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

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Date:   Tue, 2 Mar 2021 09:14:41 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Jie Deng <jie.deng@...el.com>, 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,
        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
Subject: Re: [PATCH v5] i2c: virtio: add a virtio i2c frontend driver

On 01-03-21, 14:10, Andy Shevchenko wrote:
> On Mon, Mar 01, 2021 at 02:09:25PM +0200, Andy Shevchenko wrote:
> > On Mon, Mar 01, 2021 at 05:24:41PM +0530, Viresh Kumar wrote:
> > > On 01-03-21, 14:41, Jie Deng wrote:
> > > > +/**
> > > > + * struct virtio_i2c_req - the virtio I2C request structure
> > > > + * @out_hdr: the OUT header of the virtio I2C message
> > > > + * @write_buf: contains one I2C segment being written to the device
> > > > + * @read_buf: contains one I2C segment being read from the device
> > > > + * @in_hdr: the IN header of the virtio I2C message
> > > > + */
> > > > +struct virtio_i2c_req {
> > > > +	struct virtio_i2c_out_hdr out_hdr;
> > > > +	u8 *write_buf;
> > > > +	u8 *read_buf;
> > > > +	struct virtio_i2c_in_hdr in_hdr;
> > > > +};
> > > 
> > > I am not able to appreciate the use of write/read bufs here as we
> > > aren't trying to read/write data in the same transaction. Why do we
> > > have two bufs here as well as in specs ?
> > 
> > I²C and SMBus support bidirectional transfers as well. I think two buffers is
> > the right thing to do.
> 
> Strictly speaking "half duplex".

Half duplex is what this driver implemented, i.e. only one side can
send a msg at once, we don't need two buffers for that for sure.

Though we need two buffers if we are ever going to support full
duplex.

-- 
viresh