Re: [PATCH] PCI: hotplug: Add extension driver for Ampere Altra hotplug LED control

[Rafael J. Wysocki]

On Thu, Sep 28, 2023 at 5:47 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>
> On Wed, Sep 27, 2023 at 01:23:47PM -0700, D Scott Phillips wrote:
> > On Ampere Altra, PCIe hotplug is handled through ACPI. A side interface is
> > also present to request system firmware control of attention LEDs. Add an
> > ACPI PCI Hotplug companion driver to support attention LED control.
> >
> > Signed-off-by: D Scott Phillips <scott@xxxxxxxxxxxxxxxxxxxxxx>
> > ---
> >  drivers/pci/hotplug/Kconfig                |  13 ++
> >  drivers/pci/hotplug/Makefile               |   3 +-
> >  drivers/pci/hotplug/acpiphp_ampere_altra.c | 141 +++++++++++++++++++++
> >  3 files changed, 156 insertions(+), 1 deletion(-)
> >  create mode 100644 drivers/pci/hotplug/acpiphp_ampere_altra.c
> >
> > diff --git a/drivers/pci/hotplug/Kconfig b/drivers/pci/hotplug/Kconfig
> > index 48113b210cf93..9fde600a9ad3e 100644
> > --- a/drivers/pci/hotplug/Kconfig
> > +++ b/drivers/pci/hotplug/Kconfig
> > @@ -61,6 +61,19 @@ config HOTPLUG_PCI_ACPI
> >
> >         When in doubt, say N.
> >
> > +config HOTPLUG_PCI_ACPI_AMPERE_ALTRA
> > +     tristate "ACPI PCI Hotplug driver Ampere Altra extensions"
> > +     depends on HOTPLUG_PCI_ACPI
> > +     depends on HAVE_ARM_SMCCC_DISCOVERY
> > +     depends on m
>
> Why is this restricted to being a module?  It's not unprecedented, but
> unless this only works as a module for some reason, I would leave that
> choice up to the user.
>
> > +     help
> > +       Say Y here if you have an Ampere Altra system.
> > +
> > +       To compile this driver as a module, choose M here: the
> > +       module will be called acpiphp_ampere_altra.
> > +
> > +       When in doubt, say N.
> > +
> >  config HOTPLUG_PCI_ACPI_IBM
> >       tristate "ACPI PCI Hotplug driver IBM extensions"
> >       depends on HOTPLUG_PCI_ACPI
> > diff --git a/drivers/pci/hotplug/Makefile b/drivers/pci/hotplug/Makefile
> > index 5196983220df6..29d7f6171b305 100644
> > --- a/drivers/pci/hotplug/Makefile
> > +++ b/drivers/pci/hotplug/Makefile
> > @@ -21,8 +21,9 @@ obj-$(CONFIG_HOTPLUG_PCI_RPA_DLPAR) += rpadlpar_io.o
> >  obj-$(CONFIG_HOTPLUG_PCI_ACPI)               += acpiphp.o
> >  obj-$(CONFIG_HOTPLUG_PCI_S390)               += s390_pci_hpc.o
> >
> > -# acpiphp_ibm extends acpiphp, so should be linked afterwards.
> > +# acpiphp_ibm extend acpiphp, so should be linked afterwards.
> >
> > +obj-$(CONFIG_HOTPLUG_PCI_ACPI_AMPERE_ALTRA)  += acpiphp_ampere_altra.o
> >  obj-$(CONFIG_HOTPLUG_PCI_ACPI_IBM)   += acpiphp_ibm.o
> >
> >  pci_hotplug-objs     :=      pci_hotplug_core.o
> > diff --git a/drivers/pci/hotplug/acpiphp_ampere_altra.c b/drivers/pci/hotplug/acpiphp_ampere_altra.c
> > new file mode 100644
> > index 0000000000000..8692b939dea78
> > --- /dev/null
> > +++ b/drivers/pci/hotplug/acpiphp_ampere_altra.c
> > @@ -0,0 +1,141 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * ACPI PCI Hot Plug Ampere Altra Extension
>
> Would be helpful to include a hint about what this module *does*.
> IIUC, it controls the attention indicator.
>
> > + *
> > + * Copyright (C) 2023 Ampere Computing LLC
> > + *
>
> Spurious blank line.
>
> > + */
> > +
> > +#define pr_fmt(fmt) "acpiphp_ampere_altra: " fmt
> > +
> > +#include <linux/init.h>
> > +#include <linux/module.h>
> > +#include <linux/pci.h>
> > +#include <linux/pci_hotplug.h>
> > +
> > +#include "acpiphp.h"
> > +
> > +#define HANDLE_OPEN  0xb0200000
> > +#define HANDLE_CLOSE 0xb0300000
> > +#define REQUEST              0xf0700000
> > +#define LED_CMD              0x00000004
