Re: [net-next v34 1/1] mctp pcc: Implement MCTP over PCC Transport

[Adam Young <admiyo@amperemail.onmicrosoft.com>]

I just discovered a flaw in this implementation.  Withdrawing it from 
consideration.

On 3/17/26 18:43, Adam Young wrote:
> Implementation of network driver for
> Management Component Transport Protocol(MCTP)
> over Platform Communication Channel(PCC)
>
> DMTF DSP:0292
> https://www.dmtf.org/sites/default/files/standards/documents/\
> DSP0292_1.0.0WIP50.pdf
>
> MCTP devices are specified via ACPI by entries in DSDT/SSDT and
> reference channels specified in the PCCT. Messages are sent on a type
> 3 and received on a type 4 channel.  Communication with other devices
> use the PCC based doorbell mechanism; a shared memory segment with a
> corresponding interrupt and a memory register used to trigger remote
> interrupts.
>
> Unlike the existing PCC Type 2 based drivers, the mssg parameter to
> mbox_send_msg is actively used. The data section of the struct sk_buff
> that contains the outgoing packet is sent to the mailbox, already
> properly formatted as a PCC exctended message.
>
> If the mailbox ring buffer is full, the driver stops the incoming
> packet queues until a message has been sent, freeing space in the
> ring buffer.
>
> When the Type 3 channel outbox receives a txdone response interrupt,
> it consumes the outgoing sk_buff, allowing it to be freed.
>
> Bringing up an interface creates the channel between the network driver
> and the mailbox driver. This enables communication with the remote
> endpoint, to include the receipt of new messages. Bringing down an
> interface removes the channel, and no new messages can be delivered.
> Stopping the interface also frees any packets that are cached in the
> mailbox ringbuffer.
>
> Signed-off-by: Adam Young <admiyo@os.amperecomputing.com>
>
> change 1:  size may be negative
> change 2: use constant for checking MCTP signature
> change 3: put PCC data into the skb
> ---
>   MAINTAINERS                 |   5 +
>   drivers/net/mctp/Kconfig    |  13 ++
>   drivers/net/mctp/Makefile   |   1 +
>   drivers/net/mctp/mctp-pcc.c | 366 ++++++++++++++++++++++++++++++++++++
>   4 files changed, 385 insertions(+)
>   create mode 100644 drivers/net/mctp/mctp-pcc.c
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index ff6f17458f19..43bc37f859d6 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -15288,6 +15288,11 @@ F:	include/net/mctpdevice.h
>   F:	include/net/netns/mctp.h
>   F:	net/mctp/
>   
> +MANAGEMENT COMPONENT TRANSPORT PROTOCOL (MCTP) over PCC (MCTP-PCC) Driver
> +M:	Adam Young <admiyo@os.amperecomputing.com>
> +S:	Maintained
> +F:	drivers/net/mctp/mctp-pcc.c
> +
>   MAPLE TREE
>   M:	Liam R. Howlett <Liam.Howlett@oracle.com>
>   R:	Alice Ryhl <aliceryhl@google.com>
> diff --git a/drivers/net/mctp/Kconfig b/drivers/net/mctp/Kconfig
> index cf325ab0b1ef..77cd4091050c 100644
> --- a/drivers/net/mctp/Kconfig
> +++ b/drivers/net/mctp/Kconfig
> @@ -47,6 +47,19 @@ config MCTP_TRANSPORT_I3C
>   	  A MCTP protocol network device is created for each I3C bus
>   	  having a "mctp-controller" devicetree property.
>   
> +config MCTP_TRANSPORT_PCC
> +	tristate "MCTP PCC transport"
> +	depends on ACPI
> +	help
> +	  Provides a driver to access MCTP devices over PCC transport,
> +	  A MCTP protocol network device is created via ACPI for each
> +	  entry in the DSDT/SSDT that matches the identifier. The Platform
> +	  communication channels are selected from the corresponding
> +	  entries in the PCCT.
> +
> +	  Say y here if you need to connect to MCTP endpoints over PCC. To
> +	  compile as a module, use m; the module will be called mctp-pcc.
