From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: virtio-comment-return-1583-cohuck=redhat.com@lists.oasis-open.org Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Date: Tue, 22 Dec 2020 12:29:09 +0100 From: Cornelia Huck Message-ID: <20201222122909.3168620a.cohuck@redhat.com> In-Reply-To: <20b91229-e938-66fb-464b-c85c1dbcde96@intel.com> References: <1ea117ebe46d105eda21544acf85a5c7dbe2d8ec.1606283457.git.jie.deng@intel.com> <20201216155218.GA720702@stefanha-x1.localdomain> <20201217030015-mutt-send-email-mst@kernel.org> <20201217102640.GG4338@stefanha-x1.localdomain> <1ccc5889-4e28-deac-8168-c6f0665231c1@intel.com> <20201218155526-mutt-send-email-mst@kernel.org> <20b91229-e938-66fb-464b-c85c1dbcde96@intel.com> MIME-Version: 1.0 Subject: Re: [virtio-comment] [PATCH v5] virtio-i2c: add the device specification Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable To: Jie Deng Cc: "Michael S. Tsirkin" , Stefan Hajnoczi , Paolo Bonzini , virtio-comment@lists.oasis-open.org, virtio-dev@lists.oasis-open.org, kraxel@redhat.com, wsa+renesas@sang-engineering.com, andriy.shevchenko@linux.intel.com, conghui.chen@intel.com, yu1.wang@intel.com, shuo.a.liu@intel.com List-ID: On Tue, 22 Dec 2020 14:11:24 +0800 Jie Deng wrote: > On 2020/12/20 3:05, Michael S. Tsirkin wrote: > > On Fri, Dec 18, 2020 at 10:06:45AM +0800, Jie Deng wrote: =20 > >> On 2020/12/17 18:26, Stefan Hajnoczi wrote: =20 > >>> On Thu, Dec 17, 2020 at 03:00:55AM -0500, Michael S. Tsirkin wrote: = =20 > >>>> On Thu, Dec 17, 2020 at 03:08:07PM +0800, Jie Deng wrote: =20 > >>>>> +The \field{flags} of the request is currently reserved a= s 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-b= lk 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 redun= dant. =20 > >>>> I am still not sure I understand the difference. > >>>> This point is unclear to multiple people. =20 > >>> 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; > >>> }; =20 > >> written/read are not device->driver fields. They are driver->device fi= elds. > >> They are not determined by the device but the driver(user). > >> > >> However, Michael said that the two fields may duplicate buf size avail= able > >> 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 in= to the > >> virtqueue. =20 > > 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. =20 >=20 > Then how to distinguish the request type the buffer contains. I have read through the thread and I remain confused. >=20 > 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. >=20 > If the read=3D0 and the written>0, the request is a write request > The buffer may contains 3 scatterlist: >=20 > virtio_i2c_out_hdr // scatterlist[0] So, what does virtio_i2c_{out,in}_hdr contain here? If it is the one from above, ... >=20 > =C2=A0=C2=A0=C2=A0 buf[/* this is write data, since read =3D 0 */] // sc= atterlist[1] >=20 > =C2=A0=C2=A0=C2=A0 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. >=20 > If the read>0 and the written=3D0, the request is a read request. > The buffer may contains 3 scatterlist: >=20 > virtio_i2c_out_hdr // scatterlist[0] >=20 > =C2=A0=C2=A0=C2=A0 buf[/* This is read data, since written =3D 0 */] // = scatterlist[1] >=20 > =C2=A0=C2=A0=C2=A0 virtio_i2c_in_hdr // scatterlist[2] >=20 > If the read>0 and the written>0, the request is a write-read request. > The buffer may contains 4 scatterlist: >=20 > virtio_i2c_out_hdr=C2=A0=C2=A0 // scatterlist[0] >=20 > =C2=A0=C2=A0=C2=A0 buf[/*write data*/]=C2=A0 // scatterlist[1] >=20 > =C2=A0=C2=A0=C2=A0 buf[/*read data*/] // scatterlist[2] >=20 > =C2=A0=C2=A0=C2=A0 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? >=20 > >> @Stefan @Paolo > >> > >> So what's your opinion about these two fields ? > >> =20 > >>> /* > >>> * 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 ba= ck > >>> how many bytes were written and read. The virtio_i2c_in_hdr fields ar= e > >>> really needed. > >>> > >>> Please split the struct in the spec so it's clear how this works. =20 This publicly archived list offers a means to provide input to the=0D OASIS Virtual I/O Device (VIRTIO) TC.=0D =0D In order to verify user consent to the Feedback License terms and=0D to minimize spam in the list archive, subscription is required=0D before posting.=0D =0D Subscribe: virtio-comment-subscribe@lists.oasis-open.org=0D Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org=0D List help: virtio-comment-help@lists.oasis-open.org=0D List archive: https://lists.oasis-open.org/archives/virtio-comment/=0D Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf= =0D List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lis= ts=0D Committee: https://www.oasis-open.org/committees/virtio/=0D Join OASIS: https://www.oasis-open.org/join/