lkml.org 
[lkml]   [2021]   [Aug]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v6 07/17] firmware: arm_scmi: Handle concurrent and out-of-order messages
On Mon, Jul 12, 2021 at 03:18:23PM +0100, Cristian Marussi wrote:
> Even though in case of asynchronous commands an SCMI platform server is

Drop the term "server"

> constrained to emit the delayed response message only after the related
> message response has been sent, the configured underlying transport could
> still deliver such messages together or in inverted order, causing races
> due to the concurrent or out-of-order access to the underlying xfer.
>
> Introduce a mechanism to grant exclusive access to an xfer in order to
> properly serialize concurrent accesses to the same xfer originating from
> multiple correlated messages.
>
> Add additional state information to xfer descriptors so as to be able to
> identify out-of-order message deliveries and act accordingly:
>
> - when a delayed response is expected but delivered before the related
> response, the synchronous response is considered as successfully
> received and the delayed response processing is carried on as usual.
>
> - when/if the missing synchronous response is subsequently received, it
> is discarded as not congruent with the current state of the xfer, or
> simply, because the xfer has been already released and so, now, the
> monotonically increasing sequence number carried by the late response
> is stale.
>
> Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
> ---
> v5 --> v6
> - added spinlock comment
> ---
> drivers/firmware/arm_scmi/common.h | 18 ++-
> drivers/firmware/arm_scmi/driver.c | 229 ++++++++++++++++++++++++-----
> 2 files changed, 212 insertions(+), 35 deletions(-)
>
> diff --git a/drivers/firmware/arm_scmi/common.h b/drivers/firmware/arm_scmi/common.h
> index 2233d0a188fc..9efebe1406d2 100644
> --- a/drivers/firmware/arm_scmi/common.h
> +++ b/drivers/firmware/arm_scmi/common.h
> @@ -19,6 +19,7 @@
> #include <linux/module.h>
> #include <linux/refcount.h>
> #include <linux/scmi_protocol.h>
> +#include <linux/spinlock.h>
> #include <linux/types.h>
>
> #include <asm/unaligned.h>
> @@ -145,6 +146,13 @@ struct scmi_msg {
> * @pending: True for xfers added to @pending_xfers hashtable
> * @node: An hlist_node reference used to store this xfer, alternatively, on
> * the free list @free_xfers or in the @pending_xfers hashtable
> + * @busy: An atomic flag to ensure exclusive write access to this xfer
> + * @state: The current state of this transfer, with states transitions deemed
> + * valid being:
> + * - SCMI_XFER_SENT_OK -> SCMI_XFER_RESP_OK [ -> SCMI_XFER_DRESP_OK ]
> + * - SCMI_XFER_SENT_OK -> SCMI_XFER_DRESP_OK
> + * (Missing synchronous response is assumed OK and ignored)
> + * @lock: A spinlock to protect state and busy fields.
> */
> struct scmi_xfer {
> int transfer_id;
> @@ -156,6 +164,15 @@ struct scmi_xfer {
> refcount_t users;
> bool pending;
> struct hlist_node node;
> +#define SCMI_XFER_FREE 0
> +#define SCMI_XFER_BUSY 1
> + atomic_t busy;
> +#define SCMI_XFER_SENT_OK 0
> +#define SCMI_XFER_RESP_OK 1
