Re: [PATCH 03/18] cxl/region: Support empty uuids for non-pmem regions

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

On Sun, 05 Feb 2023 17:02:45 -0800
Dan Williams <dan.j.williams@intel.com> wrote:

> Shipping versions of the cxl-cli utility expect all regions to have a
> 'uuid' attribute. In preparation for 'ram' regions, update the 'uuid'
> attribute to return an empty string which satisfies the current
> expectations of 'cxl list -R'. Otherwise, 'cxl list -R' fails in the
> presence of regions with the 'uuid' attribute missing. Force the
> attribute to be read-only as there is no facility or expectation for a
> 'ram' region to recall its uuid from one boot to the next.

This is new functionality, so do we need the backwards compatibility?
Or does that cxl list -R not just skip these, but fail hard?
It's inelegant to have this visible at all (and it confused me when
I was running tests as I assumed I needed to poke something in it).

If nothing else, good to have a comment to remind us in the code
why there is the oddity of setting it to read only.

Jonathan

> 
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>  Documentation/ABI/testing/sysfs-bus-cxl |    3 ++-
>  drivers/cxl/core/region.c               |    7 +++++--
>  2 files changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/sysfs-bus-cxl b/Documentation/ABI/testing/sysfs-bus-cxl
> index 058b0c45001f..4c4e1cbb1169 100644
> --- a/Documentation/ABI/testing/sysfs-bus-cxl
> +++ b/Documentation/ABI/testing/sysfs-bus-cxl
> @@ -317,7 +317,8 @@ Contact:	linux-cxl@vger.kernel.org
>  Description:
>  		(RW) Write a unique identifier for the region. This field must
>  		be set for persistent regions and it must not conflict with the
> -		UUID of another region.
> +		UUID of another region. For volatile ram regions this
> +		attribute is a read-only empty string.
>  
>  
>  What:		/sys/bus/cxl/devices/regionZ/interleave_granularity
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 17d2d0c12725..c9e7f05caa0f 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -45,7 +45,10 @@ static ssize_t uuid_show(struct device *dev, struct device_attribute *attr,
>  	rc = down_read_interruptible(&cxl_region_rwsem);
>  	if (rc)
>  		return rc;
> -	rc = sysfs_emit(buf, "%pUb\n", &p->uuid);
> +	if (cxlr->mode != CXL_DECODER_PMEM)
> +		rc = sysfs_emit(buf, "\n");
> +	else
> +		rc = sysfs_emit(buf, "%pUb\n", &p->uuid);
>  	up_read(&cxl_region_rwsem);
>  
>  	return rc;
> @@ -301,7 +304,7 @@ static umode_t cxl_region_visible(struct kobject *kobj, struct attribute *a,
>  	struct cxl_region *cxlr = to_cxl_region(dev);
>  
>  	if (a == &dev_attr_uuid.attr && cxlr->mode != CXL_DECODER_PMEM)
> -		return 0;
> +		return 0444;

Definitely a comment here as this is very odd and we may forget why
it is like this.

>  	return a->mode;
>  }
>  
>