> +
>   config MCTP_TRANSPORT_USB
>   	tristate "MCTP USB transport"
>   	depends on USB
> diff --git a/drivers/net/mctp/Makefile b/drivers/net/mctp/Makefile
> index c36006849a1e..0a591299ffa9 100644
> --- a/drivers/net/mctp/Makefile
> +++ b/drivers/net/mctp/Makefile
> @@ -1,4 +1,5 @@
>   obj-$(CONFIG_MCTP_SERIAL) += mctp-serial.o
>   obj-$(CONFIG_MCTP_TRANSPORT_I2C) += mctp-i2c.o
>   obj-$(CONFIG_MCTP_TRANSPORT_I3C) += mctp-i3c.o
> +obj-$(CONFIG_MCTP_TRANSPORT_PCC) += mctp-pcc.o
>   obj-$(CONFIG_MCTP_TRANSPORT_USB) += mctp-usb.o
> diff --git a/drivers/net/mctp/mctp-pcc.c b/drivers/net/mctp/mctp-pcc.c
> new file mode 100644
> index 000000000000..f8d620795349
> --- /dev/null
> +++ b/drivers/net/mctp/mctp-pcc.c
> @@ -0,0 +1,366 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * mctp-pcc.c - Driver for MCTP over PCC.
> + * Copyright (c) 2024-2026, Ampere Computing LLC
> + *
> + */
> +
> +/* Implementation of MCTP over PCC DMTF Specification DSP0256
> + * https://www.dmtf.org/sites/default/files/standards/documents/DSP0292_1.0.0WIP50.pdf
> + */
> +
> +#include <linux/acpi.h>
> +#include <linux/hrtimer.h>
> +#include <linux/if_arp.h>
> +#include <linux/init.h>
> +#include <linux/kernel.h>
> +#include <linux/mailbox_client.h>
> +#include <linux/module.h>
> +#include <linux/netdevice.h>
> +#include <linux/platform_device.h>
> +#include <linux/skbuff.h>
> +#include <linux/string.h>
> +
> +#include <acpi/acpi_bus.h>
> +#include <acpi/acpi_drivers.h>
> +#include <acpi/acrestyp.h>
> +#include <acpi/actbl.h>
> +#include <acpi/pcc.h>
> +#include <net/mctp.h>
> +#include <net/mctpdevice.h>
> +
> +#define MCTP_SIGNATURE          "MCTP"
> +#define MCTP_SIGNATURE_LENGTH   (sizeof(MCTP_SIGNATURE) - 1)
> +#define MCTP_MIN_MTU            68
> +#define PCC_DWORD_TYPE          0x0c
> +
> +struct mctp_pcc_mailbox {
> +	u32 index;
> +	struct pcc_mbox_chan *chan;
> +	struct mbox_client client;
> +};
> +
> +/* The netdev structure. One of these per PCC adapter. */
> +struct mctp_pcc_ndev {
> +	struct net_device *ndev;
> +	struct acpi_device *acpi_device;
> +	struct mctp_pcc_mailbox inbox;
> +	struct mctp_pcc_mailbox outbox;
> +};
> +
> +/**
> + * mbox_write_to_buffer - copy the contents of the data
> + * pointer to the shared buffer.  Confirms that the command
> + * flag has been set prior to writing.  Data should be a
> + * properly formatted extended data buffer.
> + * pcc_mbox_write_to_buffer
> + * @pchan: channel
> + * @len: Length of the overall buffer passed in, including the
> + *       Entire header. The length value in the shared buffer header
> + *       Will be calculated from len.
> + * @data: Client specific data to be written to the shared buffer.
> + * Return: number of bytes written to the buffer.
> + */
> +static int mbox_write_to_buffer(struct pcc_mbox_chan *pchan, int len, void *data)
> +{
> +	struct acpi_pcct_ext_pcc_shared_memory *pcc_header = data;
> +	/*
> +	 * The PCC header length includes the command field
> +	 * but not the other values from the header.
