Re: [PATCH] PCI: Add Intel remapped NVMe device support

[Daniel Drake <drake@endlessm.com>]

On Tue, Jun 18, 2019 at 3:46 PM Hannes Reinecke <hare@suse.de> wrote:
> On 6/14/19 4:26 AM, Daniel Drake wrote:
> > On Thu, Jun 13, 2019 at 4:54 PM Christoph Hellwig <hch@lst.de> wrote:
> >> So until we get very clear and good documentation from Intel on that
> >> I don't think any form of upstream support will fly.  And given that
> >> Dan who submitted the original patch can't even talk about this thing
> >> any more and apparently got a gag order doesn't really give me confidence
> >> any of this will ever work.
> >
> > I realise the architecture here seems badly thought out, and the lack
> > of a decent spec makes the situation worse, but I'd encourage you to
> > reconsider this from the perspectives of:
> >  - Are the patches really more ugly than the underlying architecture?
> >  - We strive to make Linux work well on common platforms and sometimes
> > have to accept that hardware vendors do questionable things & do not
> > fully cooperate
> >  - It works out of the box on Windows
> >
> Actually, there _is_ a register description:
>
> https://www.intel.com/content/dam/www/public/us/en/documents/datasheets/300-series-chipset-pch-datasheet-vol-2.pdf
>
> Look for section 15: Intel RST for PCIe Storage.
>
> That gives you a reasonable description of the various registers etc.

Thanks for your email! I also spotted it for the first time earlier today.

Section 15 there (D24:F0) describes a special/hidden PCI device which
I can't figure out how to access from Linux (I believe it would be at
D18:F0 in the cases where the 300 PCH is integrated into the SoC, as
it is on the Whiskey Lake platform I have here). That's probably not
important because if even if we had access all the values are probably
read-only, as the BIOS will lock them all down during early boot, as
is documented. But the docs give some interesting insights into the
design.

Section 15.2 is potentially more relevant, as it describes registers
within the AHCI BAR and we do have access to that. Some of these
registers are already used by the current code to determine the
presence of remapped devices. It might be nice to use Device Memory
BAR Length (DMBL_1) but I can't figure out what is meant by "A 1 in
the bit location indicates the corresponding lower memory BAR bit for
the PCIe SSD device is a Read/Write (RW) bit." The value is 0x3fff on
the platform I have here.

We can probably also use these registers for MSI support. I started to
experiment, doesn't quite work but I'll keep poking. The doc suggests
there is a single MSI-X vector for the AHCI SATA device, and AHCI
MSI-X Starting Vector (AMXV) has value 0x140 on this platform. No idea
how to interpret that value. From experimentation, the AHCI SATA disk
generates interrupts on vector 0.

Then there are multiple vectors for the remapped NVMe devices. Device
MSI-X Configuration (DMXC_L_1) is set up to assign vectors 1 to 19 to
NVMe on this platform. But it says "This field is only valid when
DMXC.ID indicates interrupt delivery using MSI-X" but what/where is
DMXC.ID? So far I can get NVMe-related interrupts on vector 1 but
apparently not enough of them, the driver hangs during probe.

I've nearly finished refreshing & extending Dan Williams' patches and
will send them for more discussion soon.

Daniel