Message-ID: <aMqCy670eTu-ZYUO@rric.localdomain>
Date: Wed, 17 Sep 2025 11:43:39 +0200
From: Robert Richter <rrichter@....com>
To: Jonathan Cameron <jonathan.cameron@...wei.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>,
	Dave Jiang <dave.jiang@...el.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 v3 11/11] cxl: Enable AMD Zen5 address translation using
 ACPI PRMT

On 15.09.25 11:59:48, Jonathan Cameron wrote:
> On Fri, 12 Sep 2025 16:45:13 +0200
> Robert Richter <rrichter@....com> wrote:
> 
> > Add AMD Zen5 support for address translation.
> > 
> > Zen5 systems may be configured to use 'Normalized addresses'. Then,
> > CXL endpoints use their own physical address space and are programmed
> > passthrough (DPA == HPA), the number of interleaving ways for the
> > endpoint is set to one. The Host Physical Addresses (HPAs) need to be
> > translated from the endpoint to its CXL host bridge. The HPA of a CXL
> > host bridge is equivalent to the System Physical Address (SPA).
> > 
> > ACPI Platform Runtime Mechanism (PRM) is used to translate the CXL
> > Device Physical Address (DPA) to its System Physical Address. This is
> > documented in:
> > 
> >  AMD Family 1Ah Models 00h–0Fh and Models 10h–1Fh
> >  ACPI v6.5 Porting Guide, Publication # 58088
> >  https://www.amd.com/en/search/documentation/hub.html
> > 
> > To implement AMD Zen5 address translation the following steps are
> > needed:
> > 
> > AMD Zen5 systems support the ACPI PRM CXL Address Translation firmware
> > call (Address Translation - CXL DPA to System Physical Address, see
> > ACPI v6.5 Porting Guide above) when address translation is enabled.
> > The existence of the callback can be identified using a specific GUID
> > as documented. The initialization code checks firmware and kernel
> > support of ACPI PRM.
> > 
> > Introduce a new file core/atl.c to handle ACPI PRM specific address
> > translation code. Naming is loosely related to the kernel's AMD
> > Address Translation Library (CONFIG_AMD_ATL) but implementation does
> > not dependent on it, nor it is vendor specific. Use Kbuild and Kconfig
> > options respectively to enable the code depending on architecture and
> > platform options.
> > 
> > Implement an ACPI PRM firmware call for CXL address translation in the
> > new function cxl_prm_to_hpa(). This includes sanity checks. Enable the
> > callback for applicable CXL host bridges using the new cxl_atl_init()
> > function.
> > 
> > Signed-off-by: Robert Richter <rrichter@....com>
> A few minor additions inline.  
> 
> J
> > ---
> >  drivers/cxl/Kconfig       |   4 ++
> >  drivers/cxl/core/Makefile |   1 +
> >  drivers/cxl/core/atl.c    | 138 ++++++++++++++++++++++++++++++++++++++
> >  drivers/cxl/core/core.h   |   1 +
> >  drivers/cxl/core/port.c   |   8 +++
> >  5 files changed, 152 insertions(+)
> >  create mode 100644 drivers/cxl/core/atl.c
> > 
> > diff --git a/drivers/cxl/Kconfig b/drivers/cxl/Kconfig
> > index 48b7314afdb8..31f9c96ef908 100644
> > --- a/drivers/cxl/Kconfig
> > +++ b/drivers/cxl/Kconfig
> > @@ -233,4 +233,8 @@ config CXL_MCE
> >  	def_bool y
> >  	depends on X86_MCE && MEMORY_FAILURE
> >  
> > +config CXL_ATL
> > +       def_bool y
> 
> Given no help we can't turn this off manually and it's down to
> whether ACPI_PRMT is configured or not.
> 
> To me this feels like something we should be able to control.
> Not a huge amount of code, but none the less 'so far' it only
> applies to particular AMD platforms yet ACPI_PRMT gets built
> on ARM platforms and other stuff even on AMD (CONFIG_AMD_ATL_PRM)

How about default y where possible but have a menu entry to disable
address translation?

config CXL_ATL
	bool "CXL Address Translation support"
	default y
	depends on ACPI_PRMT

I don't want to make it specific to AMD.

