linux-kernel - Re: [PATCH 07/26] cxl/port: Add dynamic capacity size support to endpoint decoders

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Date: Sun, 5 May 2024 21:07:42 -0700
From: Ira Weiny <ira.weiny@...el.com>
To: Dan Williams <dan.j.williams@...el.com>, Ira Weiny <ira.weiny@...el.com>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>
CC: Dave Jiang <dave.jiang@...el.com>, Fan Ni <fan.ni@...sung.com>, "Navneet
 Singh" <navneet.singh@...el.com>, Dan Williams <dan.j.williams@...el.com>,
	Davidlohr Bueso <dave@...olabs.net>, Alison Schofield
	<alison.schofield@...el.com>, Vishal Verma <vishal.l.verma@...el.com>,
	<linux-btrfs@...r.kernel.org>, <linux-cxl@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 07/26] cxl/port: Add dynamic capacity size support to
 endpoint decoders

Dan Williams wrote:
> Ira Weiny wrote:
> > > >  	else
> > > >  		free_pmem_start = cxlds->pmem_res.start;
> > > >  
> > > > +	/*
> > > > +	 * Limit each decoder to a single DC region to map memory with
> > > > +	 * different DSMAS entry.
> > 
> > This prevents more than 1 region per DC partition (region).
> 
> Why? Multiple regions per partition is the current status quo for other
> partition types.
> 
> > > I notice in the pmem equivalent there is a case for part of the region already mapped.
> > > Can that not happen for a DC region as well?
> > 
> > See above check.  Each DC region (partition) was to be associated with a
> > single DSMAS entry.  I'm unclear now why that decision was made.
> 
> The limitation of one DSMAS per partition makes sense otherwise that
> would indicate performance across different spans of the partition.
> 
> > It does not seem hard to add this though.  Do we really need that ability
> > considering dax devices are likely going to be the main boundry for users
> > of a DC region?
> 
> It seems like extra work to make DCD a special case compared to the
> other partition types, so the burden of proof is the other way. Why
> tolerate DCD divergence from the status quo?

I don't remember the details.  But this was discussed before in a call.  I'm ok
adding this support.  I have a test for it now.  Just need to tweek a couple of
things.

Ira