Re: [PATCH v10 21/22] cxl: Export sysfs attributes for memory device QoS class

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

On Tue, 10 Oct 2023 18:07:06 -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_class0 for the volatile
> partition and /sys/bus/cxl/devices/memX/pmem/qos_class0 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]

I'm not sure this will extent cleanly if we get a two dimensional set to describle
1) Multiple DSMAS entries for RAM (so multiple inputs to pass to the _DSM)
   One nice thing here might be to ensure we have the first one seen.
   So if in future we do need to extent it this corresponds to the 0th one
   described.
2) Want to describe less ideal QTG values from _DSM 


Maybe it's too early to come to any conclusion and the single 0 is enough.
The cynic in me suggests we call it. qos_class0_0 though to give us the space.
If we needs DSMAS ranges, then we describe those using first index,
and second is the priority index if we have multiple answers from _DSM.
For now it's always 0_0


Jonathan

> 
> [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>
> 
> ---
> v10:
> - Export only qos_class0, the first entry. Additional qos_class entries can be
>   exported later as needed. (Dan)
> - Have the sysfs attrib return -ENOENT unless driver is attached. (Dan)
> - Removed Jonathan's review tag due to code changes.
> ---
>  Documentation/ABI/testing/sysfs-bus-cxl |   34 +++++++++++++++++++++++++++++++
>  drivers/cxl/core/memdev.c               |   34 +++++++++++++++++++++++++++++++
>  2 files changed, 68 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-bus-cxl b/Documentation/ABI/testing/sysfs-bus-cxl
> index 44ffbbb36654..dd613f5987b5 100644
> --- a/Documentation/ABI/testing/sysfs-bus-cxl
> +++ b/Documentation/ABI/testing/sysfs-bus-cxl
> @@ -28,6 +28,23 @@ Description:
>  		Payload in the CXL-2.0 specification.
>  
>  
> +What:		/sys/bus/cxl/devices/memX/ram/qos_class0
> +Date:		May, 2023
> +KernelVersion:	v6.7
> +Contact:	linux-cxl@vger.kernel.org
> +Description:
> +		(RO) For CXL host platforms that support "QoS Telemmetry"
> +		this attribute conveys a comma delimited list of platform
> +		specific cookies that identifies a QoS performance class
> +		for the volatile partition of the CXL mem device. These
> +		class-ids 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 performance related
> +		side-effects that may result. First class-id is displayed.
> +
> +
>  What:		/sys/bus/cxl/devices/memX/pmem/size
>  Date:		December, 2020
>  KernelVersion:	v5.12
> @@ -38,6 +55,23 @@ Description:
>  		Payload in the CXL-2.0 specification.
>  
>  
> +What:		/sys/bus/cxl/devices/memX/pmem/qos_class0
> +Date:		May, 2023
> +KernelVersion:	v6.7
> +Contact:	linux-cxl@vger.kernel.org
> +Description:
> +		(RO) For CXL host platforms that support "QoS Telemmetry"
> +		this attribute conveys a comma delimited list of platform
> +		specific cookies that identifies a QoS performance class
> +		for the persistent partition of the CXL mem device. These
> +		class-ids 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 performance related
> +		side-effects that may result. First class-id is displayed.
> +
> +