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Subject: Re: [virtio-comment] [PATCH v5] virtio-i2c: add the device specification
On Wed, Dec 16, 2020 at 06:38:59PM +0100, Cornelia Huck wrote: > On Wed, 25 Nov 2020 13:55:18 +0800 > Jie Deng <jie.deng@intel.com> wrote: > > <some mostly editorial comments; sorry about bringing this up now> > > > virtio-i2c is a virtual I2C adapter device. It provides a way > > to ïexibly communicate with the host I2C slave devices from > > s/ïexibly/flexibly/ > > > the guest. > > > > This patch adds the specification for this device. > > > > Fixes: https://github.com/oasis-tcs/virtio-spec/issues/88 > > Signed-off-by: Jie Deng <jie.deng@intel.com> Given the comments do we want to > > --- > > conformance.tex | 28 +++++++++-- > > content.tex | 1 + > > introduction.tex | 3 ++ > > virtio-i2c.tex | 139 +++++++++++++++++++++++++++++++++++++++++++++++++++++++ > > 4 files changed, 167 insertions(+), 4 deletions(-) > > create mode 100644 virtio-i2c.tex > > > > (...) > > > > diff --git a/introduction.tex b/introduction.tex > > index cc38e29..9f016d5 100644 > > --- a/introduction.tex > > +++ b/introduction.tex > > @@ -73,6 +73,9 @@ \section{Normative References}\label{sec:Normative References} > > \phantomsection\label{intro:HDA}\textbf{[HDA]} & > > High Definition Audio Specification, > > \newline\url{https://www.intel.com/content/dam/www/public/us/en/documents/product-specifications/high-definition-audio-specification.pdf}\\ > > + \phantomsection\label{intro:I2C}\textbf{[I2C]} & > > + I2C-bus specification and user manual, > > + \newline\url{https://www.nxp.com.cn/docs/en/user-guide/UM10204.pdf}\\ > > I think this url should use www.nxp.com instead of www.nxp.com.cn (even > though both seem to lead to the same document). > > > > > \end{longtable} > > > > diff --git a/virtio-i2c.tex b/virtio-i2c.tex > > new file mode 100644 > > index 0000000..fdb0050 > > --- /dev/null > > +++ b/virtio-i2c.tex > > @@ -0,0 +1,139 @@ > > +\section{I2C Adapter Device}\label{sec:Device Types / I2C Adapter Device} > > + > > +virtio-i2c is a virtual I2C adapter device. BTW is this neessarily a virtual device? One can build a physical one to this spec, no? > It provides a way to flexibly > > +organize and use the host I2C slave devices from the guest. By attaching > > +the host ACPI I2C slave nodes to the virtual I2C adapter device, the guest can > > +communicate with them without changing or adding extra drivers for these > > +slave I2C devices. > > I know that the i2c spec talks about 'slave' devices, but can we find a > better terminology for the virtio standard? E.g. prefix this with > > "Note: This standard uses the term 'controlled I2C device' to refer to > what the I2C standard calls 'slave I2C device'." > > (Or whatever better term might exist.) > > > + > > +\subsection{Device ID}\label{sec:Device Types / I2C Adapter Device / Device ID} > > +34 > > + > > +\subsection{Virtqueues}\label{sec:Device Types / I2C Adapter Device / Virtqueues} > > + > > +\begin{description} > > +\item[0] requestq > > +\end{description} > > + > > +\subsection{Feature bits}\label{sec:Device Types / I2C Adapter Device / Feature bits} > > + > > +None currently defined. > > + > > +\subsection{Device configuration layout}\label{sec:Device Types / I2C Adapter Device / Device configuration layout} > > + > > +None currently defined. > > + > > +\subsection{Device Initialization}\label{sec:Device Types / I2C Adapter Device / Device Initialization} > > + > > +\begin{enumerate} > > +\item The virtqueue is initialized. > > +\end{enumerate} > > + > > +\subsection{Device Operation}\label{sec:Device Types / I2C Adapter Device / Device Operation} > > + > > +\subsubsection{Device Operation: Request Queue}\label{sec:Device Types / I2C Adapter Device / Device Operation: Request Queue} > > + > > +The driver queues requests to the virtqueue, and they are used by the > > +device. The request is the representation of segments of an I2C > > +transaction. Each request is of form: of the form > > + > > +\begin{lstlisting} > > +struct virtio_i2c_req { > > + le16 addr; > > + le32 flags; > > + le16 written; > > + le16 read; > > + u8 write_buf[]; > > + u8 read_buf[]; > > + u8 status; > > +}; > > +\end{lstlisting} > > + > > +The \field{addr} of the request is the address of the I2C slave device. > > + > > +The \field{flags} of the request is currently reserved as zero for future > > +feature extensibility. > > + > > +The \field{written} of the request is the number of data bytes in the \field{write_buf} > > +being written to the I2C slave address. > > + > > +The \field{read} of the request is the number of data bytes in the \field{read_buf} > > +being read from the I2C slave address. I note that read and written actually duplicate buf size available in the descriptor. Given