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Subject: Re: [PATCH v3] virtio-mmio: Specify wait needed in driver during reset
On Tue, Aug 31, 2021 at 07:27:53PM +0530, Srivatsa Vaddagiri wrote: > Reset of a virtio-mmio device is initiated by writing 0 to its Status register. > In case of some devices, the reset operation itself may not be completed > by the time write instruction completes and hence such devices would require > drivers to wait on reset operation to complete before they proceed with > remaining steps of initialization. > > Update the specification to indicate which devices would need driver to block on > reset completion. > > Suggested-by: Jason Wang <jasowang@redhat.com> > Signed-off-by: Srivatsa Vaddagiri <quic_svaddagi@quicinc.com> I have been thinking about this some more. \section{Device Reset}\label{sec:Basic Facilities of a Virtio Device / Device Reset} The driver may want to initiate a device reset at various times; notably, it is required to do so during device initialization and device cleanup. The mechanism used by the driver to initiate the reset is transport specific. \devicenormative{\subsection}{Device Reset}{Basic Facilities of a Virtio Device / Device Reset} A device MUST reinitialize \field{device status} to 0 after receiving a reset. A device MUST NOT send notifications or interact with the queues after indicating completion of the reset by reinitializing \field{device status} to 0, until the driver re-initializes the device. \drivernormative{\subsection}{Device Reset}{Basic Facilities of a Virtio Device / Device Reset} The driver SHOULD consider a driver-initiated reset complete when it reads \field{device status} as 0. So I guess we can just fix the mmio driver to read status and declare victory? Do we really need all the spec changes? > --- > v3->v2: > - Introduce a new status bit, DEVICE_RESET_IN_PROGRESS > - Limit driver need to poll for reset completion only for MMIO devices having > Version = 0x3 > > Previous version can be found at: > > https://lists.oasis-open.org/archives/virtio-dev/202108/msg00065.html > > content.tex | 23 +++++++++++++++++++---- > 1 file changed, 19 insertions(+), 4 deletions(-) > > diff --git a/content.tex b/content.tex > index 7cec1c3..e10f7d3 100644 > --- a/content.tex > +++ b/content.tex > @@ -49,6 +49,8 @@ \section{\field{Device Status} Field}\label{sec:Basic Facilities of a Virtio Dev > > \item[DEVICE_NEEDS_RESET (64)] Indicates that the device has experienced > an error from which it can't recover. > + > +\item[DEVICE_RESET_IN_PROGRESS (256)] Indicates that the device reset is in progress. > \end{description} > > The \field{device status} field starts out as 0, and is reinitialized to 0 by > @@ -1730,9 +1732,9 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi > } > \hline > \mmioreg{Version}{Device version number}{0x004}{R}{% > - 0x2. > + 0x3. > \begin{note} > - Legacy devices (see \ref{sec:Virtio Transport Options / Virtio Over MMIO / Legacy interface}~\nameref{sec:Virtio Transport Options / Virtio Over MMIO / Legacy interface}) used 0x1. > + Legacy devices (see \ref{sec:Virtio Transport Options / Virtio Over MMIO / Legacy interface}~\nameref{sec:Virtio Transport Options / Virtio Over MMIO / Legacy interface}) used 0x1. Devices that do not require drivers to poll for reset completion can use 0x2. See \ref{devicenormative:Virtio Transport Options / Virtio Over MMIO / MMIO Device Register Layout} for more details. > \end{note} > } > \hline > @@ -1916,7 +1918,15 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi > > The device MUST return 0x74726976 in \field{MagicValue}. > > -The device MUST return value 0x2 in \field{Version}. > +The device MUST return either value 0x3 or 0x2 in \field{Version} based on its reset behavior. Drivers trigger reset > +of a device by writing 0 to \field{Status}. The reset operation itself may or may not be completed by the time write > +operation is complete. Devices whose reset operation completes synchronously with the write operation are allowed to > +return value of 0x2 for \field{Version}. Other devices, whose reset operation can be incomplete by the time write > +operation completes MUST return value 0x3 as an indication for drivers to poll for reset completion. Such devices > +MUST indicate progress of reset operation in \field{Status}, with value of DEVICE_RESET_IN_PROGRESS indicating reset > +operation is still in progress, a value of 0x0 indicating reset operation is complete and a value of > +DEVICE_NEEDS_RESET indicating device has experienced an error and that reset operation could not be completed > +successfully. > > The device MUST present each event by setting the corresponding bit in \field{InterruptStatus} from the > moment it takes place, until the driver acknowledges the interrupt > @@ -1947,9 +1957,14 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi > The driver MUST ignore a device with \field{MagicValue} which is not 0x74726976, > although it MAY report an error. > > -The driver MUST ignore a device with \field{Version} which is not 0x2, > +The driver MUST ignore a device with \field{Version} which is neither 0x2 nor 0x3, > although it MAY report an error. > > +When \field{Version} is 0x3, the driver, after initiating reset of device by > +writing 0 to \field{Status}, MUST wait for device reset operation to complete. > +Further, when \field{Version} is 0x3, the driver MUST not access any register other than > +reading \field{Status} while device reset is in progress. > + > The driver MUST ignore a device with \field{DeviceID} 0x0, > but MUST NOT report any error. > > > > -- > Qualcomm Innovation Center, Inc. is submitting the attached "feedback" as a > non-member to the virtio-dev mailing list for consideration and inclusion.
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