linux-kernel - Re: [PATCH v5 1/2] usb: make xhci platform driver use 64 bit or 32 bit DMA

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Message-ID: <CADaLNDmYt+k6Sw6q+t2-FA7muk=x6h_hfYV321B03qBPMD_=AA@mail.gmail.com>
Date:	Wed, 19 Aug 2015 14:28:33 -0700
From:	Duc Dang <dhdang@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm <linux-arm-kernel@...ts.infradead.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Feng Kan <fkan@....com>, Mark Langsdorf <mlangsdo@...hat.com>,
	mathias.nyman@...el.com, Greg KH <gregkh@...uxfoundation.org>,
	USB list <linux-usb@...r.kernel.org>,
	patches <patches@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Loc Ho <lho@....com>
Subject: Re: [PATCH v5 1/2] usb: make xhci platform driver use 64 bit or 32
 bit DMA

On Sat, Aug 15, 2015 at 1:05 PM, Arnd Bergmann <arnd@...db.de> wrote:
>
> 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.

Hi Arnd,

So the check will look like this, please let me know what do you think:
        if (!pdev->dev.dma_mask) {
                WARN_ON(1);
                /* Initialize dma_mask if the broken platform code has
not done so */
                pdev->dev.dma_mask = &pdev->dev.coherent_dma_mask;
        }

>
>         Arnd




-- 
Regards,
Duc Dang.
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