> +#define SCMI_XFER_DRESP_OK 2
> + int state;
> + /* A lock to protect state and busy fields */
> + spinlock_t lock;
> };
>
> /*
> @@ -392,5 +409,4 @@ bool shmem_poll_done(struct scmi_shared_mem __iomem *shmem,
> void scmi_notification_instance_data_set(const struct scmi_handle *handle,
> void *priv);
> void *scmi_notification_instance_data_get(const struct scmi_handle *handle);
> -
> #endif /* _SCMI_COMMON_H */
> diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
> index 245ede223302..5ef33d692670 100644
> --- a/drivers/firmware/arm_scmi/driver.c
> +++ b/drivers/firmware/arm_scmi/driver.c
> @@ -369,6 +369,7 @@ static struct scmi_xfer *scmi_xfer_get(const struct scmi_handle *handle,
>
> if (!IS_ERR(xfer)) {
> refcount_set(&xfer->users, 1);
> + atomic_set(&xfer->busy, SCMI_XFER_FREE);
> xfer->transfer_id = atomic_inc_return(&transfer_last_id);
> }
> spin_unlock_irqrestore(&minfo->xfer_lock, flags);
> @@ -430,6 +431,168 @@ scmi_xfer_lookup_unlocked(struct scmi_xfers_info *minfo, u16 xfer_id)
> return xfer ?: ERR_PTR(-EINVAL);
> }
>
> +/**
> + * scmi_msg_response_validate - Validate message type against state of related
> + * xfer
> + *
> + * @cinfo: A reference to the channel descriptor.
> + * @msg_type: Message type to check
> + * @xfer: A reference to the xfer to validate against @msg_type
> + *
> + * This function checks if @msg_type is congruent with the current state of
> + * a pending @xfer; if an asynchronous delayed response is received before the
> + * related synchronous response (Out-of-Order Delayed Response) the missing
> + * synchronous response is assumed to be OK and completed, carrying on with the
> + * Delayed Response: this is done to address the case in which the underlying
> + * SCMI transport can deliver such out-of-order responses.
> + *
> + * Context: Assumes to be called with xfer->lock already acquired.
> + *
> + * Return: 0 on Success, error otherwise
> + */
> +static inline int scmi_msg_response_validate(struct scmi_chan_info *cinfo,
> + u8 msg_type,
> + struct scmi_xfer *xfer)
> +{
> + /*
> + * Even if a response was indeed expected on this slot at this point,
> + * a buggy platform could wrongly reply feeding us an unexpected
> + * delayed response we're not prepared to handle: bail-out safely
> + * blaming firmware.
> + */
> + if (msg_type == MSG_TYPE_DELAYED_RESP && !xfer->async_done) {
> + dev_err(cinfo->dev,
> + "Delayed Response for %d not expected! Buggy F/W ?\n",
> + xfer->hdr.seq);
> + return -EINVAL;
> + }
> +
> + switch (xfer->state) {
> + case SCMI_XFER_SENT_OK:
> + if (msg_type == MSG_TYPE_DELAYED_RESP) {
> + /*
> + * Delayed Response expected but delivered earlier.
> + * Assume message RESPONSE was OK and skip state.
> + */
> + xfer->hdr.status = SCMI_SUCCESS;
> + xfer->state = SCMI_XFER_RESP_OK;
> + complete(&xfer->done);
> + dev_warn(cinfo->dev,
> + "Received valid OoO Delayed Response for %d\n",
> + xfer->hdr.seq);
> + }
> + break;
> + case SCMI_XFER_RESP_OK:
> + if (msg_type != MSG_TYPE_DELAYED_RESP)
> + return -EINVAL;
> + break;
> + case SCMI_XFER_DRESP_OK:
> + /* No further message expected once in SCMI_XFER_DRESP_OK */

