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    Subject[PATCH v8 07/11] firmware: arm_scmi: Add atomic mode support to smc transport
    Date
    Add a Kernel configuration option to enable SCMI SMC transport atomic
    mode operation for selected SCMI transactions and leave it as default
    disabled.

    Substitute mutex usages with busy-waiting and declare smc transport as
    .atomic_enabled if such Kernel configuration option is enabled.

    Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
    ---
    v7 --> v8
    - removed ifdeffery, using IS_ENABLED
    v5 --> v6
    - remove usage of atomic_capable
    - removed needless union
    - reviewed Kconfig help
    v4 --> v5
    - removed RFC tag
    - add CONFIG_ARM_SCMI_TRANSPORT_SMC_ATOMIC_ENABLE option
    - add .atomic_enable support
    - make atomic_capable dependent on
    CONFIG_ARM_SCMI_TRANSPORT_SMC_ATOMIC_ENABLE
    - make also usage of mutexes vs busy-waiting dependent on
    CONFIG_ARM_SCMI_TRANSPORT_SMC_ATOMIC_ENABLE
    ---
    drivers/firmware/arm_scmi/Kconfig | 14 ++++++++
    drivers/firmware/arm_scmi/smc.c | 56 +++++++++++++++++++++++++++----
    2 files changed, 64 insertions(+), 6 deletions(-)

    diff --git a/drivers/firmware/arm_scmi/Kconfig b/drivers/firmware/arm_scmi/Kconfig
    index 638ecec89ff1..d429326433d1 100644
    --- a/drivers/firmware/arm_scmi/Kconfig
    +++ b/drivers/firmware/arm_scmi/Kconfig
    @@ -78,6 +78,20 @@ config ARM_SCMI_TRANSPORT_SMC
    If you want the ARM SCMI PROTOCOL stack to include support for a
    transport based on SMC, answer Y.

    +config ARM_SCMI_TRANSPORT_SMC_ATOMIC_ENABLE
    + bool "Enable atomic mode support for SCMI SMC transport"
    + depends on ARM_SCMI_TRANSPORT_SMC
    + help
    + Enable support of atomic operation for SCMI SMC based transport.
    +
    + If you want the SCMI SMC based transport to operate in atomic
    + mode, avoiding any kind of sleeping behaviour for selected
    + transactions on the TX path, answer Y.
    + Enabling atomic mode operations allows any SCMI driver using this
    + transport to optionally ask for atomic SCMI transactions and operate
    + in atomic context too, at the price of using a number of busy-waiting
    + primitives all over instead. If unsure say N.
    +
    config ARM_SCMI_TRANSPORT_VIRTIO
    bool "SCMI transport based on VirtIO"
    depends on VIRTIO=y || VIRTIO=ARM_SCMI_PROTOCOL
    diff --git a/drivers/firmware/arm_scmi/smc.c b/drivers/firmware/arm_scmi/smc.c
    index df0defd9f8bb..6c7871a40611 100644
    --- a/drivers/firmware/arm_scmi/smc.c
    +++ b/drivers/firmware/arm_scmi/smc.c
    @@ -7,6 +7,7 @@
    */

    #include <linux/arm-smccc.h>
    +#include <linux/atomic.h>
    #include <linux/device.h>
    #include <linux/err.h>
    #include <linux/interrupt.h>
    @@ -14,6 +15,7 @@
    #include <linux/of.h>
    #include <linux/of_address.h>
    #include <linux/of_irq.h>
    +#include <linux/processor.h>
    #include <linux/slab.h>

    #include "common.h"
    @@ -23,14 +25,20 @@
    *
    * @cinfo: SCMI channel info
    * @shmem: Transmit/Receive shared memory area
    - * @shmem_lock: Lock to protect access to Tx/Rx shared memory area
    + * @shmem_lock: Lock to protect access to Tx/Rx shared memory area.
    + * Used when NOT operating in atomic mode.
    + * @inflight: Atomic flag to protect access to Tx/Rx shared memory area.
    + * Used when operating in atomic mode.
    * @func_id: smc/hvc call function id
    */

    struct scmi_smc {
    struct scmi_chan_info *cinfo;
    struct scmi_shared_mem __iomem *shmem;
    + /* Protect access to shmem area */
    struct mutex shmem_lock;
    +#define INFLIGHT_NONE MSG_TOKEN_MAX
    + atomic_t inflight;
    u32 func_id;
    };

