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

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Message-ID: <7eea091e-f241-4305-a233-ba56a4ed5df3@intel.com>
Date: Thu, 13 Nov 2025 13:36:26 -0700
From: Dave Jiang <dave.jiang@...el.com>
To: Robert Richter <rrichter@....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 11/13/25 1:05 PM, Robert Richter wrote:
> 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.

Oh so the current implementation is auto region by BIOS but in the future it may not be? But if you add a lock flag, you wouldn't be able to remove it later anyhow since it's presented as locked?


> 
> -Robert