@apm.com>,
        Russell King - ARM Linux <linux@arm.linux.org.uk>,
        Feng Kan <fkan@apm.com>, Mark Langsdorf <mlangsdo@redhat.com>,
        mathias.nyman@intel.com, Greg KH <gregkh@linuxfoundation.org>,
        USB list <linux-usb@vger.kernel.org>, patches <patches@apm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Loc Ho <lho@apm.com>
Subject: Re: [PATCH v5 1/2] usb: make xhci platform driver use 64 bit or 32 bit DMA
Date: Sat, 15 Aug 2015 22:05:09 +0200
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On Saturday 08 August 2015 13:31:02 Duc Dang wrote:
> >
> > If we know that pdev->dev.dma_mask will always be initialised at this
> > point, then the above change is fine.  If not, it's introducing a
> > regression - dma_set_mask_and_coherent() will fail if pdev->dev.dma_mask
> > is NULL (depending on the architectures implementation of dma_set_mask()).
> >
> > Prefixing the above change with the two lines I mention above would
> > ensure equivalent behaviour.  Even if we do want to get rid of this,
> > I'd advise to do it as a separate patch after this change, which can
> > be independently reverted if there's problems with its removal.
> >
> Hi Russell,
> 
> I will add the 2 lines you mentioned back to next version of the
> patch. It is safer to do it that way as I do not see
> pdev->dev.dma_mask gets initialized before the call
> dma_set_mask_and_coherent  inside this xhci_plat.c file.

It would be good to add a WARN_ON() to the case where dma_mask
is a NULL pointer at the least. That way, we will at least
find out if there are some broken platforms that do not correctly
initialize the mask pointer.

	Arnd
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