linux-kernel - Re: [PATCH v3] usb: dwc3: support 64 bit DMA in platform driver

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Message-Id: <23348bfc-aad7-4e5f-83b1-e69463e618e5@www.fastmail.com>
Date:   Mon, 07 Jun 2021 10:01:50 +0200
From:   "Sven Peter" <sven@...npeter.dev>
To:     "Arnd Bergmann" <arnd@...db.de>
Cc:     "USB list" <linux-usb@...r.kernel.org>,
        "Felipe Balbi" <balbi@...nel.org>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] usb: dwc3: support 64 bit DMA in platform driver



On Mon, Jun 7, 2021, at 09:25, Arnd Bergmann wrote:
> On Mon, Jun 7, 2021 at 8:18 AM Sven Peter <sven@...npeter.dev> wrote:
> >
> > Currently, the dwc3 platform driver does not explicitly ask for
> > a DMA mask. This makes it fall back to the default 32-bit mask which
> > breaks the driver on systems that only have RAM starting above the
> > first 4G like the Apple M1 SoC.
> >
> > Fix this by calling dma_set_mask_and_coherent with a 64bit mask.
> >
> > Signed-off-by: Sven Peter <sven@...npeter.dev>
> > ---
> >
> > Third time's a charm I hope - this time much simpler :)
> 
> I think this is almost good, but there is still one small issue:
> 
> > diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
> > index b6e53d8212cd..ba4792b6a98f 100644
> > --- a/drivers/usb/dwc3/core.c
> > +++ b/drivers/usb/dwc3/core.c
> > @@ -1545,6 +1545,10 @@ static int dwc3_probe(struct platform_device *pdev)
> >
> >         dwc3_get_properties(dwc);
> >
> > +       ret = dma_set_mask_and_coherent(dwc->sysdev, DMA_BIT_MASK(64));
> > +       if (ret)
> > +               return ret;
> 
> This will now  fail on machines with dwc3 connected to a 32-bit bus (or a
> bus that is accidentally not annotated as supporting 64-bit) when there is
> some memory that is not addressable through that bus.
> 
> If dma_set_mask_and_coherent() fails, the platform should just fall back to
> 32-bit addressing as it did before your change. dma_alloc_*() will do that
> implicitly by allocating from ZONE_DMA32, while dma_map_*() fails
> on any non-addressable memory, or falls back to swiotlb if that is available.


Makes sense, but just to make sure I understand this correctly:
All that needs to be done is call dma_set_mask_and_coherent with a 64 bit
mask and then just ignore the return value?


Thanks,

Sven