Re: [PATCH v5 00/14] cxl: Add support for QTG ID retrieval for CXL subsystem

[Dan Williams <dan.j.williams@intel.com>]

Jonathan Cameron wrote:
> Hi Dave,
> 
> > The QTG ID for a device is retrieved via sending a _DSM method to the ACPI0017 device.
> > The _DSM expects an input package of 4 DWORDS that contains the read latency, write
> > latency, read bandwidth, and write banwidth. These are the caluclated numbers for the
> > path between the CXL device and the CPU. The QTG ID is also exported as a sysfs
> > attribute under the mem device memory partition type:
> > /sys/bus/cxl/devices/memX/ram/qos_class
> > /sys/bus/cxl/devices/memX/pmem/qos_class
> > Only the first QTG ID is exported.
> 
> The QTG DSM returning a list was done to allow for a case of mutual
> incompatibility between the first QTG that is returned for a particular
> performance point and the CFMWS that it points at.
> 
> CFMWS might say 'no pmem in here' but due to some RAM device that is a bit
> slow, we might end up with a QTG DSM response that says put it in that CFMWS.
> 
> Hence the fallback list.
> 
> That is currently hidden by this approach.  It makes things more complex, but
> I'd really like to see the whole of that list rather than just the first element
> presented for each region.  I think it's fine to let userspace then figure
> out if there is a missmatch.

There is some confusion here, the "Only the first QTG ID is exported"
statement is with respect to the case of multiple DSMAS entries per
partition. For the case of multiple platform QoS classes per single
DSMAS I would be ok if this qos_class returned a comma-separated
list/tuple.

So, for example, in a case where DSMAS0 for the 'ram' partition results
in QoS class-ids 0,1,2 and DSMAS1 for the 'ram' partition results in QoS
class-ids 3,4 then /sys/bus/cxl/devices/memX/ram/qos_class would be
allowed to report "0,1,2".