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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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@svenpeter.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?

I know you copied this from xhci-plat, but let's not duplicate bugs
please :)

thnaks,

greg k-h