[Bug] nvme blocks PC10 since v5.15 - bisected

[Rafael J. Wysocki]

Hi Keith,

It is reported that the following commit

commit e5ad96f388b765fe6b52f64f37e910c0ba4f3de7
Author: Keith Busch <kbusch at kernel.org>
Date:   Tue Jul 27 09:40:44 2021 -0700

   nvme-pci: disable hmb on idle suspend

   An idle suspend may or may not disable host memory access from devices
   placed in low power mode. Either way, it should always be safe to
   disable the host memory buffer prior to entering the low power mode, and
   this should also always be faster than a full device shutdown.

   Signed-off-by: Keith Busch <kbusch at kernel.org>
   Reviewed-by: Sagi Grimberg <sagi at grimberg.me>
   Signed-off-by: Christoph Hellwig <hch at lst.de>

is the source of a serious power regression occurring since 5.15
(please see https://bugzilla.kernel.org/show_bug.cgi?id=215467).

After this commit, the SoC on the affected system cannot enter
C-states deeper than PC2 while suspended to idle which basically
defeats the purpose of suspending.

What may be happening is that nvme_disable_prepare_reset() that is not
called any more in the ndev->nr_host_mem_descs case somehow causes the
LTR of the device to change to "no requirement" which allows deeper
C-states to be entered.

Can you have a look at this, please?

Cheers,
Rafael