linux-kernel - Re: [PATCH] cxl/hdm: allow zero sized committed decoders

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Message-ID:
 <SN7PR12MB8131CDE3909B30FCE3E24B6BBBE5A@SN7PR12MB8131.namprd12.prod.outlook.com>
Date: Sat, 4 Oct 2025 13:49:26 +0000
From: Vishal Aslot <vaslot@...dia.com>
To: Gregory Price <gourry@...rry.net>
CC: Dave Jiang <dave.jiang@...el.com>, Davidlohr Bueso <dave@...olabs.net>,
	Jonathan Cameron <jonathan.cameron@...wei.com>, 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>, Li Ming
	<ming.li@...omail.com>, Peter Zijlstra <peterz@...radead.org>, Dan Carpenter
	<dan.carpenter@...aro.org>, Zijun Hu <zijun.hu@....qualcomm.com>,
	"linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cxl/hdm: allow zero sized committed decoders

> ________________________________________
> From: Gregory Price <gourry@...rry.net>
> Sent: Friday, October 3, 2025 9:23 AM
> To: Vishal Aslot
> Cc: Dave Jiang; Davidlohr Bueso; Jonathan Cameron; Alison Schofield; Vishal Verma; Ira Weiny; Dan Williams; Li Ming; Peter Zijlstra; Dan Carpenter; Zijun Hu; linux-cxl@...r.kernel.org; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH] cxl/hdm: allow zero sized committed decoders
>
> External email: Use caution opening links or attachments
>
>
> On Fri, Oct 03, 2025 at 01:03:16AM +0000, Vishal Aslot wrote:
>> > ________________________________________
>> > From: Dave Jiang <dave.jiang@...el.com>
>> > Sent: Thursday, October 2, 2025 10:32 AM
>> > To: Gregory Price; Vishal Aslot
>> > Cc: Davidlohr Bueso; Jonathan Cameron; Alison Schofield; Vishal Verma; Ira Weiny; Dan Williams; Li Ming; Peter Zijlstra; Dan Carpenter; Zijun Hu; linux-cxl@...r.kernel.org; linux-kernel@...r.kernel.org
>> > Subject: Re: [PATCH] cxl/hdm: allow zero sized committed decoders
>> >
>> > External email: Use caution opening links or attachments
>> >
>> >
>> > On 10/1/25 10:48 PM, Gregory Price wrote:
>> >> On Wed, Oct 01, 2025 at 08:37:26PM +0000, Vishal Aslot wrote:
>> >>> @@ -1210,6 +1210,11 @@ int devm_cxl_enumerate_decoders(struct cxl_hdm *cxlhdm,
>> >>>                 rc = init_hdm_decoder(port, cxld, target_map, hdm, i,
>> >>>                                       &dpa_base, info);
>> >>>                 if (rc) {
>> >>> +                       if (rc == -ENOSPC) {
>> >>> +                               put_device(&cxld->dev);
>> >>> +                               rc = 0;
>> >>> +                               continue;
>> >>> +                       }
>> >>
>> >> How do you suggest actually testing this? I briefly poked at this in
>> >> QEMU trying to commit decoders, but i found myself incapable of
>> >> exercising this path.
>>
>> I tested it locally with our BIOS (UEFI) where we commit and lock all decoders and
>> all except decoder 0 are zero-sized.
>>
>
> Ahhh, so are you saying that you will only ever observe the following
> (as an example)
>
> endpoint decoders...
> decoder2.0   ->  available and can be programmed
> decoder2.1   ->  size=0, locked
> ...
> decoder2.N   ->  size=0, locked
>
> or are you suggesting the following is valid:
>
> decoder2.0   ->  size=0, locked
> decoder2.1   ->  available and can be programmed
> ...
> decoder2.N   ->  available and can be programmed
>
> ~Gregory

The first case is what we've got. In our case, the HB has 4 decoders. The end point (a Montage card with single port) has two decoders. For both, we commit & lock all decoders and all decoders except decoder<port>.0 are zero-sized.