From:   Akhil R <akhilrajeev@nvidia.com>
To:     Thierry Reding <thierry.reding@gmail.com>
CC:     "christian.koenig@amd.com" <christian.koenig@amd.com>,
        "digetx@gmail.com" <digetx@gmail.com>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        Laxman Dewangan <ldewangan@nvidia.com>,
        "linux-i2c@vger.kernel.org" <linux-i2c@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-tegra@vger.kernel.org" <linux-tegra@vger.kernel.org>,
        "sumit.semwal@linaro.org" <sumit.semwal@linaro.org>,
        "wsa@kernel.org" <wsa@kernel.org>,
        "robh+dt@kernel.org" <robh+dt@kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>
Subject: RE: [PATCH v2] i2c: tegra: Share same DMA channel for Rx and Tx
Thread-Topic: [PATCH v2] i2c: tegra: Share same DMA channel for Rx and Tx
Thread-Index: AQHZRqhoys7GDrpzlk6j6Sg5IxRKrK7cPgMAgAAO5xA=
Date:   Thu, 23 Feb 2023 10:04:43 +0000
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References: <20230222102759.23165-1-akhilrajeev@nvidia.com>
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> > Allocate only one DMA channel for I2C and share it for both Tx and Rx.
> > Since I2C supports only half duplex, there is no impact on perf with
> > this.
> >
> > Signed-off-by: Akhil R <akhilrajeev@nvidia.com>
> > ---
> > v1->v2: Remove WARN_ON for DMA channel mismatch. There is only one
> > channel in use with this change.
> >
> >  drivers/i2c/busses/i2c-tegra.c | 54 ++++++++++------------------------
> >  1 file changed, 15 insertions(+), 39 deletions(-)
>=20
> I'm a little confused by this. All device trees already list the very
> same reference for both TX and RX DMA channels in the I2C nodes, so
> these channels are already effectively shared, aren't they?
The value given in DT refers to the slave-id of an I2C instance and not the=
=20
channel number. Each I2C instance was in-effect using two different DMA=20
channels with the same slave-id.
This change is to free up the extra DMA channel since both will not be
used simultaneously.=20

>=20
> So all this does is to get rid of the duplicated pointer? In practice,
> is the DMA channel pointer going to point to the exact same memory or
> are these separate objects that happen to point to the same hardware
> resource?
They are indeed separate hardware resource itself. As I described above,
two separate DMA channels were in use for each I2C instance.

>=20
> In either case, I think the commit message should clarify that. Also, a
> few minor nits below...

Agreed with the other comments.

Regards,
Akhil