netdev - Re: [PATCH v19 18/22] cxl: Allow region creation by type2 drivers

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Message-ID: <127311bc-d3cd-48e9-9fc3-f19853bb766b@amd.com>
Date: Thu, 16 Oct 2025 08:23:18 -0500
From: "Cheatham, Benjamin" <benjamin.cheatham@....com>
To: Dave Jiang <dave.jiang@...el.com>, <alejandro.lucero-palau@....com>
CC: Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	<linux-cxl@...r.kernel.org>, <netdev@...r.kernel.org>,
	<dan.j.williams@...el.com>, <edward.cree@....com>, <davem@...emloft.net>,
	<kuba@...nel.org>, <pabeni@...hat.com>, <edumazet@...gle.com>
Subject: Re: [PATCH v19 18/22] cxl: Allow region creation by type2 drivers

On 10/15/2025 4:42 PM, Dave Jiang wrote:
> 
> 
> On 10/9/25 1:56 PM, Cheatham, Benjamin wrote:
>> On 10/6/2025 5:01 AM, alejandro.lucero-palau@....com wrote:
>>> From: Alejandro Lucero <alucerop@....com>
>>>
>>> Creating a CXL region requires userspace intervention through the cxl
>>> sysfs files. Type2 support should allow accelerator drivers to create
>>> such cxl region from kernel code.
>>>
>>> Adding that functionality and integrating it with current support for
>>> memory expanders.
>>>
>>> Support an action by the type2 driver to be linked to the created region
>>> for unwinding the resources allocated properly.
>>>
>>> Based on https://lore.kernel.org/linux-cxl/168592159835.1948938.1647215579839222774.stgit@dwillia2-xfh.jf.intel.com/
>>>
>>> Signed-off-by: Alejandro Lucero <alucerop@....com>
>>> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
>>> ---
>>
>> Fix for this one should be split between 13/22 and this patch, but the majority of it is in this one. The idea is
>> if we don't find a free decoder we check for pre-programmed decoders and use that instead. Unfortunately, this
>> invalidates some of the assumptions made by __construct_new_region().
> 
> Wouldn't you look for a pre-programmed decoder first and construct the auto region before you try to manually create one? Also for a type 2 device, would the driver know what it wants and what the region configuration should look like? Would it be a single region either it's auto or manual, or would there be a configuration of multiple regions possible? To me a type 2 region is more intentional where the driver would know exactly what it needs and thus trying to get that from the cxl core. 
> 

Since this is a fix I didn't want to supersede the current behavior. A better solution would've been to add a flag to allow the type 2 driver
to set up an expected region type.

As for multiple regions, I have no clue. I haven't heard of any reason why a type 2 device would need multiple regions, but it's still very
early days. I don't think there's anything in this set that keeps you from using multiple regions though.

Thanks,
Ben

> DJ
>