On Fri, Jun 29, 2012 at 8:23 PM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Jun 29, 2012 at 12:36:30PM -0700, Andrew Boie wrote:
>> Android userspace tells the kernel that it wants to boot into recovery
>> or some other non-default OS environment by passing a string argument
>> to reboot(). It is left to the OEM to hook this up to their specific
>> bootloader.
>
> How does userspace "tell" the kernel this? You are creating a new
> user/kernel api here, and haven't documented it at all. That's not
> generally a wise idea.
Android's libcutils uses the existing reboot syscall with
reboot(LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2,
LINUX_REBOOT_CMD_MAGIC2, <string>). According to "man 2 reboot":
LINUX_REBOOT_CMD_RESTART2
(0xa1b2c3d4; since 2.1.30). The message "Restarting system with
command '%s'" is printed, and a restart (using the command
string given in arg) is performed immediately. If not preceded
by a sync(2), data will be lost.
>> This driver uses the bootloader control block (BCB) in the 'misc'
>> partition, which is the AOSP mechanism used to communicate with
>> the bootloader. Writing 'bootonce-NNN' to the command field
>> will cause the bootloader to do a oneshot boot into an alternate
>> boot label NNN if it exists. The device and partition number are
>> passed in via kernel command line.
>
> I don't understand still, why userspace can't just do this as it is the
> one that knws where the device and partition is, and it knows what it
> wants to have written in this area, why have the kernel do this? Why
> not just modify userspace to do it instead?
In this particular case, userspace could handle writing the data. On
many SoCs, reboot mode is written to a register or to internal IO
mapped memory. Supporting the REBOOT2 argument to the syscall
requires something in the kernel to save that message, this driver
seems to commonize saving that to a disk partition, which is the least
common denominator way to handle it.
<snip>
>>
>> diff --git a/drivers/staging/android/Kconfig b/drivers/staging/android/Kconfig
>> index 9a99238..c30fd20 100644
>> --- a/drivers/staging/android/Kconfig
>> +++ b/drivers/staging/android/Kconfig
>> @@ -78,6 +78,17 @@ config ANDROID_INTF_ALARM_DEV
>> elapsed realtime, and a non-wakeup alarm on the monotonic clock.
>> Also exports the alarm interface to user-space.
>>
>> +config BOOTLOADER_CONTROL
>> + tristate "Bootloader Control Block module"
>> + default n
>> + help
>> + This driver installs a reboot hook, such that if reboot() is invoked
>> + with a string argument NNN, "bootonce-NNN" is copied to the command
>> + field in the Bootloader Control Block on the /misc partition, to be
>> + read by the bootloader. If the string matches one of the boot labels
>> + defined in its configuration, it will boot once into that label. The
>> + device and partition number are specified on the kernel command line.
>> +
>> endif # if ANDROID
>>
>> endmenu
Most of this driver is not unique to Android. If you remove the
status and recovery fields in the partition and the list of possible
command values, it becomes a neutral way to pass the REBOOT2 argument
from userspace to the bootloader. status and recovery are a separate
contract between userspace and the bootloader, and could go into a
separate block in the same partition for systems that need it.
_______________________________________________
devel mailing list
devel@xxxxxxxxxxxxxxxxxxxxxx
http://driverdev.linuxdriverproject.org/mailman/listinfo/devel
