Re: [PATCH v2 00/13] mpt3sas driver NVMe support:

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Mon, 2017-08-07 at 19:26 +0530, Kashyap Desai wrote:
> > 
> > -----Original Message-----
> > From: James Bottomley [mailto:James.Bottomley@HansenPartnership.com
> > ]
> > Sent: Saturday, August 05, 2017 8:12 PM
> > To: Christoph Hellwig; Hannes Reinecke
> > Cc: Suganath Prabu S; martin.petersen@oracle.com; linux-
> > scsi@vger.kernel.org; Sathya.Prakash@broadcom.com;
> > kashyap.desai@broadcom.com; linux-kernel@vger.kernel.org;
> > chaitra.basappa@broadcom.com; sreekanth.reddy@broadcom.com; linux-
> > nvme@lists.infradead.org
> > Subject: Re: [PATCH v2 00/13] mpt3sas driver NVMe support:
> > 
> > On Sat, 2017-08-05 at 06:53 -0700, Christoph Hellwig wrote:
> > > 
> > > On Wed, Aug 02, 2017 at 10:14:40AM +0200, Hannes Reinecke wrote:
> > > > 
> > > > 
> > > > I'm not happy with this approach.
> > > > NVMe devices should _not_ appear as SCSI devices; this will
> > > > just
> > > > confuse matters _and_ will be incompatible with 'normal' NVMe
> > > > devices.
> > > > 
> > > > Rather I would like to see the driver to hook into the existing
> > > > NVMe
> > > > framework (which essentially means to treat the mpt3sas as a
> > > > weird
> > > > NVMe-over-Fabrics HBA), and expose the NVMe devices like any
> > > > other
> > > > NVMe HBA.
> > > 
> > > That doesn't make any sense.  The devices behind the mpt adapter
> > > don't
> > > look like NVMe devices at all for the hosts - there are no NVMe
> > > commands or queues involved at all, they hide behind the same
> > > somewhat
> > > leaky scsi abstraction as other devices behind the mpt
> > > controller.
> > 
> > You might think about what we did for SAS: split the generic
> > handler into
> > two
> > pieces, libsas for driving the devices, which mpt didn't need
> > because of
> > the fat
> > firmware and the SAS transport class so mpt could at least show the
> > same
> > sysfs
> > files as everything else for SAS devices.
> 
>  Ventura generation of controllers are adding connectivity of NVME
>  drives seamlessly and protocol handling is in Firmware.
>  Same as SCSI to ATA translation done in firmware, Ventura controller
>  is doing SCSI to NVME translation and for end user protocol handling
>  is abstracted.
> 
>  This product handles new Transport protocol (NVME) same as ATA and
>  transport is abstracted for end user.
> 
> NVME pass-through related driver code, it is just a big tunnel for
> user space application. It is just a basic framework like SATA PASS-
> Through in existing mpt3sas driver.

I know how it works ... and I'm on record as not liking your SATL
approach because we keep tripping across bugs in the SATL that we have
to fix in the driver.

However, at least for bot SAS and SATA they appear to the system as
SCSI devices regardless of HBA, so we've largely smoothed over any
problems if you transfer from mp3sas to another SAS/SATA controller.

I believe your current proposal is to have NVMe devices appear as SCSI,
which isn't how the native NVMe driver handles them at all.  This is
going to have to be special cased in any tool designed to handle nvme
devices and it's going to cause big problems if someone changes
controller (or moves the device).  What's the proposal for making this
as painless as possible?

James