> +	 */
> +	pcc_header->length = len - sizeof(struct acpi_pcct_ext_pcc_shared_memory) + sizeof(u32);
> +
> +	if (len > pchan->shmem_size)
> +		return 0;
> +
> +	memcpy_toio(pchan->shmem,  data, len);
> +
> +	return len;
> +}
> +
> +static void mctp_pcc_client_rx_callback(struct mbox_client *cl, void *mssg)
> +{
> +	struct acpi_pcct_ext_pcc_shared_memory pcc_header;
> +	struct mctp_pcc_ndev *mctp_pcc_ndev;
> +	struct mctp_pcc_mailbox *inbox;
> +	struct mctp_skb_cb *cb;
> +	struct sk_buff *skb;
> +	int size;
> +
> +	mctp_pcc_ndev = container_of(cl, struct mctp_pcc_ndev, inbox.client);
> +	inbox = &mctp_pcc_ndev->inbox;
> +	memcpy_fromio(&pcc_header, inbox->chan->shmem, sizeof(pcc_header));
> +	size = pcc_header.length - sizeof(pcc_header.command) + sizeof(pcc_header);
> +
> +	if (size <= sizeof(pcc_header)) {
> +		dev_dstats_rx_dropped(mctp_pcc_ndev->ndev);
> +		return;
> +	}
> +	if (memcmp(&pcc_header.command, MCTP_SIGNATURE, MCTP_SIGNATURE_LENGTH) != 0) {
> +		dev_dstats_rx_dropped(mctp_pcc_ndev->ndev);
> +		return;
> +	}
> +
> +	if (size > mctp_pcc_ndev->ndev->mtu)
> +		dev_dbg(cl->dev, "MCTP_PCC bytes available exceeds MTU");
> +
> +	skb = netdev_alloc_skb(mctp_pcc_ndev->ndev, size);
> +	if (!skb) {
> +		dev_dstats_rx_dropped(mctp_pcc_ndev->ndev);
> +		return;
> +	}
> +	skb_put(skb, size);
> +	skb->protocol = htons(ETH_P_MCTP);
> +	memcpy_fromio(skb->data, inbox->chan->shmem, size);
> +	dev_dstats_rx_add(mctp_pcc_ndev->ndev, size);
> +	skb_pull(skb, sizeof(pcc_header));
> +	skb_reset_mac_header(skb);
> +	skb_reset_network_header(skb);
> +	cb = __mctp_cb(skb);
> +	cb->halen = 0;
> +	netif_rx(skb);
> +}
> +
> +static netdev_tx_t mctp_pcc_tx(struct sk_buff *skb, struct net_device *ndev)
> +{
> +	struct acpi_pcct_ext_pcc_shared_memory *pcc_header;
> +	struct mctp_pcc_ndev *mpnd = netdev_priv(ndev);
> +	int len = skb->len;
> +	int rc;
> +
> +	rc = skb_cow_head(skb, sizeof(*pcc_header));
> +	if (rc) {
> +		dev_dstats_tx_dropped(ndev);
> +		kfree_skb(skb);
> +		return NETDEV_TX_OK;
> +	}
> +
> +	pcc_header = skb_push(skb, sizeof(*pcc_header));
> +	pcc_header->signature = PCC_SIGNATURE | mpnd->outbox.index;
> +	pcc_header->flags = PCC_CMD_COMPLETION_NOTIFY;
> +	memcpy(&pcc_header->command, MCTP_SIGNATURE, MCTP_SIGNATURE_LENGTH);
> +	pcc_header->length = len + MCTP_SIGNATURE_LENGTH;
> +
> +	rc = mbox_send_message(mpnd->outbox.chan->mchan, skb);
> +	if (rc < 0) {
> +		//Remove the header in case it gets sent again
> +		skb_pull(skb, sizeof(*pcc_header));
> +		netif_stop_queue(ndev);
> +		return NETDEV_TX_BUSY;
> +	}
> +
> +	return NETDEV_TX_OK;
> +}
> +
> +static void mctp_pcc_tx_prepare(struct mbox_client *cl, void *mssg)
> +{
> +	struct mctp_pcc_ndev *mctp_pcc_ndev;
> +	struct mctp_pcc_mailbox *outbox;
> +	struct sk_buff *skb = mssg;
> +	int len_sent;
> +
> +	mctp_pcc_ndev = container_of(cl, struct mctp_pcc_ndev, outbox.client);
