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

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Tue, 16 May 2023 14:49:46 -0700
Dan Williams <dan.j.williams@intel.com> wrote:

> 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".
> 
Great, that works nicely.

Jonathan