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Re: Pause GRUB booting

From: Robert Middleton
Subject: Re: Pause GRUB booting
Date: Tue, 11 Apr 2017 17:29:59 -0400

On Tue, Apr 11, 2017 at 1:58 PM, Andrei Borzenkov <address@hidden> wrote:
> 11.04.2017 18:59, Robert Middleton пишет:
>> I'm trying to modify GRUB to pause booting if a user presses a button
>> that is connected to a USB<->Serial converter.  This is a bit of an
>> odd situation, so here's a quick overview:
>> On occasion, we need to allow end-users to pause booting to allow for
>> HDD recovery.  This can only happen before GRUB attempts to write to
>> the HDD,
> GRUB does not write anything; the only possible write is save_env
> command that must be called explicitly in grub.cfg.

Yes that's what I meant; GRUB tries to write the environment.  The
first few bytes of the data that gets sent to disk start with

Is there a specific time that the save_env has to be called?  For
example, can I just re-arrange what gets generated from
/etc/grub.d/00_header to have all save_env calls after a timeout?

>> otherwise the system stays broken.  The system has no
>> keyboard, but it does have hard buttons that send a serial message
>> over an FTDI usb to serial converter.
> Note that right now GRUB ftdi driver only supports QEMU emulated USB
> serial, so you will likely need to extend it.
>> I have tried editing grub.cfg
>> to insert the FTDI module, but I don't see exactly how to read data
>> from an arbitrary serial port and continue on if there is no data on
>> the serial line.
> This is similar to what was requested before. I think we need to add
> support for timeout as well as raw character to read. Actually there was
> patch at least for timeout as far as I remember. Currently I'm afraid
> there is no way to poll for character with timeout on CLI level.

I was afraid of that, but that's good to know.  One suggestion if
anybody is going to implement this: also have the ability to wait for
a specific string of data, not just a character.

>> There are also about 6 FTDIs on the system, so
>> knowing which one to read from is important.
> Actually GRUB supports multiple input (and output) devices so you could
> add all of them as input device and read would poll all of them. Vut see
> below.
> I would not know how to distinguish between 6 identical USB devices in
> Linux either.

In Linux, I have several udev rules that read the device descriptors
from the chips and create the proper symlinks.  We program the FTDI
devices so that we know which one is which.

>>  My backup plan is to
>> pause the boot for 30 seconds each time, but I'm trying to avoid
>> that(plus the wait still has to happen before any data gets written to
>> the disk).
>> Is this possible to achieve, or would it require hacking of GRUB code 
>> directly?
> You will need to hack code. The most obvious is to extend read to
> timeout or return after fixed amount of characters (e.g. 1) are seen.
> See grub-core/commands/read.c.

After talking with our customer, they're actually okay with just
delaying the boot for a few seconds each time.  It's not ideal, but is
less likely to cause problems with systems.

-Robert Middleton

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