Messages in this thread | | | Date | Tue, 21 Oct 2014 15:05:50 -0400 | From | Peter Hurley <> | Subject | pnp bus disables serial console despite no_console_suspend |
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Hi,
I'm looking into a cpu lockup while resuming from S3.
I can't paste the warning here because the serial console comes back scrambled. There are two issues: 1. The 16550A serial port is a pnp device that the pnp bus disables on suspend, despite having no_console_suspend set, and 2. ACPI suspend powers off the device anyway.
I've managed to workaround these 2 issues by brute force hack, but I think that it's worthwhile to consider making no_console_suspend work properly in mainline.
Not disabling the pnp device of the serial console is trivial: option 1 is to special case some return value from pnp_dev->suspend() which prevent further action by the pnp bus but does not error out of suspending, or option 2 is for the device to override pnp_dev->capabilities and turn off PNP_DISABLE. This option also requires a new capability to prevent the pnpacpi protocol from powering off the device. Option 2 is the workaround I'm using right now and it's pretty messy.
The second issue is less straightforward.
Since ACPI powers off the device anyway, on return from acpi_suspend_lowlevel(), the device needs some minimal re-programming. The workaround right now is to re-check a device register on every console write, and perform the re-programming if it's been cleared. This is not very robust or desirable; the power-on register value is a valid (but unlikely) state for the device.
What's needed is a way to distinguish between a console write immediately before and immediately after resume, before resume starts using printk :) Ideally, consoles would get a notification before being used; optionally, a system-wide value which identifies power transition states could work.
Thoughts?
Regards, Peter Hurley
PS - This situation is not limited to pnpacpi on x86. Several ARM designs power off the serial port, and require explicit re-programming for resume using no_console_suspend.
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