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

[Dave Jiang <dave.jiang@intel.com>]

On 10/11/23 06:26, Jonathan Cameron wrote:
> 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

I talked to Dan and it seems he prefers the simplest form for the current version until we have a need to move towards something more complex. So qos_class0 -> qos_class. We can move to qos_classN_M when there is a need.


> 
> 
> 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.
>> +
>> +
> 
>