linux-kernel - Re: [PATCH v3 08/11] cxl/region: Implement endpoint decoder address translation

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Message-ID: <10d3ed8d-7688-4ced-841a-ce4e6d0b7f31@intel.com>
Date: Wed, 17 Sep 2025 13:57:59 -0700
From: Dave Jiang <dave.jiang@...el.com>
To: Gregory Price <gourry@...rry.net>,
 Jonathan Cameron <jonathan.cameron@...wei.com>
Cc: Robert Richter <rrichter@....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>, Davidlohr Bueso
 <dave@...olabs.net>, linux-cxl@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 "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 v3 08/11] cxl/region: Implement endpoint decoder address
 translation



On 9/17/25 1:51 PM, Gregory Price wrote:
> On Mon, Sep 15, 2025 at 11:46:14AM +0100, Jonathan Cameron wrote:
>>> +	/*
>>> +	 * Since translated addresses include the interleaving
>>> +	 * offsets, align the range to 256 MB.
>>
>> So we pass in an HPA range without interleaving offsets and get back
>> one with them?  Is that unavoidable, or can we potentially push
>> this bit into the callback?  Probably with separate callbacks to
>> get the interleave details.
>>
>> Overall I'm not really following what is going on here.  Maybe
>> some ascii art would help?
>>
> 
> The endpoints in this case are encoded with "normalized" (base-0) with
> a size of only the memory they provide. As a result, the decoder
> interleave settings will always be passthrough (iw=1, ig=ignored).
> 
> This chunk translates the normalized address region to the relevant SPA
> region, and translates the IW/IG to what it actually is (i.e. what it 
> *would have* been on a "normal" system).
> 
> Took me a while when i originally reviewed and tested this set.
> 
> Example - this is how you'd expect a real system supported by this code
> to be programmed:
> 
> region {
>     .start = 0x20000000
>     .end   = 0x3fffffff
>     .iw    = 2
>     .ig    = 256
> }
> 
> endpoint1_decoder {
>     .start = 0x0
>     .end   = 0xfffffff
>     .iw    = 1
>     .ig    = 256
> }
> 
> endpoint2_decoder {
>     .start = 0x0
>     .end   = 0xfffffff
>     .iw    = 1
>     .ig    = 256
> }
> 
> when you do the translation from either decoder's hpa start/end,
> you want the following output:
> 
> range {
>     .start = 0x20000000
>     .end   = 0x3fffffff
>     .iw    = 2
>     .ig    = 256
> }
> 
> If you assume a "normal" system - this is the settings the decoders
> would have been programmed with in the first place.
> 
> You have to do the alignment because the translation function (may)
> only work on granularity alignment.
> 
> Example:
> endpoint1->to_hpa(0)         => 0x0
> endpoint1->to_hpa(0xfffffff) => 0xffffe00
> endpoint2->to_hpa(0)         => 0x100
> endpoint2->to_hpa(0xfffffff) => 0xfffff00
> 
> So this code applies the appropriate alignment and returns the
> translated iw/ig for use elsewhere in the stack when validating the rest
> of the decoders.

Having this explanation added to the Conventions document would be good to have.

> 
> (haven't gotten to later commits, but iirc it was eventually used)
> 
> ~Gregory
> 
>>> +	 */
>>> +	range.start = ALIGN_DOWN(range.start, SZ_256M);
>>> +	range.end = ALIGN(range.end, SZ_256M) - 1;
>>> +
>>> +	spa_len = range_len(&range);
>>> +	if (!len || !spa_len || spa_len % len) {
>>> +		dev_warn(&port->dev,
>>> +			"CXL address translation: HPA range not contiguous: %#llx-%#llx:%#llx-%#llx(%s)\n",
>>> +			range.start, range.end, ctx->hpa_range.start,
>>> +			ctx->hpa_range.end, dev_name(&cxld->dev));
>>> +		return -ENXIO;
>>> +	}
>>> +
>>> +	ways = spa_len / len;
>>> +	gran = SZ_256;
>>> +