linux-kernel - Re: [PATCH v5] i2c: virtio: add a virtio i2c frontend driver

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Date:   Tue, 2 Mar 2021 12:54:31 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Jie Deng <jie.deng@...el.com>
Cc:     linux-i2c@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, mst@...hat.com, wsa@...nel.org,
        jasowang@...hat.com, wsa+renesas@...g-engineering.com,
        andriy.shevchenko@...ux.intel.com, conghui.chen@...el.com,
        arnd@...db.de, kblaiech@...lanox.com,
        jarkko.nikula@...ux.intel.com, Sergey.Semin@...kalelectronics.ru,
        rppt@...nel.org, loic.poulain@...aro.org, tali.perry1@...il.com,
        u.kleine-koenig@...gutronix.de, bjorn.andersson@...aro.org,
        yu1.wang@...el.com, shuo.a.liu@...el.com,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v5] i2c: virtio: add a virtio i2c frontend driver

On 02-03-21, 14:24, Jie Deng wrote:
> Not for the full duplex. As Paolo explained in those links.
> We defined a combined request called "write-read request"
> 
> "
> This is when a write is followed by a read: the master
> starts off the transmission with a write, then sends a second START,
> then continues with a read from the same address.

I think above talks about the real I2C protocol at bus level ?

> In theory there's no difference between one multi-segment transaction
> and many single-segment transactions _in a single-master scenario_.
> 
> However, it is a plausible configuration to have multiple guests sharing
> an I2C host device as if they were multiple master.
> 
> So the spec should provide a way at least to support for transactions with
> 1 write and 1 read segment in one request to the same address.
> "

> From the perspective of specification design, it hopes to provide more
> choices
> while from the perspective of specific implementation, we can choose what we
> need
> as long as it does not violate the specification.

That is fine, but what I was not able to understand was how do we get
to know if we should expect both write and read bufs after the out
header or only one of them ?

I think I have understood that part now, we need to look at incnt and
outcnt and make out what kind of transfer we need to do.

- If outcnt == 1 and incnt == 2, then it is read operation.
- If outcnt == 2 and incnt == 1, then it is write operation.
- If outcnt == 2 and incnt == 2, then it is read-write operation.

Anything else is invalid. Is my understanding correct here ?

> Since the current Linux driver doesn't use this mechanism. I'm considering
> to move
> the "struct virtio_i2c_req" into the driver and use one "buf" instead.

Linux can very much have its own definition of the structure and so I
am not in favor of any such structure in the spec as well, it confuses
people (like me) :).

-- 
viresh