> +	outbox = &mctp_pcc_ndev->outbox;
> +
> +	if (!skb)
> +		return;
> +
> +	len_sent = mbox_write_to_buffer(outbox->chan, skb->len, skb->data);
> +	if (len_sent == 0)
> +		dev_dstats_tx_dropped(mctp_pcc_ndev->ndev);
> +	else
> +		dev_dstats_tx_add(mctp_pcc_ndev->ndev, len_sent);
> +}
> +
> +static void mctp_pcc_tx_done(struct mbox_client *c, void *mssg, int r)
> +{
> +	struct mctp_pcc_ndev *mctp_pcc_ndev;
> +	struct sk_buff *skb = mssg;
> +
> +	mctp_pcc_ndev = container_of(c, struct mctp_pcc_ndev, outbox.client);
> +	dev_consume_skb_any(skb);
> +	netif_wake_queue(mctp_pcc_ndev->ndev);
> +}
> +
> +static int mctp_pcc_ndo_open(struct net_device *ndev)
> +{
> +	struct mctp_pcc_ndev *mctp_pcc_ndev = netdev_priv(ndev);
> +	struct mctp_pcc_mailbox *outbox, *inbox;
> +
> +	outbox = &mctp_pcc_ndev->outbox;
> +	inbox = &mctp_pcc_ndev->inbox;
> +
> +	outbox->chan = pcc_mbox_request_channel(&outbox->client, outbox->index);
> +	if (IS_ERR(outbox->chan))
> +		return PTR_ERR(outbox->chan);
> +
> +	inbox->client.rx_callback = mctp_pcc_client_rx_callback;
> +	inbox->chan = pcc_mbox_request_channel(&inbox->client, inbox->index);
> +	if (IS_ERR(inbox->chan)) {
> +		pcc_mbox_free_channel(outbox->chan);
> +		return PTR_ERR(inbox->chan);
> +	}
> +	return 0;
> +}
> +
> +static int mctp_pcc_ndo_stop(struct net_device *ndev)
> +{
> +	struct mctp_pcc_ndev *mctp_pcc_ndev = netdev_priv(ndev);
> +
> +	pcc_mbox_free_channel(mctp_pcc_ndev->outbox.chan);
> +	pcc_mbox_free_channel(mctp_pcc_ndev->inbox.chan);
> +	return 0;
> +}
> +
> +static const struct net_device_ops mctp_pcc_netdev_ops = {
> +	.ndo_open = mctp_pcc_ndo_open,
> +	.ndo_stop = mctp_pcc_ndo_stop,
> +	.ndo_start_xmit = mctp_pcc_tx,
> +};
> +
> +static void mctp_pcc_setup(struct net_device *ndev)
> +{
> +	ndev->type = ARPHRD_MCTP;
> +	ndev->hard_header_len = sizeof(struct acpi_pcct_ext_pcc_shared_memory);
> +	ndev->tx_queue_len = 0;
> +	ndev->flags = IFF_NOARP;
> +	ndev->netdev_ops = &mctp_pcc_netdev_ops;
> +	ndev->needs_free_netdev = true;
> +	ndev->pcpu_stat_type = NETDEV_PCPU_STAT_DSTATS;
> +}
> +
> +struct mctp_pcc_lookup_context {
> +	int index;
> +	u32 inbox_index;
> +	u32 outbox_index;
> +};
> +
> +static acpi_status lookup_pcct_indices(struct acpi_resource *ares,
> +				       void *context)
> +{
> +	struct mctp_pcc_lookup_context *luc = context;
> +	struct acpi_resource_address32 *addr;
> +
> +	if (ares->type != PCC_DWORD_TYPE)
> +		return AE_OK;
> +
> +	addr = ACPI_CAST_PTR(struct acpi_resource_address32, &ares->data);
> +	switch (luc->index) {
> +	case 0:
> +		luc->outbox_index = addr[0].address.minimum;
> +		break;
> +	case 1:
> +		luc->inbox_index = addr[0].address.minimum;
> +		break;
> +	}
> +	luc->index++;
> +	return AE_OK;
> +}
> +
> +static void mctp_cleanup_netdev(void *data)
> +{
> +	struct net_device *ndev = data;
> +
> +	mctp_unregister_netdev(ndev);
> +}
> +
> +static int initialize_MTU(struct net_device *ndev)
