Changes since v0:

Introduce a new feature bit, VIRTIO_F_MMIO_RESET_WAIT, which controls whether a driver polls
for reset completion or not.

===

Reset of a virtio-mmio device is accomplished by writing 0 to its Status register.
For devices that are emulated in software, writes to Status register are trapped
and resumed only after the reset operation is complete. Thus a driver can be
assured of reset completion as soon as its write completes.

That may not be the case for virtio-mmio devices implemented in hardware
directly. A write could complete before the reset operation inside device is
completed. Introduce a new feature, VIRTIO_F_MMIO_RESET_WAIT, that such devices
will offer as means to indicate to driver that they need to poll for reset
completion, indicated by a read of Status register returning 0.

Signed-off-by: Srivatsa Vaddagiri <svaddagi@qti.qualcomm.com>

diff --git a/content.tex b/content.tex
index 5c70a3c..1990a5c 100644
--- a/content.tex
+++ b/content.tex
@@ -1924,7 +1924,10 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi
 do not represent events which took place MUST be zero.

 Upon reset, the device MUST clear all bits in \field{InterruptStatus} and ready bits in the
-\field{QueueReady} register for all queues in the device.
+\field{QueueReady} register for all queues in the device. The device MUST also offer
+VIRTIO_F_MMIO_RESET_WAIT if it requires driver to poll for reset completion
+indicated by a read of \field{Status} returning 0. Such a device MUST also fail to accept
+FEATURES_OK bit if driver does not negotiate VIRTIO_F_MMIO_RESET_WAIT.

 The device MUST change value returned in \field{ConfigGeneration} if there is any risk of a
 driver seeing an inconsistent configuration state.
@@ -1944,6 +1947,10 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi
 8 bit wide fields, 16 bit wide and aligned accesses for 16 bit wide fields and 32 bit wide and
 aligned accesses for 32 and 64 bit wide fields.

+The driver MUST accept VIRTIO_F_MMIO_RESET_WAIT if offered by device. During
+reset of such a device that offers VIRTIO_F_MMIO_RESET_WAIT, driver MUST poll for
+reset completion indicated by a read of \field{Status} returning 0.
+
 The driver MUST ignore a device with \field{MagicValue} which is not 0x74726976,
 although it MAY report an error.

@@ -6672,6 +6679,10 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
   transport specific.
   For more details about driver notifications over PCI see \ref{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Available Buffer Notifications}.

+  \item[VIRTIO_F_MMIO_RESET_WAIT(40)] This feature could be offered by a device
+  using MMIO transport and indicates that the driver needs to poll for reset
+  completion indicated by a read of \field{Status} returning 0.
+
 \end{description}

 \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
@@ -6708,6 +6719,8 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}

 A driver SHOULD accept VIRTIO_F_NOTIF_CONFIG_DATA if it is offered.

+A driver SHOULD accept VIRTIO_F_MMIO_RESET_WAIT if it is offered.
+
 \devicenormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}

 A device MUST offer VIRTIO_F_VERSION_1.  A device MAY fail to operate further


---

Qualcomm Innovation Center, Inc. is submitting the attached "feedback" as a
non-member to the virtio-dev mailing list for consideration and inclusion.