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

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Date:   Sun, 06 Jun 2021 12:08:25 +0200
From:   "Sven Peter" <sven@...npeter.dev>
To:     "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>
Cc:     linux-usb@...r.kernel.org, "Felipe Balbi" <balbi@...nel.org>,
        linux-kernel@...r.kernel.org, "Arnd Bergmann" <arnd@...db.de>
Subject: Re: [PATCH] usb: dwc3: support 64 bit DMA in platform driver



On Sun, Jun 6, 2021, at 11:52, Greg Kroah-Hartman wrote:
> On Sun, Jun 06, 2021 at 11:36:29AM +0200, Sven Peter 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 using the same logic already present in xhci-plat.c:
> > First, try to set a coherent dma mask for 64-bit, and then attempt
> > again with a 32-bit mask if this fails.
> > 
> > Signed-off-by: Sven Peter <sven@...npeter.dev>
> > ---
> > 
> > I have taken the code directly from the xhci-plat.c driver so
> > I think this change should be fairly low risk.
> > Unfortunately I only have the Apple M1 to test this on but here
> > the driver still works with the iommu enabled which limits the
> > address space to 32 bit. It also enables to use this with the iommu
> > in bypass mode which requires 64 bit addresses.
> > 
> > I believe this has been working fine so far since the dwc3 driver
> > only uses a few very small buffers in host mode which might still
> > fit within the first 4G of address space on many devices. The
> > majority of DMA buffers are allocated inside the xhci driver which
> > will already call dma_set_mask_and_coherent.
> > 
> > Best,
> > 
> > Sven
> > 
> >  drivers/usb/dwc3/core.c | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> > 
> > diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
> > index b6e53d8212cd..ef6bb6aaffd8 100644
> > --- a/drivers/usb/dwc3/core.c
> > +++ b/drivers/usb/dwc3/core.c
> > @@ -1545,6 +1545,21 @@ static int dwc3_probe(struct platform_device *pdev)
> >  
> >  	dwc3_get_properties(dwc);
> >  
> > +	/* Try to set 64-bit DMA first */
> > +	if (WARN_ON(!dwc->sysdev->dma_mask))
> 
> This will cause systems to reboot if they have panic-on-warn set.  Are
> you sure you want that to happen?

Probably not :) I'll remove the WARN_ON.

thanks,

Sven