Re: [PATCH v4 1/4] firmware: arm_scmi: Add polling support for completion in smc

[Brian Masney]

On Mon, Sep 11, 2023 at 12:43:56PM -0700, Nikunj Kela wrote:
> Currently, the return from the smc call assumes the completion of
> the scmi request. However this may not be true in virtual platforms
> that are using hvc doorbell.
> 
> This change adds a Kconfig to enable the polling for the request
> completion.
> 
> Signed-off-by: Nikunj Kela <quic_nkela@xxxxxxxxxxx>
> ---
>  drivers/firmware/arm_scmi/Kconfig | 14 ++++++++++++++
>  drivers/firmware/arm_scmi/smc.c   | 15 ++++++++++++++-
>  2 files changed, 28 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/firmware/arm_scmi/Kconfig b/drivers/firmware/arm_scmi/Kconfig
> index ea0f5083ac47..771d60f8319f 100644
> --- a/drivers/firmware/arm_scmi/Kconfig
> +++ b/drivers/firmware/arm_scmi/Kconfig
> @@ -125,6 +125,20 @@ config ARM_SCMI_TRANSPORT_SMC_ATOMIC_ENABLE
>  	  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_SMC_POLL_COMPLETION
> +	bool "Enable polling support for SCMI SMC transport"
> +	depends on ARM_SCMI_TRANSPORT_SMC
> +	help
> +	  Enable completion polling support for SCMI SMC based transport.
> +
> +	  If you want the SCMI SMC based transport to poll for the completion,
> +	  answer Y.
> +	  Enabling completion polling might be desired in the absence of the a2p
> +	  irq when the return from smc/hvc call doesn't indicate the completion
> +	  of the SCMI requests. This might be useful for instances used in
> +	  virtual platforms.
> +	  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 c193516a254d..0a0b7e401159 100644
> --- a/drivers/firmware/arm_scmi/smc.c
> +++ b/drivers/firmware/arm_scmi/smc.c
> @@ -250,6 +250,16 @@ static void smc_mark_txdone(struct scmi_chan_info *cinfo, int ret,
>  	smc_channel_lock_release(scmi_info);
>  }
>  
> +#ifdef CONFIG_ARM_SCMI_TRANSPORT_SMC_POLL_COMPLETION
> +static bool
> +smc_poll_done(struct scmi_chan_info *cinfo, struct scmi_xfer *xfer)
> +{
> +	struct scmi_smc *scmi_info = cinfo->transport_info;
> +
> +	return shmem_poll_done(scmi_info->shmem, xfer);
> +}
> +#endif
> +
>  static const struct scmi_transport_ops scmi_smc_ops = {
>  	.chan_available = smc_chan_available,
>  	.chan_setup = smc_chan_setup,
> @@ -257,6 +267,9 @@ static const struct scmi_transport_ops scmi_smc_ops = {
>  	.send_message = smc_send_message,
>  	.mark_txdone = smc_mark_txdone,
>  	.fetch_response = smc_fetch_response,
> +#ifdef CONFIG_ARM_SCMI_TRANSPORT_SMC_POLL_COMPLETION
> +	.poll_done = smc_poll_done,
> +#endif
>  };
>  
>  const struct scmi_desc scmi_smc_desc = {
> @@ -272,6 +285,6 @@ const struct scmi_desc scmi_smc_desc = {
>  	 * for the issued command will be immmediately ready to be fetched
>  	 * from the shared memory area.
>  	 */
> -	.sync_cmds_completed_on_ret = true,
> +	.sync_cmds_completed_on_ret = !IS_ENABLED(CONFIG_ARM_SCMI_TRANSPORT_SMC_POLL_COMPLETION),
>  	.atomic_enabled = IS_ENABLED(CONFIG_ARM_SCMI_TRANSPORT_SMC_ATOMIC_ENABLE),

>From a Linux distributor viewpoint, it would be nice if this was
determined at runtime, rather than at compile time. We generate a single
kernel binary that's used on systems from multiple hardware
manufacturers. We'd run into an issue if one company required this, but
another one didn't. We may potentially run into this same type of issue
with the upstream arm64 defconfig.

Brian