we no longer mirror i2c_msg 1:1 do we still want to do this? It will be trivial for the host device to populate these fields correctly for linux. Duplication of information iten leads to errors ... > > + > > +The \field{write_buf} of the request contains one segment of an I2C transaction > > +being written to the device. > > + > > +The \field{read_buf} of the request contains one segment of an I2C transaction > > +being read from the device. > > + > > +The final \field{status} byte of the request is written by the device: either > > +VIRTIO_I2C_MSG_OK for success or VIRTIO_I2C_MSG_ERR for error. > > + > > +\begin{lstlisting} > > +#define VIRTIO_I2C_MSG_OK 0 > > +#define VIRTIO_I2C_MSG_ERR 1 > > +\end{lstlisting} > > + > > +If the \field{read}=0 and the \field{written}>0, the request is called write request. > > + > > +If the \field{read}>0 and the \field{written}=0, the request is called read request. > > + > > +If the \field{read}>0 and the \field{written}>0, the request is called write-read request. > > +It means an I2C write segment followed by a read segment. Usually, the write segment > > +provides the number of an I2C slave device register to be read. > > + > > +The \field{read}=0 and at the same time the \field{written}=0 doesn't make any sense. > > +The driver SHOULD never send such request. > > 'SHOULD' makes this a normative statement. > > > + > > +\subsubsection{Device Operation: Operation Status}\label{sec:Device Types / I2C Adapter Device / Device Operation: Operation Status} > > + > > +A driver may send one request or multiple requests to the device at a time. > > +The requests in the virtqueue MUST be queued and processed in order. > > Same here, 'MUST' makes this a normative statement. I think lowercasing > these terms would make it correct. > It won't, the relevant rfc includes MUST and must etc. If this is normative move it to a normative statement. Or preferably, both change this to e.g. The requests in the virtqueue are both queued and processed in order. and add a normative statement in the normative section. > > + > > +If a driver sends multiple requests at a time and a device fails to process > > +some of them, then the first failed request MUST have its \field{status} > > +being set to VIRTIO_I2C_MSG_ERR by the device and the requests after the first > > +failed one MUST also be treated as VIRTIO_I2C_MSG_ERR by the driver, > > +no matter what \field{status} of them. In this case, the number of successfully > > +sent requests this time is the number of the last request being successfully > > +processed. > > + > > +\drivernormative{\subsubsection}{Device Operation}{Device Types / I2C Adapter Device / Device Operation} > > + > > +A driver MUST set \field{addr}, \field{flags}(currently reserved as zero), > > I'd drop "(currently reserved as zero)" here and add > > "A driver MUST set \field{flags} to zero." > > > > +\field{written} and \field{read} before sending the request. > > + > > +A driver MUST place one segment of an I2C transaction into \field{write_buf} if the > > s/if the/if/ > > > +\field{written}>0. > > + > > +A driver MUST NOT use \field{read_buf} if the final \field{status} returned > > +from the device is VIRTIO_I2C_MSG_ERR. > > + > > +A driver MAY queue one request or multiple requests at a time. > > + > > +A driver MUST queue the requests in order if multiple requests are going to > > +be sent at a time. > > + > > +If multiple requests are sent at a time and some of them returned from the > > +device have the \field{status} being VIRTIO_I2C_MSG_ERR, a driver MUST treat > > +the first failed request and the requests after the first failed one as > > +VIRTIO_I2C_MSG_ERR. > > + > > +\devicenormative{\subsubsection}{Device Operation}{Device Types / I2C Adapter Device / Device Operation} > > + > > +A device SHOULD keep consistent behaviors with the hardware as described in > > +\hyperref[intro:I2C]{I2C}. > > + > > +A device MUST NOT change the value of \field{addr}, \field{flags}, \field{written}, > > +\field{read} and \field{write_buf}. > > + > > +A device MUST place one segment of an I2C transaction into \field{read_buf} if the > > s/if/if the/ > > > +\field{read}>0. > > + > > +A device MUST guarantee the requests in the virtqueue being processed in order > > +if multiple requests are received at a time. > > + > > +If multiple requests are received at a time and some of them being processed failed, and processing of some of the requests fails > > +a device MUST set the \field{status} of the first failed request to be > > +VIRTIO_I2C_MSG_ERR and MAY set the \field{status} of the requests after > > +the first failed one to be VIRTIO_I2C_MSG_ERR. So what is it saying? That VIRTIO_I2C_MSG_ERR can also mean "previous request failed"? -- MST
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