Why would you do this Dell?

On 29-Sep-2021 14:22:22:

This is a followup to my previous post about issues with the ACPI implementation in my Dell Latitude 7410. I had been hoping that some firmware update would fix it but nothing has. Then I saw an article on Hacker News titled Do not leave XPS laptop in any sleep/hibernate/standby mode when placed in a bag which linked to an article on Dell's website titled FAQ Modern Standby.

The main point from Dell's FAQ is this quote:

Under no circumstances should you leave a laptop powered on and in any sleep/hibernate/standby mode when placed in a bag, backpack, or in an overhead bin. The laptop will overheat as a result of that action.

Wait, what? The entire point of sleep and/or hibernate is being able to put the laptop in a bag while moving from one location to another! While this "Modern" Standby may be fine (and even desirable) for people who put their laptop on a desk and then never move it, its very much not okay for people who actually use their laptops as portable computers.

Modern Standby is, in technical terms, called Low Power S0 Idle. This is quite a bit different from the traditional S1 Standby, S3 Sleep (suspend-to-ram), or S4 Hibernate (suspend-to-disk). In fact traditionally S0 meant the system was fully powered on. This new Low Power S0 Idle may consume less power than the fully powered on S0 state, but it consumes a lot more power than the traditional S3 sleep. More power == more heat. This is why Dell is now saying you can't store a sleeping laptop in a backpack.

You can probably guess how this relates to my laptop's sleep issues at this point...

Support for Lower Power S0 Idle is incdicated by a bit in the Fixed ACPI Descriptor Table or FADT. And if I dump the table on my laptop I see this:

                      Low Power S0 Idle (V5) : 1

So the laptop's firmware is advertising support for this new Modern Standby. Taking a look at the contents of /sys/power/mem_sleep indicates that Linux has detected this and is using it for the preferred sleep mode:

    [s2idle] deep

And this is also reflected in the dmesg output when the kernel goes into sleep mode:

    PM: suspend entry (s2idle)


Luckily this is Linux, not Windows, and we can easily override crap like this. After running echo deep > /sys/power/mem_sleep it now shows:

    s2idle [deep]

And this is also reflected in the dmesg output:

    PM: suspend entry (deep)

And like magic the laptop no longer comes out of sleep mode when the lid is closed! The amount of power consumed and heat generated is also greatly reduced.

Bad news: The fix above only works as long as the laptop still supports S3 sleep. My coworker has a Dell Latitude 7420 (one model newer than mine) and reports that it no longer advertises S3 sleep capability. Dumping the ACPI DSDT off of his laptop suggests that it does support S3 sleep but its disabled. He is going to poke through the firmware settings to see if it can be re-enabled.

So in summary:

For two decades its been safe to put a sleeping laptop in a backpack. Now vendors have decided to change the default behavior of sleep mode in a way that makes this unsafe. This can easily cause the laptop to overheat, reducing the lifespan of the laptop. This is a clear violation of the principle of least surprise.

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