linux-kernel - Re: [PATCH v4 11/14] cxl/atl: Lock decoders that need address translation

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Message-ID: <aRY6EaTALW4SvkaW@rric.localdomain>
Date: Thu, 13 Nov 2025 21:05:37 +0100
From: Robert Richter <rrichter@....com>
To: Dave Jiang <dave.jiang@...el.com>
Cc: Alison Schofield <alison.schofield@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Ira Weiny <ira.weiny@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	Davidlohr Bueso <dave@...olabs.net>, linux-cxl@...r.kernel.org,
	linux-kernel@...r.kernel.org, Gregory Price <gourry@...rry.net>,
	"Fabio M. De Francesco" <fabio.m.de.francesco@...ux.intel.com>,
	Terry Bowman <terry.bowman@....com>,
	Joshua Hahn <joshua.hahnjy@...il.com>
Subject: Re: [PATCH v4 11/14] cxl/atl: Lock decoders that need address
 translation

On 12.11.25 09:34:34, Dave Jiang wrote:
> 
> 
> On 11/11/25 5:54 AM, Robert Richter wrote:
> > On 04.11.25 10:13:34, Dave Jiang wrote:
> >>
> >>
> >> On 11/3/25 11:47 AM, Robert Richter wrote:
> >>> There is only support to translate addresses from an endpoint to its
> >>> CXL host bridge, but not in the opposite direction from the bridge to
> >>> the endpoint. Thus, the endpoint address range cannot be determined
> >>> and setup manually for a given SPA range of a region. If the endpoint
> >>> has address translation enabled, lock it to prevent the kernel from
> >>> reconfiguring it.
> >>>
> >>> Reviewed-by: Gregory Price <gourry@...rry.net>
> >>> Signed-off-by: Robert Richter <rrichter@....com>
> >>> ---
> >>>  drivers/cxl/core/atl.c | 10 ++++++++++
> >>>  1 file changed, 10 insertions(+)
> >>>
> >>> diff --git a/drivers/cxl/core/atl.c b/drivers/cxl/core/atl.c
> >>> index d6aa7e6d0ac5..5c15e4d12193 100644
> >>> --- a/drivers/cxl/core/atl.c
> >>> +++ b/drivers/cxl/core/atl.c
> >>> @@ -158,6 +158,16 @@ static int cxl_prm_translate_hpa_range(struct cxl_root *cxl_root, void *data)
> >>>  		return -ENXIO;
> >>>  	}
> >>>  
> >>> +	/*
> >>> +	 * There is only support to translate from the endpoint to its
> >>> +	 * parent port, but not in the opposite direction from the
> >>> +	 * parent to the endpoint. Thus, the endpoint address range
> >>> +	 * cannot be determined and setup manually. If the address range
> >>> +	 * was translated and modified, forbid reprogramming of the
> >>> +	 * decoders and lock them.
> >>> +	 */
> >>> +	cxld->flags |= CXL_DECODER_F_LOCK;
> >>
> >  
> >> Feels like this should be something the BIOS should enforce if that
> >> is the expectation? And the kernel checks and warns if that is not
> >> the case.
> > 
> > I think this is more a limitation of the kernel implementation rather
> > than the BIOS. The BIOS provides enought information by CFMWS, PRM,
> > HDM and PCI topology. In theory and if there is demand for it, support
> > could be added for driver region setup.
> 

> But shouldn't the BIOS set the decoder lock rather than the kernel
> setting a software lock flag based on assumption of the PRM based
> setup?

If BIOS locks the decoders, it cannot be removed even for the case
there the OS can actually handle it.

-Robert