linux-kernel - [PATCH v2] usb: dwc3: support 64 bit DMA in platform driver

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Date:   Sun,  6 Jun 2021 12:36:56 +0200
From:   Sven Peter <sven@...npeter.dev>
To:     linux-usb@...r.kernel.org
Cc:     Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Sven Peter <sven@...npeter.dev>
Subject: [PATCH v2] usb: dwc3: support 64 bit DMA in platform driver

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

changes from v1:
 - removed WARN_ON around !dwc->sysdev->dma_mask; pointed out by greg k-h

 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..4930541a8984 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 (!dwc->sysdev->dma_mask)
+		/* Platform did not initialize dma_mask */
+		ret = dma_coerce_mask_and_coherent(dwc->sysdev,
+						   DMA_BIT_MASK(64));
+	else
+		ret = dma_set_mask_and_coherent(dwc->sysdev, DMA_BIT_MASK(64));
+
+	/* If seting 64-bit DMA mask fails, fall back to 32-bit DMA mask */
+	if (ret) {
+		ret = dma_set_mask_and_coherent(dwc->sysdev, DMA_BIT_MASK(32));
+		if (ret)
+			return ret;
+	}
+
 	dwc->reset = devm_reset_control_array_get_optional_shared(dev);
 	if (IS_ERR(dwc->reset))
 		return PTR_ERR(dwc->reset);
-- 
2.25.1