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I am upgrading the kernel on a platform using an ARM SOC (AT91SAM9G25) from 3.2 to 4.4. This is a sysv system. The previous 3.2 works fine, but when booting the 4.4 kernel, it hangs after the exec of /sbin/init. I can specify 'init=/bin/sh' on the U-Boot bootargs and it successfully execs the shell (I get a shell prompt). From there, things look proper; I can mount /proc, verify that the rootfs is mounted, bring up a NIC interface, etc.

I have successfully performed this upgrade on a different platform running a different ARM SOC (AT91SAM9G45). I compared the kernel configs between this working other platform and the one that hangs. The only differences are those related to the different SOCs. Kernel configuration differences follow:

300,301c300,301
< CONFIG_PLATFORM_SLK1=y
< # CONFIG_PLATFORM_SLK2 is not set
---
> # CONFIG_PLATFORM_SLK1 is not set
> CONFIG_PLATFORM_SLK2=y
421c421
< CONFIG_ARM_APPENDED_DTB_FILE="arch/arm/boot/dts/slk1.dtb"
---
> CONFIG_ARM_APPENDED_DTB_FILE="arch/arm/boot/dts/slk2.dtb"
1061c1061
< # CONFIG_MTD_M25P80 is not set
---
> CONFIG_MTD_M25P80=y
1311,1314c1311
< CONFIG_NET_VENDOR_MICREL=y
< # CONFIG_KS8842 is not set
< # CONFIG_KS8851 is not set
< # CONFIG_KS8851_MLL is not set
---
> # CONFIG_NET_VENDOR_MICREL is not set
2414c2411,2450
< # CONFIG_USB_GADGET is not set
---
> CONFIG_USB_GADGET=y
> # CONFIG_USB_GADGET_DEBUG is not set
> # CONFIG_USB_GADGET_DEBUG_FILES is not set
> # CONFIG_USB_GADGET_DEBUG_FS is not set
> CONFIG_USB_GADGET_VBUS_DRAW=2
> CONFIG_USB_GADGET_STORAGE_NUM_BUFFERS=2
> 
> #
> # USB Peripheral Controller
> #
> # CONFIG_USB_AT91 is not set
> CONFIG_USB_ATMEL_USBA=y
> # CONFIG_USB_FUSB300 is not set
> # CONFIG_USB_FOTG210_UDC is not set
> # CONFIG_USB_GR_UDC is not set
> # CONFIG_USB_R8A66597 is not set
> # CONFIG_USB_PXA27X is not set
> # CONFIG_USB_MV_UDC is not set
> # CONFIG_USB_MV_U3D is not set
> # CONFIG_USB_M66592 is not set
> # CONFIG_USB_BDC_UDC is not set
> # CONFIG_USB_NET2272 is not set
> # CONFIG_USB_GADGET_XILINX is not set
> # CONFIG_USB_DUMMY_HCD is not set
> # CONFIG_USB_CONFIGFS is not set
> # CONFIG_USB_ZERO is not set
> # CONFIG_USB_AUDIO is not set
> # CONFIG_USB_ETH is not set
> # CONFIG_USB_G_NCM is not set
> # CONFIG_USB_GADGETFS is not set
> # CONFIG_USB_FUNCTIONFS is not set
> # CONFIG_USB_MASS_STORAGE is not set
> # CONFIG_USB_G_SERIAL is not set
> # CONFIG_USB_MIDI_GADGET is not set
> # CONFIG_USB_G_PRINTER is not set
> # CONFIG_USB_CDC_COMPOSITE is not set
> # CONFIG_USB_G_ACM_MS is not set
> # CONFIG_USB_G_MULTI is not set
> # CONFIG_USB_G_HID is not set
> # CONFIG_USB_G_DBGP is not set
2584,2585c2620,2621
< # CONFIG_RTC_DRV_AT91RM9200 is not set
< CONFIG_RTC_DRV_AT91SAM9=y
---
> CONFIG_RTC_DRV_AT91RM9200=y
> # CONFIG_RTC_DRV_AT91SAM9 is not set
2599c2635
< # CONFIG_AT_HDMAC is not set
---
> CONFIG_AT_HDMAC=y
3058c3094
< CONFIG_DEBUG_UART_PHYS=0xffffee00
---
> CONFIG_DEBUG_UART_PHYS=0xfffff200

If I boot to a shell (with init=/bin/sh), I can run '/sbin/init -i' and the system boots normally (init has a PID other than 1). However, 'exec /sbin/init' or 'exec /sbin/init -i' hang.

Any ideas on how to figure out where init is hung?

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    please add to the question hw used and modifications to configurations done. Jul 17, 2017 at 20:48

1 Answer 1

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I think I have found the cause of this kernel hang. The atmel_serial driver is disabling the UART transmitter when init closes the console device (/dev/console). The next printk from the kernel then hangs, busy waiting for the THR to be empty. This bug has been fixed in the upstream 4.4-at91 branch (https://github.com/linux4sam/linux-at91/commit/0a1757cfa5ba3b46f6ee7a74ddb7a5c0bd5d7c2f)

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