Re: [PATCH v2 1/8] cxl/mem: Introduce a driver for CXL-2.0-Type-3 endpoints

Jonathan Cameron
2021-02-10T16:19:11Z

On Tue, 9 Feb 2021 16:02:52 -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.
>
> With the CXL-2.0 specification a PCI endpoint can implement a "Type-3"
> device interface and give the operating system control over "Host
> Managed Device Memory". See section 2.3 Type 3 CXL Device.
>
> 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 mailbox protocol defined in section
> 8.2.8.4 Mailbox Registers.
>
> For now just land the initial driver boiler-plate and Documentation/
> infrastructure.
>
> Link: https://www.computeexpresslink.org/download-the-specification
> Cc: Jonathan Corbet <corbet@lwn.net>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
> Acked-by: David Rientjes <rientjes@google.com> (v1)

A few trivial bits inline but nothing that I feel that strongly about.
It is probably a good idea to add a note about generic dvsec code
somewhere in this patch description (to avoid people raising it on
future versions!)

With the define of PCI_EXT_CAP_ID_DVSEC dropped (it's in the generic
header already).

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

> ---
> Documentation/driver-api/cxl/index.rst | 12 ++++
> .../driver-api/cxl/memory-devices.rst | 29 +++++++++
> Documentation/driver-api/index.rst | 1 +
> drivers/Kconfig | 1 +
> drivers/Makefile | 1 +
> drivers/cxl/Kconfig | 35 +++++++++++
> drivers/cxl/Makefile | 4 ++
> drivers/cxl/mem.c | 63 +++++++++++++++++++
> drivers/cxl/pci.h | 18 ++++++
> include/linux/pci_ids.h | 1 +
> 10 files changed, 165 insertions(+)
> create mode 100644 Documentation/driver-api/cxl/index.rst
> create mode 100644 Documentation/driver-api/cxl/memory-devices.rst
> create mode 100644 drivers/cxl/Kconfig
> create mode 100644 drivers/cxl/Makefile
> create mode 100644 drivers/cxl/mem.c
> create mode 100644 drivers/cxl/pci.h
>
> diff --git a/Documentation/driver-api/cxl/index.rst b/Documentation/driver-api/cxl/index.rst
> new file mode 100644
> index 000000000000..036e49553542
> --- /dev/null
> +++ b/Documentation/driver-api/cxl/index.rst
> @@ -0,0 +1,12 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +====================
> +Compute Express Link
> +====================
> +
> +.. toctree::
> + :maxdepth: 1
> +
> + memory-devices
> +
> +.. only:: subproject and html
> diff --git a/Documentation/driver-api/cxl/memory-devices.rst b/Documentation/driver-api/cxl/memory-devices.rst
> new file mode 100644
> index 000000000000..43177e700d62
> --- /dev/null
> +++ b/Documentation/driver-api/cxl/memory-devices.rst
> @@ -0,0 +1,29 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +.. include:: <isonum.txt>
> +
> +===================================
> +Compute Express Link Memory Devices
> +===================================
> +
> +A Compute Express Link Memory Device is a CXL component that implements the
> +CXL.mem protocol. It contains some amount of volatile memory, persistent memory,
> +or both. It is enumerated as a PCI device for configuration and passing
> +messages over an MMIO mailbox. Its contribution to the System Physical
> +Address space is handled via HDM (Host Managed Device Memory) decoders
> +that optionally define a device's contribution to an interleaved address
> +range across multiple devices underneath a host-bridge or interleaved
> +across host-bridges.
> +
> +Driver Infrastructure
> +=====================
> +
> +This section covers the driver infrastructure for a CXL memory device.
> +
> +CXL Memory Device
> +-----------------
> +
> +.. kernel-doc:: drivers/cxl/mem.c
> + :doc: cxl mem
> +
> +.. kernel-doc:: drivers/cxl/mem.c
> + :internal:
> diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
> index 2456d0a97ed8..d246a18fd78f 100644
> --- a/Documentation/driver-api/index.rst
> +++ b/Documentation/driver-api/index.rst
> @@ -35,6 +35,7 @@ available subsections can be seen below.
> usb/index
> firewire
> pci/index
> + cxl/index
> spi
> i2c
> ipmb
> diff --git a/drivers/Kconfig b/drivers/Kconfig
> index dcecc9f6e33f..62c753a73651 100644
> --- a/drivers/Kconfig
> +++ b/drivers/Kconfig
> @@ -6,6 +6,7 @@ menu "Device Drivers"
> source "drivers/amba/Kconfig"
> source "drivers/eisa/Kconfig"
> source "drivers/pci/Kconfig"
> +source "drivers/cxl/Kconfig"
> source "drivers/pcmcia/Kconfig"
> source "drivers/rapidio/Kconfig"
>
> diff --git a/drivers/Makefile b/drivers/Makefile
> index fd11b9ac4cc3..678ea810410f 100644
> --- a/drivers/Makefile
> +++ b/drivers/Makefile
> @@ -73,6 +73,7 @@ obj-$(CONFIG_NVM) += lightnvm/
> obj-y += base/ block/ misc/ mfd/ nfc/
> obj-$(CONFIG_LIBNVDIMM) += nvdimm/
> obj-$(CONFIG_DAX) += dax/
> +obj-$(CONFIG_CXL_BUS) += cxl/
> obj-$(CONFIG_DMA_SHARED_BUFFER) += dma-buf/
> obj-$(CONFIG_NUBUS) += nubus/
> obj-y += macintosh/
> diff --git a/drivers/cxl/Kconfig b/drivers/cxl/Kconfig
> new file mode 100644
> index 000000000000..9e80b311e928
> --- /dev/null
> +++ b/drivers/cxl/Kconfig
> @@ -0,0 +1,35 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +menuconfig CXL_BUS
> + tristate "CXL (Compute Express Link) Devices Support"
> + depends on PCI
> + help
> + CXL is a bus that is electrically compatible with PCI Express, but
> + layers three protocols on that signalling (CXL.io, CXL.cache, and
> + CXL.mem). The CXL.cache protocol allows devices to hold cachelines
> + locally, the CXL.mem protocol allows devices to be fully coherent
> + memory targets, the CXL.io protocol is equivalent to PCI Express.
> + Say 'y' to enable support for the configuration and management of
> + devices supporting these protocols.
> +
> +if CXL_BUS
> +
> +config CXL_MEM
> + tristate "CXL.mem: Memory Devices"
> + 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" when built as
> + a module) that will attach to CXL.mem devices for
> + configuration, provisioning, and health monitoring. This
> + driver is required for dynamic provisioning of CXL.mem
> + attached memory which is a prerequisite for persistent memory
> + support. Typically volatile memory is mapped by platform
> + firmware and included in the platform memory map, but in some
> + cases the OS is responsible for mapping that memory. See
> + Chapter 2.3 Type 3 CXL Device in the CXL 2.0 specification.
> +
> + If unsure say 'm'.
> +endif
> diff --git a/drivers/cxl/Makefile b/drivers/cxl/Makefile
> new file mode 100644
> index 000000000000..4a30f7c3fc4a
> --- /dev/null
> +++ b/drivers/cxl/Makefile
> @@ -0,0 +1,4 @@
> +# SPDX-License-Identifier: GPL-2.0
> +obj-$(CONFIG_CXL_MEM) += cxl_mem.o
> +
> +cxl_mem-y := mem.o
> diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> new file mode 100644
> index 000000000000..99a6571508df
> --- /dev/null
> +++ b/drivers/cxl/mem.c
> @@ -0,0 +1,63 @@
> +// 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 "pci.h"
> +
> +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_HEADER1, &vendor);
> + pci_read_config_word(pdev, pos + PCI_DVSEC_HEADER2, &id);
> + if (vendor == PCI_DVSEC_VENDOR_ID_CXL && dvsec == id)
> + return pos;
> +
> + pos = pci_find_next_ext_capability(pdev, pos,
> + PCI_EXT_CAP_ID_DVSEC);
> + }
> +
> + return 0;

