Re: [PATCH v14 2/8] cxl/mem: Read dynamic capacity configuration from the device

[Jonathan Cameron <jic23@kernel.org>]

On Tue, 22 Sep 2026 11:44:16 +0800
Richard Cheng <icheng@nvidia.com> wrote:

> On Fri, Sep 18, 2026 at 01:30:11PM +0800, Anisa Su wrote:
> > From: Ira Weiny <iweiny@kernel.org>
> > 
> > Devices which support Dynamic Capacity (DC) are configured
> > via mailbox commands. CXL r4.0 section 9.13.3 describes the use of
> > the Get DC Configuration command in order to obtain DCD partition
> > characteristics.
> > 
> > Implement the DC mailbox commands as specified in CXL 4.0 section
> > 8.2.10.9.9 (opcodes 48XXh) to read and store the DCD configuration
> > information. Disable DCD if an invalid configuration is found.
> > 
> > Initial enablement for Linux only supports one dynamic capacity
> > partition. Read all the partitions the device reports but validate
> > only the first, and configure it as 'dynamic ram 1'.
> > 
> > The specification requires that volatile capacity starts at DPA 0 and pmem
> > starts at the DPA immediately following it, but DC partitions only need
> > to be 256MB aligned according to CXL r4.0 section 8.2.10.9.9.1 Table 8-347.
> > So a device could leave a gap between ram/pmem (static) capacity and its first
> > DC partition, or between one DC partition and the next.
> > 
> > However, Linux chooses not to support gaps between static and dynamic
> > capacity, and requires the first DC partition to begin at the DPA
> > immediately following static capacity.
> > 
> > Based on an original patch by Navneet Singh.
> > 
> > Signed-off-by: Ira Weiny <iweiny@kernel.org>
> > Co-developed-by: Anisa Su <anisa.su@samsung.com>
> > Signed-off-by: Anisa Su <anisa.su@samsung.com>
> > Tested-by: Wonjae Lee <wj28.lee@samsung.com>
> > Tested-by: Junhee Park <jh9934.park@samsung.com>
> > Tested-by: Heesoo Kim <habil.kim@samsung.com>
> > Reviewed-by: Alison Schofield <alison.schofield@intel.com>
> >  
> 
> Reviewed-by: Richard Cheng <icheng@nvidia.com>
> 
> I wonder if we can have comments in code explaining why linux doesn't support
> gaps between static and DC, otherwise this can be a bit confusing why
> the 1st DC partition needs to be immediately after the static part, at least
> for me.

Because we are lazy?  :)

Or maybe because the more we constrain the hardware that is supported
the less variations we have deal with and test!

Jonathan