Re: [PATCH v1 3/7] PCI: New Primary to Sideband (P2SB) bridge support library

Bjorn Helgaas
2021-03-08T18:53:06Z

On Mon, Mar 08, 2021 at 02:20:16PM +0200, Andy Shevchenko wrote:
> From: Jonathan Yong <jonathan.yong@intel.com>
>
> There is already one and at least one more user is coming which
> requires an access to Primary to Sideband bridge (P2SB) in order to
> get IO or MMIO bar hidden by BIOS. Create a library to access P2SB
> for x86 devices.

Can you include a spec reference? I'm trying to figure out why this
belongs in drivers/pci/. It looks very device-specific.

> Signed-off-by: Jonathan Yong <jonathan.yong@intel.com>
> Co-developed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> ---
> drivers/pci/Kconfig | 8 ++++
> drivers/pci/Makefile | 1 +
> drivers/pci/pci-p2sb.c | 83 ++++++++++++++++++++++++++++++++++++++++
> include/linux/pci-p2sb.h | 28 ++++++++++++++
> 4 files changed, 120 insertions(+)
> create mode 100644 drivers/pci/pci-p2sb.c
> create mode 100644 include/linux/pci-p2sb.h
>
> diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
> index 0c473d75e625..740e5b30d6fd 100644
> --- a/drivers/pci/Kconfig
> +++ b/drivers/pci/Kconfig
> @@ -252,6 +252,14 @@ config PCIE_BUS_PEER2PEER
>
> endchoice
>
> +config PCI_P2SB
> + bool "Primary to Sideband (P2SB) bridge access support"
> + depends on PCI && X86
> + help
> + The Primary to Sideband bridge is an interface to some PCI
> + devices connected through it. In particular, SPI NOR
> + controller in Intel Apollo Lake SoC is one of such devices.

This doesn't sound like a "bridge". If it's a bridge, what's on the
primary (upstream) side? What's on the secondary side? What
resources are passed through the bridge, i.e., what transactions does
it transfer from one side to the other?

> source "drivers/pci/hotplug/Kconfig"
> source "drivers/pci/controller/Kconfig"
> source "drivers/pci/endpoint/Kconfig"
> diff --git a/drivers/pci/Makefile b/drivers/pci/Makefile
> index d62c4ac4ae1b..eee8d5dda7d9 100644
> --- a/drivers/pci/Makefile
> +++ b/drivers/pci/Makefile
> @@ -23,6 +23,7 @@ obj-$(CONFIG_PCI_IOV) += iov.o
> obj-$(CONFIG_PCI_BRIDGE_EMUL) += pci-bridge-emul.o
> obj-$(CONFIG_PCI_LABEL) += pci-label.o
> obj-$(CONFIG_X86_INTEL_MID) += pci-mid.o
> +obj-$(CONFIG_PCI_P2SB) += pci-p2sb.o
> obj-$(CONFIG_PCI_SYSCALL) += syscall.o
> obj-$(CONFIG_PCI_STUB) += pci-stub.o
> obj-$(CONFIG_PCI_PF_STUB) += pci-pf-stub.o
> diff --git a/drivers/pci/pci-p2sb.c b/drivers/pci/pci-p2sb.c
> new file mode 100644
> index 000000000000..68d7dad48cdb
> --- /dev/null
> +++ b/drivers/pci/pci-p2sb.c
> @@ -0,0 +1,83 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Primary to Sideband bridge (P2SB) access support
> + *
> + * Copyright (c) 2017, 2021 Intel Corporation.
> + *
> + * Authors: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> + * Jonathan Yong <jonathan.yong@intel.com>
> + */
> +
> +#include <linux/bitops.h>
> +#include <linux/export.h>
> +#include <linux/pci-p2sb.h>
> +
> +#include <asm/cpu_device_id.h>
> +#include <asm/intel-family.h>
> +
> +#include "pci.h"
> +
> +#define P2SBC_HIDE_BYTE 0xe1
> +#define P2SBC_HIDE_BIT BIT(0)
> +
> +static const struct x86_cpu_id p2sb_cpu_ids[] = {
> + X86_MATCH_INTEL_FAM6_MODEL(ATOM_GOLDMONT, PCI_DEVFN(13, 0)),
> + {}
> +};
> +
> +static int pci_p2sb_devfn(unsigned int *devfn)
> +{
> + const struct x86_cpu_id *id;
> +
> + id = x86_match_cpu(p2sb_cpu_ids);
> + if (!id)
> + return -ENODEV;
> +
> + *devfn = (unsigned int)id->driver_data;
> + return 0;
> +}
> +
> +/**
> + * pci_p2sb_bar - Get Primary to Sideband bridge (P2SB) device BAR
> + * @pdev: PCI device to get a PCI bus to communicate with
> + * @devfn: PCI slot and function to communicate with
> + * @mem: memory resource to be filled in
> + *
> + * The BIOS prevents the P2SB device from being enumerated by the PCI
> + * subsystem, so we need to unhide and hide it back to lookup the BAR.
> + *
> + * Caller must provide a valid pointer to @mem.
> + *
> + * Locking is handled by pci_rescan_remove_lock mutex.
> + *
> + * Return:
> + * 0 on success or appropriate errno value on error.
> + */
> +int pci_p2sb_bar(struct pci_dev *pdev, unsigned int devfn, struct resource *mem)
> +{
> + struct pci_bus *bus = pdev->bus;
> + unsigned int df;
> + int ret;
> +
> + /* Get devfn for P2SB device itself */
> + ret = pci_p2sb_devfn(&df);
> + if (ret)
> + return ret;
> +
> + pci_lock_rescan_remove();
> +
> + /* Unhide the P2SB device */
> + pci_bus_write_config_byte(bus, df, P2SBC_HIDE_BYTE, 0);
> +
> + /* Read the first BAR of the device in question */
> + __pci_bus_read_base(bus, devfn, pci_bar_unknown, mem, PCI_BASE_ADDRESS_0, true);

I don't get this. Apparently this normally hidden device is consuming
PCI address space. The PCI core needs to know about this. If it
doesn't, the PCI core may assign this space to another device.

> + /* Hide the P2SB device */
> + pci_bus_write_config_byte(bus, df, P2SBC_HIDE_BYTE, P2SBC_HIDE_BIT);
> +
> + pci_unlock_rescan_remove();
> +
> + pci_bus_info(bus, devfn, "BAR: %pR\n", mem);
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(pci_p2sb_bar);
> diff --git a/include/linux/pci-p2sb.h b/include/linux/pci-p2sb.h
> new file mode 100644
> index 000000000000..15dd42737c84
> --- /dev/null
> +++ b/include/linux/pci-p2sb.h
> @@ -0,0 +1,28 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Primary to Sideband bridge (P2SB) access support
> + */
> +
> +#ifndef _PCI_P2SB_H
> +#define _PCI_P2SB_H
> +
> +#include <linux/errno.h>
> +
> +struct pci_dev;
> +struct resource;
> +
> +#if IS_BUILTIN(CONFIG_PCI_P2SB)
> +
> +int pci_p2sb_bar(struct pci_dev *pdev, unsigned int devfn, struct resource *mem);
> +
> +#else /* CONFIG_PCI_P2SB is not set */
> +
> +static inline
> +int pci_p2sb_bar(struct pci_dev *pdev, unsigned int devfn, struct resource *mem)
> +{
> + return -ENODEV;
> +}
> +
> +#endif /* CONFIG_PCI_P2SB */
> +
> +#endif /* _PCI_P2SB_H */
> --
> 2.30.1
>