    @@ -54,6 +62,41 @@ static bool smc_chan_available(struct device *dev, int idx)
    return true;
    }

    +static inline void smc_channel_lock_init(struct scmi_smc *scmi_info)
    +{
    + if (IS_ENABLED(CONFIG_ARM_SCMI_TRANSPORT_SMC_ATOMIC_ENABLE))
    + atomic_set(&scmi_info->inflight, INFLIGHT_NONE);
    + else
    + mutex_init(&scmi_info->shmem_lock);
    +}
    +
    +static bool smc_xfer_inflight(struct scmi_xfer *xfer, atomic_t *inflight)
    +{
    + int ret;
    +
    + ret = atomic_cmpxchg(inflight, INFLIGHT_NONE, xfer->hdr.seq);
    +
    + return ret == INFLIGHT_NONE;
    +}
    +
    +static inline void
    +smc_channel_lock_acquire(struct scmi_smc *scmi_info,
    + struct scmi_xfer *xfer __maybe_unused)
    +{
    + if (IS_ENABLED(CONFIG_ARM_SCMI_TRANSPORT_SMC_ATOMIC_ENABLE))
    + spin_until_cond(smc_xfer_inflight(xfer, &scmi_info->inflight));
    + else
    + mutex_lock(&scmi_info->shmem_lock);
    +}
    +
    +static inline void smc_channel_lock_release(struct scmi_smc *scmi_info)
    +{
    + if (IS_ENABLED(CONFIG_ARM_SCMI_TRANSPORT_SMC_ATOMIC_ENABLE))
    + atomic_set(&scmi_info->inflight, INFLIGHT_NONE);
    + else
    + mutex_unlock(&scmi_info->shmem_lock);
    +}
    +
    static int smc_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
    bool tx)
    {
    @@ -114,7 +157,7 @@ static int smc_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,

    scmi_info->func_id = func_id;
    scmi_info->cinfo = cinfo;
    - mutex_init(&scmi_info->shmem_lock);
    + smc_channel_lock_init(scmi_info);
    cinfo->transport_info = scmi_info;

    return 0;
    @@ -140,10 +183,10 @@ static int smc_send_message(struct scmi_chan_info *cinfo,
    struct arm_smccc_res res;

    /*
    - * Channel lock will be released only once response has been
    + * Channel will be released only once response has been
    * surely fully retrieved, so after .mark_txdone()
    */
    - mutex_lock(&scmi_info->shmem_lock);
    + smc_channel_lock_acquire(scmi_info, xfer);

    shmem_tx_prepare(scmi_info->shmem, xfer);

    @@ -151,7 +194,7 @@ static int smc_send_message(struct scmi_chan_info *cinfo,

    /* Only SMCCC_RET_NOT_SUPPORTED is valid error code */
    if (res.a0) {
    - mutex_unlock(&scmi_info->shmem_lock);
    + smc_channel_lock_release(scmi_info);
    return -EOPNOTSUPP;
    }

    @@ -170,7 +213,7 @@ static void smc_mark_txdone(struct scmi_chan_info *cinfo, int ret)
    {
    struct scmi_smc *scmi_info = cinfo->transport_info;

    - mutex_unlock(&scmi_info->shmem_lock);
    + smc_channel_lock_release(scmi_info);
    }

    static const struct scmi_transport_ops scmi_smc_ops = {
    @@ -196,4 +239,5 @@ const struct scmi_desc scmi_smc_desc = {
    * from the shared memory area.
    */
    .sync_cmds_completed_on_ret = true,
    + .atomic_enabled = IS_ENABLED(CONFIG_ARM_SCMI_TRANSPORT_SMC_ATOMIC_ENABLE),
    };
    --
    2.17.1
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