Christopher Hellwig raised this in v1.

https://lore.kernel.org/linux-pci/20201104201141.GA399378@bjorn-Precision-5520/

+CC Dave Jiang for update on that.

This wants to move towards a generic helper. We can do the deduplication
later as Bjorn suggested.

> +}
> +
> +static int cxl_mem_probe(struct pci_dev *pdev, const struct pci_device_id *id)
> +{
> + struct device *dev = &pdev->dev;
> + int regloc;
> +
> + regloc = cxl_mem_dvsec(pdev, PCI_DVSEC_ID_CXL_REGLOC_OFFSET);
> + if (!regloc) {
> + dev_err(dev, "register location dvsec not found\n");
> + return -ENXIO;
> + }
> +
> + return 0;
> +}
> +
> +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 << 8 | CXL_MEMORY_PROGIF, 0xffffff, 0 },

Having looked at this and thought 'thats a bit tricky to check'
I did a quick grep and seems the kernel is split between this approach
and people going with the mor readable c99 style initiators
.class = .. etc

Personally I'd find the c99 approach easier to read.

> + { /* 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,
> + .driver = {
> + .probe_type = PROBE_PREFER_ASYNCHRONOUS,
> + },
> +};
> +
> +MODULE_LICENSE("GPL v2");
> +module_pci_driver(cxl_mem_driver);
> diff --git a/drivers/cxl/pci.h b/drivers/cxl/pci.h
> new file mode 100644
> index 000000000000..f135b9f7bb21
> --- /dev/null
> +++ b/drivers/cxl/pci.h
> @@ -0,0 +1,18 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/* Copyright(c) 2020 Intel Corporation. All rights reserved. */
> +#ifndef __CXL_PCI_H__
> +#define __CXL_PCI_H__
> +
> +#define CXL_MEMORY_PROGIF 0x10
> +
> +/*
> + * See section 8.1 Configuration Space Registers in the CXL 2.0
> + * Specification
> + */
> +#define PCI_EXT_CAP_ID_DVSEC 0x23

This is already in include/uapi/linux/pci_regs.h

> +#define PCI_DVSEC_VENDOR_ID_CXL 0x1E98
> +#define PCI_DVSEC_ID_CXL 0x0
> +
> +#define PCI_DVSEC_ID_CXL_REGLOC_OFFSET 0x8
> +
> +#endif /* __CXL_PCI_H__ */
> diff --git a/include/linux/pci_ids.h b/include/linux/pci_ids.h
> index d8156a5dbee8..766260a9b247 100644
> --- a/include/linux/pci_ids.h
> +++ b/include/linux/pci_ids.h
> @@ -51,6 +51,7 @@
> #define PCI_BASE_CLASS_MEMORY 0x05
> #define PCI_CLASS_MEMORY_RAM 0x0500
> #define PCI_CLASS_MEMORY_FLASH 0x0501
> +#define PCI_CLASS_MEMORY_CXL 0x0502
> #define PCI_CLASS_MEMORY_OTHER 0x0580
>
> #define PCI_BASE_CLASS_BRIDGE 0x06