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

[Rafael J. Wysocki]

On Fri, Jan 21, 2022 at 10:09 PM Keith Busch <kbusch at kernel.org> wrote:
>
> On Fri, Jan 21, 2022 at 08:00:49PM +0100, Rafael J. Wysocki wrote:
> > 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?
>
> I thought platforms that wanted full device shutdown  behaviour would
> always set acpi_storage_d3. Is that not happening here?

Evidently, it isn't.

Also that flag is about putting the device into D3, which need not be
necessary as long as the LTR is set to "don't care".