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

Cristian Marussi
2021-12-14T10:57:08Z

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