Re: [PATCH v5 13/14] cxl: Export sysfs attributes for memory device QoS class

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

On Mon, 08 May 2023 13:48:10 -0700
Dave Jiang <dave.jiang@intel.com> wrote:

> Export qos_class sysfs attributes for the CXL memory device. The QoS clas
> should show up as /sys/bus/cxl/devices/memX/ram/qos_class for the volatile
> partition and /sys/bus/cxl/devices/memX/pmem/qos_class for the persistent
> partition. The QTG ID is retrieved via _DSM after supplying the
> calculated bandwidth and latency for the entire CXL path from device to
> the CPU. This ID is used to match up to the root decoder QoS class to
> determine which CFMWS the memory range of a hotplugged CXL mem device
> should be assigned under.
> 
> While there may be multiple DSMAS exported by the device CDAT, the driver
> will only expose the first QTG ID per partition in sysfs for now. In the
> future when multiple QTG IDs are necessary, they can be exposed. [1]
> 
> [1]: https://lore.kernel.org/linux-cxl/167571650007.587790.10040913293130712882.stgit@djiang5-mobl3.local/T/#md2a47b1ead3e1ba08f50eab29a4af1aed1d215ab
> 
> Suggested-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> 

Reading this patch made me notice the whole issue of we may be exporting
a useless QTG for the device due to a missmatch of memory type and CFMWS
(as not using the full list provided by the _DSM)

> ---
> v5:
> - Change qtg_id to qos_class
> v4:
> - Change kernel version for documentation to v6.5
> v3:
> - Expand description of qtg_id. (Alison)
> ---
>  Documentation/ABI/testing/sysfs-bus-cxl |   32 +++++++++++++++++++++++++++++++
>  drivers/cxl/core/memdev.c               |   26 +++++++++++++++++++++++++
>  2 files changed, 58 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-bus-cxl b/Documentation/ABI/testing/sysfs-bus-cxl
> index 2f24e42ef36d..c13ae49dd9aa 100644
> --- a/Documentation/ABI/testing/sysfs-bus-cxl
> +++ b/Documentation/ABI/testing/sysfs-bus-cxl
> @@ -28,6 +28,22 @@ Description:
>  		Payload in the CXL-2.0 specification.
>  
>  
> +What:		/sys/bus/cxl/devices/memX/ram/qos_class
> +Date:		May, 2023
> +KernelVersion:	v6.5
> +Contact:	linux-cxl@vger.kernel.org
> +Description:
> +		(RO) For CXL host platforms that support "QoS Telemmetry"
> +		this attribute conveys a platform specific cookie that
> +		identifies a QoS performance class for the volatile
> +		partition of the CXL mem device. This class-id can be
> +		compared against a similar "qos_class" published for
> +		a root decoder. While it is not required that the endpoints
> +		map their local memory-class to a matching platform class,
> +		mismatches are not recommended and there are platform specific
> +		side-effects that may result.

I think this needs to be the full ordered list from the _DSM with text suggesting
that a compatible CFMWS matching the earliest entry possible should be used.