linux-kernel - Re: [PATCH v5 08/27] cxl/mem: Read dynamic capacity configuration from the device

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Message-ID: <672398d5caa17_7b1f129472@iweiny-mobl.notmuch>
Date: Thu, 31 Oct 2024 09:48:53 -0500
From: Ira Weiny <ira.weiny@...el.com>
To: Davidlohr Bueso <dave@...olabs.net>, <ira.weiny@...el.com>
CC: Dave Jiang <dave.jiang@...el.com>, Fan Ni <fan.ni@...sung.com>, "Jonathan
 Cameron" <Jonathan.Cameron@...wei.com>, Navneet Singh
	<navneet.singh@...el.com>, Jonathan Corbet <corbet@....net>, Andrew Morton
	<akpm@...ux-foundation.org>, Dan Williams <dan.j.williams@...el.com>, "Alison
 Schofield" <alison.schofield@...el.com>, Vishal Verma
	<vishal.l.verma@...el.com>, <linux-cxl@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <nvdimm@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, Kees Cook <kees@...nel.org>, "Gustavo A. R.
 Silva" <gustavoars@...nel.org>, <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH v5 08/27] cxl/mem: Read dynamic capacity configuration
 from the device

Davidlohr Bueso wrote:
> On Tue, 29 Oct 2024, ira.weiny@...el.com wrote:
> 
> >Linux has no use for the trailing fields of the Get Dynamic Capacity
> >Configuration Output Payload (Total number of supported extents, number
> >of available extents, total number of supported tags, and number of
> >available tags).  Avoid defining those fields to use the more useful
> >dynamic C array.
> 
> More of a general question, if anyone knows: why does the spec define
> the fields in that order, and not put the region config structure(s)
> at the end, as with all other cmds, afaik.

Because the 3.0 spec only left 7 bytes of reserved space before the region
config structure array.  One might argue that no one would ever implement
the 3.0 spec for DCD but at the time it was seen as an unbreakable
structure.

While it is not impossible to utilize those fields they are not required
right now.  So in this implementation it was better to use the dynamic C
array's and leave the use of those fields for another day.

Ira