> 
> 
> 
> > +       depends on ACPI_PRMT
> > +
> >  endif
> 
> > diff --git a/drivers/cxl/core/atl.c b/drivers/cxl/core/atl.c
> > new file mode 100644
> > index 000000000000..5fc21eddaade
> > --- /dev/null
> > +++ b/drivers/cxl/core/atl.c
> 
> > +struct prm_cxl_dpa_spa_data {
> > +	u64 dpa;
> > +	u8 reserved;
> > +	u8 devfn;
> > +	u8 bus;
> > +	u8 segment;
> > +	void *out;
> 
> If reality is out is always a u64 * maybe just give it that type.

Will check that.

> 
> > +} __packed;
> > +
> > +static u64 prm_cxl_dpa_spa(struct pci_dev *pci_dev, u64 dpa)
> > +{
> > +	struct prm_cxl_dpa_spa_data data;
> > +	u64 spa;
> > +	int rc;
> > +
> > +	data = (struct prm_cxl_dpa_spa_data) {
> > +		.dpa     = dpa,
> > +		.devfn   = pci_dev->devfn,
> > +		.bus     = pci_dev->bus->number,
> > +		.segment = pci_domain_nr(pci_dev->bus),
> > +		.out     = &spa,
> > +	};
> > +
> > +	rc = acpi_call_prm_handler(prm_cxl_dpa_spa_guid, &data);
> > +	if (rc) {
> > +		pci_dbg(pci_dev, "failed to get SPA for %#llx: %d\n", dpa, rc);
> > +		return ULLONG_MAX;
> > +	}
> > +
> > +	pci_dbg(pci_dev, "PRM address translation: DPA -> SPA: %#llx -> %#llx\n", dpa, spa);
> > +
> > +	return spa;
> > +}
> > +
> > +static u64 cxl_prm_to_hpa(struct cxl_decoder *cxld, u64 hpa)
> > +{
> 
> > +	pci_dev = to_pci_dev(cxlmd->dev.parent);
> 
> 
> 	return prm_cxl_dpa_spa(to_pci_dev(cxlmd->dev.parent), hpa);
> seem fine to me and shortens things a little.

Ok.

> 
> > +
> > +	return prm_cxl_dpa_spa(pci_dev, hpa);
> > +}
> > +
> > +static void cxl_prm_init(struct cxl_port *port)
> > +{
> > +	u64 spa;
> > +	struct prm_cxl_dpa_spa_data data = { .out = &spa, };
> > +	int rc;
> > +
> > +	if (!check_prm_address_translation(port))
> > +		return;
> > +
> > +	/* Check kernel (-EOPNOTSUPP) and firmware support (-ENODEV) */
> > +	rc = acpi_call_prm_handler(prm_cxl_dpa_spa_guid, &data);
> > +	if (rc == -EOPNOTSUPP || rc == -ENODEV)
> > +		return;
> 
> So other error values are fine?  IF they don't occur no need to be explicit
> just check rc < 0 and return.

This is just to check the existence of the PRM, but it will fail (if
exists) here as parameters are a stub only. Both error codes are
reserved for firmware or kernel support respectively. Else, it returns
the PRM's error code, which is ignored here.

> 
> > +
> > +	port->to_hpa = cxl_prm_to_hpa;
> > +
> > +	dev_dbg(port->host_bridge, "PRM address translation enabled for %s.\n",
> > +		dev_name(&port->dev));
> > +}
> > +
> > +void cxl_atl_init(struct cxl_port *port)
> > +{
> > +	cxl_prm_init(port);
> Why not just rename cxl_prm_init() to cxl_atl_init() and get rid of this wrapper?

cxl_prm_init() handles the PRM specifics, while cxl_atl_init() is used
as an entry for the core module to enable address translation. I
thought it would be misleading to name cxl_prm_init() different. The
compiler result should be the same for both.

-Robert

> 
> > +}
> 
> 

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