Re: [PATCH] Documentation: PCI: add vmd documentation

[Paul M Stillwell Jr <paul.m.stillwell.jr@intel.com>]

On 4/17/2024 4:51 PM, Keith Busch wrote:
> On Wed, Apr 17, 2024 at 01:15:42PM -0700, Paul M Stillwell Jr wrote:
>> +=================================================================
>> +Linux Base Driver for the Intel(R) Volume Management Device (VMD)
>> +=================================================================
>> +
>> +Intel vmd Linux driver.
>> +
>> +Contents
>> +========
>> +
>> +- Overview
>> +- Features
>> +- Limitations
>> +
>> +The Intel VMD provides the means to provide volume management across separate
>> +PCI Express HBAs and SSDs without requiring operating system support or
>> +communication between drivers. It does this by obscuring each storage
>> +controller from the OS, but allowing a single driver to be loaded that would
>> +control each storage controller. A Volume Management Device (VMD) provides a
>> +single device for a single storage driver. The VMD resides in the IIO root
>> +complex and it appears to the OS as a root bus integrated endpoint. In the IIO,
>> +the VMD is in a central location to manipulate access to storage devices which
>> +may be attached directly to the IIO or indirectly through the PCH. Instead of
>> +allowing individual storage devices to be detected by the OS and allow it to
>> +load a separate driver instance for each, the VMD provides configuration
>> +settings to allow specific devices and root ports on the root bus to be
>> +invisible to the OS.
> 
> This doesn't really capture how the vmd driver works here, though. The
> linux driver doesn't control or hide any devices connected to it; it
> just creates a host bridge to its PCI domain, then exposes everything to
> the rest of the OS for other drivers to bind to individual device
> instances that the pci bus driver finds. Everything functions much the
> same as if VMD was disabled.

I was trying more to provide an overview of how the VMD device itself 
works here and not the driver. The driver is fairly simple; it's how the 
device works that seems to confuse people :).

Do you have a suggestion on what you would like to see here?

Paul