USB/swiotlb failure on arm64/RPi3

[Aaro Koskinen]

Hi,

Booting Debian rootfs from USB hard disk (ext4) using 64-bit 4.9 kernel
on Raspberry Pi 3 fails early in the boot as follows:

[   15.162466] systemd[1]: Detected architecture arm64.

Welcome to Debian GNU/Linux 9 (stretch)!

[   15.200822] systemd[1]: Set hostname to <raspberrypi-3>.
[   46.135227] usb 1-1.5: reset high-speed USB device number 4 using dwc2
[   76.844211] usb 1-1.5: reset high-speed USB device number 4 using dwc2
[  105.257888] systemd[1]: system-generators terminated by signal ALRM.
[  108.087234] usb 1-1.5: reset high-speed USB device number 4 using dwc2
[  138.796224] usb 1-1.5: reset high-speed USB device number 4 using dwc2
[  170.039222] usb 1-1.5: reset high-speed USB device number 4 using dwc2
[  201.261222] usb 1-1.5: reset high-speed USB device number 4 using dwc2
[  201.405906] sd 0:0:0:0: [sda] tag#0 UNKNOWN(0x2003) Result: hostbyte=0x05 driverbyte=0x00
[  201.414234] sd 0:0:0:0: [sda] tag#0 CDB: opcode=0x28 28 00 02 c7 c5 90 00 00 68 00
[  201.421951] blk_update_request: I/O error, dev sda, sector 46646672

Boot hangs and I/O does not recover.

I first suspected the dwc2 driver, but the cause turns out to be DMA
mapping error in usb_hcd_map_urb_for_dma(). This will cause usb_sg_wait()
to loop forever trying to re-try. On RPi3 dma_mapping_error() is:

int
swiotlb_dma_mapping_error(struct device *hwdev, dma_addr_t dma_addr)
{
	return (dma_addr == phys_to_dma(hwdev, io_tlb_overflow_buffer));
}

On arm64, swiotlb is not initialized by default, so io_tlb_overflow_buffer
is 0. But phys_to_dma(hwdev, 0) should be a valid DMA address and not
be rejected. I tested this by initializing io_tlb_overflow_buffer with
INVALID_PHYS_ADDR, and then the boot passes and system runs fine.

Any ideas how this should be fixed properly?

A.