Re: [PATCH v7 14/16] firmware: arm_scmi: Add atomic mode support to virtio transport

[Cristian Marussi <cristian.marussi@arm.com>]

On Mon, Dec 13, 2021 at 11:34:56AM +0000, Sudeep Holla wrote:
> On Mon, Nov 29, 2021 at 07:11:54PM +0000, Cristian Marussi wrote:
> > Add support for .mark_txdone and .poll_done transport operations to SCMI
> > VirtIO transport as pre-requisites to enable atomic operations.
> > 
> > Add a Kernel configuration option to enable SCMI VirtIO transport polling
> > and atomic mode for selected SCMI transactions while leaving it default
> > disabled.
> > 
> > Cc: "Michael S. Tsirkin" <mst@redhat.com>
> > Cc: Igor Skalkin <igor.skalkin@opensynergy.com>
> > Cc: Peter Hilber <peter.hilber@opensynergy.com>
> > Cc: virtualization@lists.linux-foundation.org
> > Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
> > ---
> > V6 --> V7
> > - added a few comments about virtio polling internals
> > - fixed missing list_del on pending_cmds_list processing
> > - shrinked spinlocked areas in virtio_poll_done
> > - added proper spinlocking to scmi_vio_complete_cb while scanning list
> >   of pending cmds
> > ---
> >  drivers/firmware/arm_scmi/Kconfig  |  15 ++
> >  drivers/firmware/arm_scmi/virtio.c | 241 +++++++++++++++++++++++++++--
> >  2 files changed, 243 insertions(+), 13 deletions(-)
> > 
> > diff --git a/drivers/firmware/arm_scmi/Kconfig b/drivers/firmware/arm_scmi/Kconfig
> > index d429326433d1..7794bd41eaa0 100644
> > --- a/drivers/firmware/arm_scmi/Kconfig
> > +++ b/drivers/firmware/arm_scmi/Kconfig
> > @@ -118,6 +118,21 @@ config ARM_SCMI_TRANSPORT_VIRTIO_VERSION1_COMPLIANCE
> >  	  the ones implemented by kvmtool) and let the core Kernel VirtIO layer
> >  	  take care of the needed conversions, say N.
> >  
> > +config ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
> > +	bool "Enable atomic mode for SCMI VirtIO transport"
> > +	depends on ARM_SCMI_TRANSPORT_VIRTIO
> > +	help
> > +	  Enable support of atomic operation for SCMI VirtIO based transport.
> > +
> > +	  If you want the SCMI VirtIO 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.
> > +
> >  endif #ARM_SCMI_PROTOCOL
> >  
> >  config ARM_SCMI_POWER_DOMAIN
> > diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
> > index fd0f6f91fc0b..0598e185a786 100644
> > --- a/drivers/firmware/arm_scmi/virtio.c
> > +++ b/drivers/firmware/arm_scmi/virtio.c
> > @@ -38,6 +38,7 @@
> >   * @vqueue: Associated virtqueue
> >   * @cinfo: SCMI Tx or Rx channel
> >   * @free_list: List of unused scmi_vio_msg, maintained for Tx channels only
> > + * @pending_cmds_list: List of pre-fetched commands queueud for later processing
> >   * @is_rx: Whether channel is an Rx channel
> >   * @ready: Whether transport user is ready to hear about channel
> >   * @max_msg: Maximum number of pending messages for this channel.
> > @@ -49,6 +50,9 @@ struct scmi_vio_channel {
> >  	struct virtqueue *vqueue;
> >  	struct scmi_chan_info *cinfo;
> >  	struct list_head free_list;
> > +#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
> > +	struct list_head pending_cmds_list;
> > +#endif
> >  	bool is_rx;
> >  	bool ready;
> >  	unsigned int max_msg;
> > @@ -65,12 +69,22 @@ struct scmi_vio_channel {
> >   * @input: SDU used for (delayed) responses and notifications
> >   * @list: List which scmi_vio_msg may be part of
> >   * @rx_len: Input SDU size in bytes, once input has been received
> > + * @poll_idx: Last used index registered for polling purposes if this message
> > + *	      transaction reply was configured for polling.
> > + *	      Note that virtqueue used index is an unsigned 16-bit.
> > + * @poll_lock: Protect access to @poll_idx.
> >   */
> >  struct scmi_vio_msg {
> >  	struct scmi_msg_payld *request;
> >  	struct scmi_msg_payld *input;
> >  	struct list_head list;
> >  	unsigned int rx_len;
> > +#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
> 
> Do we really need the #ifdefery for struct definition ? TBH I don't like
> the way it is. I would avoid it as much as possible. I assume some are
> added to avoid build warnings ?
> 
> Doesn't __maybe_unused help to remove some of them like the functions
> mark_txdone and poll_done. I haven't tried but thought of checking.
> 

Ok, I'll remove ifdeffery while addressing Peter concerns.

Thanks,
Cristian