Do we really need this case ? If so, how can this happen.

> + return -EINVAL;
> + }
> +
> + return 0;
> +}
> +
> +static bool scmi_xfer_is_free(struct scmi_xfer *xfer)
> +{
> + int ret;
> +
> + ret = atomic_cmpxchg(&xfer->busy, SCMI_XFER_FREE, SCMI_XFER_BUSY);
> +
> + return ret == SCMI_XFER_FREE;
> +}
> +
> +/**
> + * scmi_xfer_command_acquire - Helper to lookup and acquire a command xfer
> + *
> + * @cinfo: A reference to the channel descriptor.
> + * @msg_hdr: A message header to use as lookup key
> + *
> + * When a valid xfer is found for the sequence number embedded in the provided
> + * msg_hdr, reference counting is properly updated and exclusive access to this
> + * xfer is granted till released with @scmi_xfer_command_release.
> + *
> + * Return: A valid @xfer on Success or error otherwise.
> + */
> +static inline struct scmi_xfer *
> +scmi_xfer_command_acquire(struct scmi_chan_info *cinfo, u32 msg_hdr)
> +{
> + int ret;
> + unsigned long flags;
> + struct scmi_xfer *xfer;
> + struct scmi_info *info = handle_to_scmi_info(cinfo->handle);
> + struct scmi_xfers_info *minfo = &info->tx_minfo;
> + u8 msg_type = MSG_XTRACT_TYPE(msg_hdr);
> + u16 xfer_id = MSG_XTRACT_TOKEN(msg_hdr);
> +
> + /* Are we even expecting this? */
> + spin_lock_irqsave(&minfo->xfer_lock, flags);
> + xfer = scmi_xfer_lookup_unlocked(minfo, xfer_id);
> + if (IS_ERR(xfer)) {
> + dev_err(cinfo->dev,
> + "Message for %d type %d is not expected!\n",
> + xfer_id, msg_type);
> + spin_unlock_irqrestore(&minfo->xfer_lock, flags);
> + return xfer;
> + }
> + refcount_inc(&xfer->users);
> + spin_unlock_irqrestore(&minfo->xfer_lock, flags);
> +
> + spin_lock_irqsave(&xfer->lock, flags);
> + ret = scmi_msg_response_validate(cinfo, msg_type, xfer);
> + /*
> + * If a pending xfer was found which was also in a congruent state with
> + * the received message, acquire exclusive access to it setting the busy
> + * flag.
> + * Spins only on the rare limit condition of concurrent reception of
> + * RESP and DRESP for the same xfer.
> + */
> + if (!ret) {
> + spin_until_cond(scmi_xfer_is_free(xfer));

I agree with the discussion between you and Peter around this, so I assume
it will be renamed or handled accordingly.

> + xfer->hdr.type = msg_type;
> + }
> + spin_unlock_irqrestore(&xfer->lock, flags);
> +
> + if (ret) {
> + dev_err(cinfo->dev,
> + "Invalid message type:%d for %d - HDR:0x%X state:%d\n",
> + msg_type, xfer_id, msg_hdr, xfer->state);
> + /* On error the refcount incremented above has to be dropped */
> + __scmi_xfer_put(minfo, xfer);
> + xfer = ERR_PTR(-EINVAL);
> + }
> +
> + return xfer;
> +}
> +
> +static inline void scmi_xfer_command_release(struct scmi_info *info,
> + struct scmi_xfer *xfer)
> +{
> + atomic_set(&xfer->busy, SCMI_XFER_FREE);
> + __scmi_xfer_put(&info->tx_minfo, xfer);
> +}
> +
> +/**
> + * scmi_xfer_state_update - Update xfer state
> + *
> + * @xfer: A reference to the xfer to update
> + *
> + * Context: Assumes to be called on an xfer exclusively acquired using the
> + * busy flag.
> + */
> +static inline void scmi_xfer_state_update(struct scmi_xfer *xfer)
> +{
> + switch (xfer->hdr.type) {
> + case MSG_TYPE_COMMAND:
> + xfer->state = SCMI_XFER_RESP_OK;
> + break;
> + case MSG_TYPE_DELAYED_RESP:
> + xfer->state = SCMI_XFER_DRESP_OK;
> + break;
> + }
> +}

Can't this be if () .. else if(), switch sounds unnecessary for 2 conditions.

Other than the things already discussed with you and Peter, don't have much to
add ATM. I may look at this with fresh eyes once again in the next version.

--
Regards,
Sudeep

\
 
 \ /
  Last update: 2021-08-02 12:12    [W:0.254 / U:0.548 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site