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Subject: Re: [virtio-comment] [PATCH v5] virtio-i2c: add the device specification
On Tue, Dec 22, 2020 at 12:29:09PM +0100, Cornelia Huck wrote: > On Tue, 22 Dec 2020 14:11:24 +0800 > Jie Deng <jie.deng@intel.com> wrote: > > > On 2020/12/20 3:05, Michael S. Tsirkin wrote: > > > On Fri, Dec 18, 2020 at 10:06:45AM +0800, Jie Deng wrote: > > >> On 2020/12/17 18:26, Stefan Hajnoczi wrote: > > >>> On Thu, Dec 17, 2020 at 03:00:55AM -0500, Michael S. Tsirkin wrote: > > >>>> On Thu, Dec 17, 2020 at 03:08:07PM +0800, Jie Deng wrote: > > >>>>> +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. > > >>>>> > > >>>>> This field seems redundant since the device can determine the size of > > >>>>> write_buf implicitly from the total out buffer size. virtio-blk takes > > >>>>> this approach. > > >>>>> > > >>>>> The read/write are the actual number of data bytes being read from or written > > >>>>> to the device > > >>>>> which is not determined by the device. So I don't think it is redundant. > > >>>> I am still not sure I understand the difference. > > >>>> This point is unclear to multiple people. > > >>> I think I get it now. This is made clear by splitting the struct: > > >>> > > >>> /* Driver->device fields */ > > >>> struct virtio_i2c_out_hdr > > >>> { > > >>> le16 addr; > > >>> le16 padding; > > >>> le32 flags; > > >>> }; > > >>> > > >>> /* Device->driver fields */ > > >>> struct virtio_i2c_in_hdr > > >>> { > > >>> le16 written; > > >>> le16 read; > > >>> u8 status; > > >>> }; > > >> written/read are not device->driver fields. They are driver->device fields. > > >> They are not determined by the device but the driver(user). > > >> > > >> However, Michael said that the two fields may duplicate buf size available > > >> in the descriptor. He intended to remove them. > > >> > > >> " > > >> 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 ... > > >> " > > >> > > >> But there is a corner case I'm not sure if you have noticed. > > >> > > >> read and written can be 0. I think we may not put a buf with size 0 into the > > >> virtqueue. > > > You always have the header and the status, right? > > > E.g. with the below, the total buffer size is virtio_i2c_out_hdr size + > > > write size for writes and read size + virtio_i2c_in_hdr size for reads. > > > Neither result is ever 0. > > > > Then how to distinguish the request type the buffer contains. > > I have read through the thread and I remain confused. > > > > > Each type will have both virtio_i2c_out_hdr and virtio_i2c_in_hdr. > > the backend can know the type by checking the read/written. > > > > If the read=0 and the written>0, the request is a write request > > The buffer may contains 3 scatterlist: > > > > virtio_i2c_out_hdr // scatterlist[0] > > So, what does virtio_i2c_{out,in}_hdr contain here? If it is the one from > above, ... > > > > >    buf[/* this is write data, since read = 0 */] // scatterlist[1] > > > >    virtio_i2c_in_hdr // scatterlist[2] > > ...we do not know whether there's read data, write data, or what their > length is, until we've actually consumed the whole buffer, and then we > have to go backwards. > > > > > If the read>0 and the written=0, the request is a read request. > > The buffer may contains 3 scatterlist: > > > > virtio_i2c_out_hdr // scatterlist[0] > > > >    buf[/* This is read data, since written = 0 */] // scatterlist[1] > > > >    virtio_i2c_in_hdr // scatterlist[2] > > > > If the read>0 and the written>0, the request is a write-read request. > > The buffer may contains 4 scatterlist: > > > > virtio_i2c_out_hdr  // scatterlist[0] > > > >    buf[/*write data*/] // scatterlist[1] > > > >    buf[/*read data*/] // scatterlist[2] > > > >    virtio_i2c_in_hdr // scatterlist[3] > > Is there any reason why we need to infer the type of the request by > checking some lengths? Can't we just specify explicit flags for read > and write? What am I missing? Point is descriptors already have flags for read/write. If there is a read buffer and length > sizeof virtio_i2c_in_hdr then we know it's a read request. If there is a write buffer and length > sizeof virtio_i2c_out_hdr then we know it's a write request. If both then both. All this is known before buffer itself is consumed, which is nice. Putting this info in flags will duplicate info which is often a source of errors. > > > > >> @Stefan @Paolo > > >> > > >> So what's your opinion about these two fields ? > > >> > > >>> /* > > >>> * Virtqueue element layout looks like this: > > >>> * > > >>> * struct virtio_i2c_out_hdr out_hdr; /* OUT */ > > >>> * u8 write_buf[]; /* OUT */ > > >>> * u8 read_buf[]; /* IN */ > > >>> * struct virtio_i2c_in_hdr in_hdr; /* IN */ > > >>> */ > > >>> > > >>> This makes sense to me: a bi-directional request has both write_buf[] > > >>> and read_buf[] so the vring used.len field is not enough to report back > > >>> how many bytes were written and read. The virtio_i2c_in_hdr fields are > > >>> really needed. > > >>> > > >>> Please split the struct in the spec so it's clear how this works.
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