Re: [RFC PATCH 3/9] cxl/mem: Add a driver for the type-3 mailbox

Ben Widawsky
2020-11-14T01:09:19Z

On 20-11-13 12:17:28, Bjorn Helgaas wrote:
> On Tue, Nov 10, 2020 at 09:43:50PM -0800, Ben Widawsky wrote:
> > From: Dan Williams <dan.j.williams@intel.com>
> >
> > The CXL.mem protocol allows a device to act as a provider of "System
> > RAM" and/or "Persistent Memory" that is fully coherent as if the memory
> > was attached to the typical CPU memory controller.
> >
> > The memory range exported by the device may optionally be described by
> > the platform firmware memory map, or by infrastructure like LIBNVDIMM to
> > provision persistent memory capacity from one, or more, CXL.mem devices.
> >
> > A pre-requisite for Linux-managed memory-capacity provisioning is this
> > cxl_mem driver that can speak the "type-3 mailbox" protocol.
>
> "Type 3" to indicate that this is a proper adjective that can be
> looked up in the spec and to match the usage there.
>
> The r1.1 spec I have doesn't mention "mailbox". Is that also
> something defined in the 2.0 spec?

Yes, these device types are new to 2.0.

>
> A URL or similar citation for the spec would be nice somewhere.
>

Agreed. For the patches I authored at least, it seemed repetitive to put a Link:
in each one to the spec. It was meant to be in the cover letter, but obviously I
missed that. Do you have a suggestion there, is cover letter good enough?

> > For now just land the driver boiler-plate and fill it in with
> > functionality in subsequent commits.
> >
> > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> > Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
> > ---
> > drivers/cxl/Kconfig | 20 +++++++++++
> > drivers/cxl/Makefile | 2 ++
> > drivers/cxl/mem.c | 82 ++++++++++++++++++++++++++++++++++++++++++++
> > drivers/cxl/pci.h | 15 ++++++++
> > 4 files changed, 119 insertions(+)
> > create mode 100644 drivers/cxl/mem.c
> > create mode 100644 drivers/cxl/pci.h
> >
> > diff --git a/drivers/cxl/Kconfig b/drivers/cxl/Kconfig
> > index dd724bd364df..15548f5c77ff 100644
> > --- a/drivers/cxl/Kconfig
> > +++ b/drivers/cxl/Kconfig
> > @@ -27,4 +27,24 @@ config CXL_ACPI
> > resources described by the CEDT (CXL Early Discovery Table)
> >
> > Say 'y' to enable CXL (Compute Express Link) drivers.
> > +
> > +config CXL_MEM
> > + tristate "CXL.mem Device Support"
> > + depends on PCI && CXL_BUS_PROVIDER != n
> > + default m if CXL_BUS_PROVIDER
> > + help
> > + The CXL.mem protocol allows a device to act as a provider of
> > + "System RAM" and/or "Persistent Memory" that is fully coherent
> > + as if the memory was attached to the typical CPU memory
> > + controller.
> > +
> > + Say 'y/m' to enable a driver named "cxl_mem.ko" that will attach
> > + to CXL.mem devices for configuration, provisioning, and health
> > + monitoring, the so called "type-3 mailbox". Note, this driver
>
> "Type 3"
>
> > + is required for dynamic provisioning of CXL.mem attached
> > + memory, a pre-requisite for persistent memory support, but
> > + devices that provide volatile memory may be fully described by
> > + existing platform firmware memory enumeration.
> > +
> > + If unsure say 'n'.
> > endif
> > diff --git a/drivers/cxl/Makefile b/drivers/cxl/Makefile
> > index d38cd34a2582..97fdffb00f2d 100644
> > --- a/drivers/cxl/Makefile
> > +++ b/drivers/cxl/Makefile
> > @@ -1,5 +1,7 @@
> > # SPDX-License-Identifier: GPL-2.0
> > obj-$(CONFIG_CXL_ACPI) += cxl_acpi.o
> > +obj-$(CONFIG_CXL_MEM) += cxl_mem.o
> >
> > ccflags-y += -DDEFAULT_SYMBOL_NAMESPACE=CXL
> > cxl_acpi-y := acpi.o
> > +cxl_mem-y := mem.o
> > diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> > new file mode 100644
> > index 000000000000..aa7d881fa47b
> > --- /dev/null
> > +++ b/drivers/cxl/mem.c
> > @@ -0,0 +1,82 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +// Copyright(c) 2020 Intel Corporation. All rights reserved.
> > +#include <linux/module.h>
> > +#include <linux/pci.h>
> > +#include <linux/io.h>
> > +#include "acpi.h"
> > +#include "pci.h"
> > +
> > +struct cxl_mem {
> > + void __iomem *regs;
> > +};
>
> Unused, maybe move it to the patch that adds the use?
>

This is a remnant from when Dan gave me the basis to do the mmio work. I agree
it can be removed now.

