linux-kernel - Re: [PATCH 2/5] cxl: Rename CXL_DECODER

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Message-ID: <6712d07f3300b_10a03294bd@dwillia2-mobl3.amr.corp.intel.com.notmuch>
Date: Fri, 18 Oct 2024 14:17:51 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: "Huang, Ying" <ying.huang@...el.com>, Dan Williams
	<dan.j.williams@...el.com>
CC: Dave Jiang <dave.jiang@...el.com>, <linux-cxl@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Davidlohr Bueso <dave@...olabs.net>, "Gregory
 Price" <gourry@...rry.net>, Jonathan Cameron <jonathan.cameron@...wei.com>,
	Alison Schofield <alison.schofield@...el.com>, Vishal Verma
	<vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>, Alejandro Lucero
	<alucerop@....com>, Ben Cheatham <benjamin.cheatham@....com>
Subject: Re: [PATCH 2/5] cxl: Rename CXL_DECODER_HOSTONLYMEM/DEVMEM

Huang, Ying wrote:
> Hi, Dan,
> 
> Dan Williams <dan.j.williams@...el.com> writes:
> 
> > Huang Ying wrote:
> >> Previously, CXL type3 devices (memory expanders) use host only
> >> coherence (HDM-H), while CXL type2 devices (accelerators) use dev
> >> coherence (HDM-D).  So the name of the target device type of a cxl
> >> decoder is CXL_DECODER_HOSTONLYMEM for type3 devices and
> >> CXL_DECODER_DEVMEM for type2 devices.  However, this isn't true
> >> anymore.  CXL type3 devices can use dev coherence + back
> >> invalidation (HDM-DB) too.
> >> 
> >> To avoid confusion between the device type and coherence, the patch
> >> renames CXL_DECODER_HOSTONLYMEM/DEVMEM to CXL_DECODER_EXPANDER/ACCEL.
> >
> > This does not look like an improvement to me. Type-3 devices that
> > support back-invalidate are DEVMEM devices. The device plays a role in
> > the coherence. 
> >
> > Your explanation is the reverse of this commit:
> >
> > 5aa39a9165cf cxl/port: Rename CXL_DECODER_{EXPANDER, ACCELERATOR} => {HOSTONLYMEM, DEVMEM}
> >
> > ...so I am confused what motivated this rename?
> 
> Sorry, I am confused about the target_type and coherence and forgot to
> check the history.  In some places, current kernel still hints
> target_type (CXL_DECODER_HOSTONLYMEM/DEVMEM) as expander/accelerator.
> Should we change them to avoid confusion in the future?
> 
> $ grep expander -r drivers/cxl/
> drivers/cxl/cxl.h:346: * @target_type: accelerator vs expander (type2 vs type3) selector
> drivers/cxl/core/region.c:2450: * @type: select whether this is an expander or accelerator (type-2 or type-3)
> drivers/cxl/core/port.c:141:		return sysfs_emit(buf, "expander\n");
> 
> The last one is
> 
> static ssize_t target_type_show(struct device *dev,
> 				struct device_attribute *attr, char *buf)
> {
> 	struct cxl_decoder *cxld = to_cxl_decoder(dev);
> 
> 	switch (cxld->target_type) {
> 	case CXL_DECODER_DEVMEM:
> 		return sysfs_emit(buf, "accelerator\n");
> 	case CXL_DECODER_HOSTONLYMEM:
> 		return sysfs_emit(buf, "expander\n");
> 	}
> 	return -ENXIO;
> }
> static DEVICE_ATTR_RO(target_type);
> 
> for decoder device.  This is a testing ABI documented in,
> 
> Documentation/ABI/testing/sysfs-bus-cxl
> 
> Is it OK to change this?

No, why does it need to change?

It is unfortunate, but ABI's are forever. The place to clarify that this
decoder is participating in HDM-D[B] vs HDM-H protocol rather than being
an "accelerator" or "expander" device would be in user tooling like
cxl-cli. sysfs is just a transport, not a UI.