> > +#define LED_ATTENTION        0x00000002
> > +#define LED_SET_ON   0x00000001
> > +#define LED_SET_OFF  0x00000002
> > +#define LED_SET_BLINK        0x00000003
> > +
> > +static const struct acpi_device_id acpi_ids[] = {
> > +     {"AMPC0008", 0}, {}
> > +};
> > +MODULE_DEVICE_TABLE(acpi, acpi_ids);
> > +
> > +static u32 led_service_id[4];
> > +
> > +static int led_status(u8 status)
> > +{
> > +     switch (status) {
> > +     case 1: return LED_SET_ON;
> > +     case 2: return LED_SET_BLINK;
> > +     default: return LED_SET_OFF;
> > +     }
> > +}
> > +
> > +static int set_attention_status(struct hotplug_slot *slot, u8 status)
> > +{
> > +     struct arm_smccc_res res;
> > +     struct pci_bus *bus;
> > +     struct pci_dev *root_port;
> > +     unsigned long flags;
> > +     u32 handle;
> > +     int ret = 0;
> > +
> > +     bus = slot->pci_slot->bus;
> > +     root_port = pcie_find_root_port(bus->self);
> > +     if (!root_port)
> > +             return -ENODEV;
> > +
> > +     local_irq_save(flags);
> > +     arm_smccc_smc(HANDLE_OPEN, led_service_id[0], led_service_id[1],
> > +                   led_service_id[2], led_service_id[3], 0, 0, 0, &res);
> > +     if (res.a0) {
> > +             ret = -ENODEV;
> > +             goto out;
> > +     }
> > +     handle = res.a1 & 0xffff0000;
> > +
> > +     arm_smccc_smc(REQUEST, LED_CMD, led_status(status), LED_ATTENTION,
> > +                   pci_domain_nr(bus) | ((root_port->devfn >> 3) << 4), 0, 0,
>
> PCI_SLOT(root_port->devfn)
>
> > +                   handle, &res);
> > +     if (res.a0)
> > +             ret = -ENODEV;
> > +
> > +     arm_smccc_smc(HANDLE_CLOSE, handle, 0, 0, 0, 0, 0, 0, &res);
> > +
> > + out:
> > +     local_irq_restore(flags);
> > +     return ret;
> > +}
> > +
> > +static int get_attention_status(struct hotplug_slot *slot, u8 *status)
> > +{
> > +     return -EINVAL;
> > +}
> > +
> > +static struct acpiphp_attention_info ampere_altra_attn = {
> > +     .set_attn = set_attention_status,
> > +     .get_attn = get_attention_status,
> > +     .owner = THIS_MODULE,
> > +};
> > +
> > +static acpi_status __init get_acpi_handle(acpi_handle handle, u32 level,
> > +                                       void *context, void **return_value)
> > +{
> > +     *(acpi_handle *)return_value = handle;
> > +     return AE_CTRL_TERMINATE;
> > +}
> > +
> > +static int __init acpiphp_ampere_altra_init(void)
> > +{
> > +     struct fwnode_handle *fwnode;
> > +     acpi_handle leds_handle = NULL;
> > +     struct acpi_device *leds;
> > +     acpi_status status;
> > +     int ret;
> > +
> > +     status = acpi_get_devices("AMPC0008", get_acpi_handle, NULL,
> > +                               &leds_handle);
>
> Rafael, can you comment on whether we should use acpi_get_devices(),
> acpi_bus_register_driver(), acpi_acpi_scan_add_handler(), or something
> else here?

Personally, I would go for a platform driver, because the ACPI core
should create a platform device for this object.

acpi_get_devices() carries out a namespace walk that is costly and
entirely avoidable.

>  I try to avoid pci_get_device() because it subverts the
> driver model (no hotplug support, no driver/device binding).
>
> I see Documentation/driver-api/acpi/scan_handlers.rst, but I'm not
> clear on when to use acpi_bus_register_driver() vs

Never.

> acpi_acpi_scan_add_handler().

When you don't want the ACPI core to create a platform device for your
ACPI device object.  There are cases like that, but they are rare.

> I guess the only ACPI connection here is to retrieve the "uuid"
> property once, and there's no need for any ACPI services after that.

That's in agreement with my understanding.