linux-kernel - Re: [RFC PATCH v8 05/10] cxl/memscrub: Add CXL device patrol scrub control feature

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Date: Thu, 9 May 2024 17:26:46 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: <shiju.jose@...wei.com>, <linux-cxl@...r.kernel.org>,
	<linux-acpi@...r.kernel.org>, <linux-mm@...ck.org>,
	<dan.j.williams@...el.com>, <dave@...olabs.net>,
	<jonathan.cameron@...wei.com>, <dave.jiang@...el.com>,
	<alison.schofield@...el.com>, <vishal.l.verma@...el.com>,
	<ira.weiny@...el.com>
CC: <linux-edac@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<david@...hat.com>, <Vilas.Sridharan@....com>, <leo.duran@....com>,
	<Yazen.Ghannam@....com>, <rientjes@...gle.com>, <jiaqiyan@...gle.com>,
	<tony.luck@...el.com>, <Jon.Grimm@....com>, <dave.hansen@...ux.intel.com>,
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	<wschwartz@...erecomputing.com>, <dferguson@...erecomputing.com>,
	<wbs@...amperecomputing.com>, <nifan.cxl@...il.com>, <tanxiaofei@...wei.com>,
	<prime.zeng@...ilicon.com>, <kangkang.shen@...urewei.com>,
	<wanghuiqiang@...wei.com>, <linuxarm@...wei.com>, <shiju.jose@...wei.com>
Subject: Re: [RFC PATCH v8 05/10] cxl/memscrub: Add CXL device patrol scrub
 control feature

shiju.jose@ wrote:
> From: Shiju Jose <shiju.jose@...wei.com>
> 
> CXL spec 3.1 section 8.2.9.9.11.1 describes the device patrol scrub control
> feature. The device patrol scrub proactively locates and makes corrections
> to errors in regular cycle.
> 
> Allow specifying the number of hours within which the patrol scrub must be
> completed, subject to minimum and maximum limits reported by the device.
> Also allow disabling scrub allowing trade-off error rates against
> performance.
> 
> Register with scrub subsystem to provide scrub control attributes to the
> user.
> 
> Co-developed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> Signed-off-by: Shiju Jose <shiju.jose@...wei.com>
[..]
> diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> index 0c79d9ce877c..399e43463626 100644
> --- a/drivers/cxl/mem.c
> +++ b/drivers/cxl/mem.c
> @@ -117,6 +117,12 @@ static int cxl_mem_probe(struct device *dev)
>  	if (!cxlds->media_ready)
>  		return -EBUSY;
>  
> +	rc = cxl_mem_patrol_scrub_init(cxlmd);
> +	if (rc) {
> +		dev_dbg(&cxlmd->dev, "CXL patrol scrub init failed\n");
> +		return rc;
> +	}

2 concerns:

* Why should cxl_mem_probe() fail just because this optional
  scrub interface did not register?

* Why is this not located in cxl_region_probe()? If the ras2 scrub is an
  HPA-based scrub I think CXL should do the work to interface with the scrub
  interface at the same level. This also provides another in-kernel user
  for all the DPA-to-HPA translation infrastructure that the CXL driver
  contains. Pretty much the only reason the CXL driver needs to exist at
  all is address translation, so at a minimum it seems a waste to inflict
  more need to understand DPAs on userspace.