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

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Message-ID: <CAK8P3a0j=vowKpdJxt-GBsFuwqNJZv-dB-XoZihg=XHey1VoCg@mail.gmail.com>
Date:   Mon, 7 Jun 2021 10:22:55 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Sven Peter <sven@...npeter.dev>
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 10:01 AM Sven Peter <sven@...npeter.dev> wrote:
> 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?

If the driver never calls dma_map_*() on the device, that is correct, otherwise
it has to be careful about what pointers it passes in there to avoid
failing later.
Since it is already working without the dma_set_mask(), I don't expect a
problem there.

I suppose in theory, the dwc3_alloc_scratch_buffers() should use GFP_DMA32
if dma_set_mask_and_coherent() failed. On arm32, it won't matter since
all kernel pointers are generall within ZONE_DMA32, and on arm64 we always
build with SWIOTLB enabled. Not sure where else you'd typically find dwc3,
or if any of them are broken without changing this.

        Arnd