> > +static int cxl_mem_dvsec(struct pci_dev *pdev, int dvsec)
> > +{
> > + int pos;
> > +
> > + pos = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_DVSEC);
> > + if (!pos)
> > + return 0;
> > +
> > + while (pos) {
> > + u16 vendor, id;
> > +
> > + pci_read_config_word(pdev, pos + PCI_DVSEC_VENDOR_OFFSET, &vendor);
> > + pci_read_config_word(pdev, pos + PCI_DVSEC_ID_OFFSET, &id);
> > + if (vendor == PCI_DVSEC_VENDOR_CXL && dvsec == id)
> > + return pos;
> > +
> > + pos = pci_find_next_ext_capability(pdev, pos, PCI_EXT_CAP_ID_DVSEC);
> > + }
> > +
> > + return 0;
> > +}
>
> I assume we'll refactor and move this into the PCI core after we
> resolve the several places this is needed. When we do that, the
> vendor would be passed in, so maybe we should do that here to make it
> simpler to move this to the PCI core.
>

I think we'll need to keep this in order to try to keep the dream alive of
loading a CXL kernel module on an older kernel. However, PCI code would benefit
from having it (in an ideal world, it'd only be there).

> > +static int cxl_mem_probe(struct pci_dev *pdev, const struct pci_device_id *id)
> > +{
> > + struct device *dev = &pdev->dev;
> > + struct cxl_mem *cxlm;
> > + int rc, regloc;
> > +
> > + rc = cxl_bus_prepared(pdev);
> > + if (rc != 0) {
> > + dev_err(dev, "failed to acquire interface\n");
>
> Interesting naming: apparently when cxl_bus_prepared() returns a
> non-zero ("true") value, it is actually *not* prepared?
>

This looks like a rebase fail to me, but I'll let Dan answer.

> > + return rc;
> > + }
> > +
> > + regloc = cxl_mem_dvsec(pdev, PCI_DVSEC_ID_CXL_REGLOC);
> > + if (!regloc) {
> > + dev_err(dev, "register location dvsec not found\n");
> > + return -ENXIO;
> > + }
> > +
> > + cxlm = devm_kzalloc(dev, sizeof(*cxlm), GFP_KERNEL);
> > + if (!cxlm)
> > + return -ENOMEM;
>
> Unused. And [4/9] removes it before it's *ever* used :)
>

Same as a few above, I think Dan was providing this for me to implement the
reset. It could go away...

> > + return 0;
> > +}
> > +
> > +static void cxl_mem_remove(struct pci_dev *pdev)
> > +{
> > +}
> > +
> > +static const struct pci_device_id cxl_mem_pci_tbl[] = {
> > + /* PCI class code for CXL.mem Type-3 Devices */
> > + { PCI_ANY_ID, PCI_ANY_ID, PCI_ANY_ID, PCI_ANY_ID,
> > + PCI_CLASS_MEMORY_CXL, 0xffffff, 0 },
> > + { /* terminate list */ },
> > +};
> > +MODULE_DEVICE_TABLE(pci, cxl_mem_pci_tbl);
> > +
> > +static struct pci_driver cxl_mem_driver = {
> > + .name = KBUILD_MODNAME,
> > + .id_table = cxl_mem_pci_tbl,
> > + .probe = cxl_mem_probe,
> > + .remove = cxl_mem_remove,
> > +};
> > +
> > +MODULE_LICENSE("GPL v2");
> > +MODULE_AUTHOR("Intel Corporation");
> > +module_pci_driver(cxl_mem_driver);
> > +MODULE_IMPORT_NS(CXL);
> > diff --git a/drivers/cxl/pci.h b/drivers/cxl/pci.h
> > new file mode 100644
> > index 000000000000..beb03921e6da
> > --- /dev/null
>
> > +++ b/drivers/cxl/pci.h
> > @@ -0,0 +1,15 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +// Copyright(c) 2020 Intel Corporation. All rights reserved.
>
> /* SPDX-... */
> /* Copyright ...*/
>
> The SPDX rules are a bit arcane and annoyingly hard to grep for, but
> I found them in Documentation/process/license-rules.rst
>
> > +#ifndef __CXL_PCI_H__
> > +#define __CXL_PCI_H__
> > +
> > +#define PCI_CLASS_MEMORY_CXL 0x050210
> > +
> > +#define PCI_EXT_CAP_ID_DVSEC 0x23
> > +#define PCI_DVSEC_VENDOR_CXL 0x1E98
> > +#define PCI_DVSEC_VENDOR_OFFSET 0x4
> > +#define PCI_DVSEC_ID_OFFSET 0x8
> > +#define PCI_DVSEC_ID_CXL 0x0
> > +#define PCI_DVSEC_ID_CXL_REGLOC 0x8
>
> I assume these will go in include/linux/pci_ids.h (PCI_CLASS_...) and
> include/uapi/linux/pci_regs.h (the rest) eventually, after we get the
> merge issues sorted out. But if they're only used in cxl/mem.c, I'd
> put them there for now.
>
> > +#endif /* __CXL_PCI_H__ */
> > --
> > 2.29.2
> >