> +{
> +	struct mctp_pcc_ndev *mctp_pcc_ndev;
> +	struct mctp_pcc_mailbox *outbox;
> +	struct pcc_mbox_chan *pchan;
> +	int mctp_pcc_mtu;
> +
> +	mctp_pcc_mtu = MCTP_MIN_MTU;
> +	mctp_pcc_ndev = netdev_priv(ndev);
> +	outbox = &mctp_pcc_ndev->outbox;
> +	pchan = pcc_mbox_request_channel(&outbox->client, outbox->index);
> +	if (IS_ERR(pchan))
> +		return -1;
> +
> +	mctp_pcc_mtu = pchan->shmem_size;
> +	pcc_mbox_free_channel(pchan);
> +
> +	mctp_pcc_mtu = mctp_pcc_mtu - sizeof(struct acpi_pcct_ext_pcc_shared_memory);
> +	ndev->mtu = MCTP_MIN_MTU;
> +	ndev->max_mtu = mctp_pcc_mtu;
> +	ndev->min_mtu = MCTP_MIN_MTU;
> +
> +	return 0;
> +}
> +
> +static int mctp_pcc_driver_add(struct acpi_device *acpi_dev)
> +{
> +	struct mctp_pcc_lookup_context context = {0};
> +	struct mctp_pcc_ndev *mctp_pcc_ndev;
> +	struct device *dev = &acpi_dev->dev;
> +	struct net_device *ndev;
> +	acpi_handle dev_handle;
> +	acpi_status status;
> +	char name[32];
> +	int rc;
> +
> +	dev_dbg(dev, "Adding mctp_pcc device for HID %s\n",
> +		acpi_device_hid(acpi_dev));
> +	dev_handle = acpi_device_handle(acpi_dev);
> +	status = acpi_walk_resources(dev_handle, "_CRS", lookup_pcct_indices,
> +				     &context);
> +	if (!ACPI_SUCCESS(status)) {
> +		dev_err(dev, "FAILED to lookup PCC indexes from CRS\n");
> +		return -EINVAL;
> +	}
> +
> +	snprintf(name, sizeof(name), "mctppcc%d", context.inbox_index);
> +	ndev = alloc_netdev(sizeof(*mctp_pcc_ndev), name, NET_NAME_PREDICTABLE,
> +			    mctp_pcc_setup);
> +	if (!ndev)
> +		return -ENOMEM;
> +
> +	mctp_pcc_ndev = netdev_priv(ndev);
> +
> +	mctp_pcc_ndev->inbox.index = context.inbox_index;
> +	mctp_pcc_ndev->inbox.client.dev = dev;
> +	mctp_pcc_ndev->outbox.index = context.outbox_index;
> +	mctp_pcc_ndev->outbox.client.dev = dev;
> +
> +	mctp_pcc_ndev->outbox.client.tx_prepare = mctp_pcc_tx_prepare;
> +	mctp_pcc_ndev->outbox.client.tx_done = mctp_pcc_tx_done;
> +	mctp_pcc_ndev->acpi_device = acpi_dev;
> +	mctp_pcc_ndev->ndev = ndev;
> +	acpi_dev->driver_data = mctp_pcc_ndev;
> +
> +	rc = initialize_MTU(ndev);
> +	if (rc)
> +		goto free_netdev;
> +
> +	rc = mctp_register_netdev(ndev, NULL, MCTP_PHYS_BINDING_PCC);
> +	if (rc)
> +		goto free_netdev;
> +
> +	return devm_add_action_or_reset(dev, mctp_cleanup_netdev, ndev);
> +free_netdev:
> +	free_netdev(ndev);
> +	return rc;
> +}
> +
> +static const struct acpi_device_id mctp_pcc_device_ids[] = {
> +	{ "DMT0001" },
> +	{}
> +};
> +
> +static struct acpi_driver mctp_pcc_driver = {
> +	.name = "mctp_pcc",
> +	.class = "Unknown",
> +	.ids = mctp_pcc_device_ids,
> +	.ops = {
> +		.add = mctp_pcc_driver_add,
> +	},
> +};
> +
> +module_acpi_driver(mctp_pcc_driver);
> +
> +MODULE_DEVICE_TABLE(acpi, mctp_pcc_device_ids);
> +
> +MODULE_DESCRIPTION("MCTP PCC ACPI device");
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Adam Young <admiyo@os.amperecomputing.com>");