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

Jonathan Cameron
2020-11-17T14:50:00Z

On Tue, 10 Nov 2020 21:43:50 -0800
Ben Widawsky <ben.widawsky@intel.com> 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.
>
> 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>

I've tried to avoid repeats, so mostly this is me moaning about naming!

Jonathan

> ---
> 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
> + 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;
> +};
> +
> +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);

This is good generic code and wouldn't cause much backport effort (even if needed
to bring in a local copy), so perhaps make it a generic function and move to
core PCI code?

Mind you I guess that can happen the 'second' time someone wants to find a DVSEC.

> + }
> +
> + return 0;
> +}
> +
> +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");
> + 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;
> +
> + return 0;
> +}
> +
> +static void cxl_mem_remove(struct pci_dev *pdev)
> +{
> +}

I'd bring this in only when needed in later patch.

> +
> +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.
> +#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

Hmm. The magic question of what to call a vendor ID that isn't a vendor
ID but just a magic number that talks like a duck and quacks like a duck
(for anyone wondering what I'm talking about, there is a nice bit of legal
boilerplate on this in the CXL spec)

This name is definitely not accurate however.

PCI_UNIQUE_VALUE_CXL maybe? It is used for other things than DVSEC (VDMs etc),
though possibly this is the only software visible use.

> +#define PCI_DVSEC_VENDOR_OFFSET 0x4
> +#define PCI_DVSEC_ID_OFFSET 0x8

Put a line break here perhaps and maybe a spec reference to where to find
the various DVSEC IDs.

> +#define PCI_DVSEC_ID_CXL 0x0

That's definitely a confusing name as well.

PCI_DEVSEC_ID_CXL_DEVICE maybe?

> +#define PCI_DVSEC_ID_CXL_REGLOC 0x8
> +
> +#endif /* __CXL_PCI_H__ */