linux-kernel - Re: [PATCH 5/5] iio: adc: ad7380: use spi_optimize

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Message-ID: <e03968102b92b3711808eb532685bc9e05fc3c8d.camel@gmail.com>
Date: Tue, 13 Feb 2024 17:08:19 +0100
From: Nuno Sá <noname.nuno@...il.com>
To: David Lechner <dlechner@...libre.com>
Cc: Mark Brown <broonie@...nel.org>, Martin Sperl <kernel@...tin.sperl.org>,
  David Jander <david@...tonic.nl>, Jonathan Cameron <jic23@...nel.org>,
 Michael Hennerich <michael.hennerich@...log.com>, Nuno
 Sá <nuno.sa@...log.com>,  Alain Volmat
 <alain.volmat@...s.st.com>, Maxime Coquelin <mcoquelin.stm32@...il.com>,
 Alexandre Torgue <alexandre.torgue@...s.st.com>, linux-spi@...r.kernel.org,
  linux-kernel@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com, 
 linux-arm-kernel@...ts.infradead.org, linux-iio@...r.kernel.org
Subject: Re: [PATCH 5/5] iio: adc: ad7380: use spi_optimize_message()

On Tue, 2024-02-13 at 09:27 -0600, David Lechner wrote:
> On Tue, Feb 13, 2024 at 3:47 AM Nuno Sá <noname.nuno@...il.com> wrote:
> > 
> > On Mon, 2024-02-12 at 17:26 -0600, David Lechner wrote:
> > > This modifies the ad7380 ADC driver to use spi_optimize_message() to
> > > optimize the SPI message for the buffered read operation. Since buffered
> > > reads reuse the same SPI message for each read, this can improve
> > > performance by reducing the overhead of setting up some parts the SPI
> > > message in each spi_sync() call.
> > > 
> > > Signed-off-by: David Lechner <dlechner@...libre.com>
> > > ---
> > >  drivers/iio/adc/ad7380.c | 52 +++++++++++++++++++++++++++++++++++++++++--
> > > ----
> > > -
> > >  1 file changed, 45 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/drivers/iio/adc/ad7380.c b/drivers/iio/adc/ad7380.c
> > > index abd746aef868..5c5d2642a474 100644
> > > --- a/drivers/iio/adc/ad7380.c
> > > +++ b/drivers/iio/adc/ad7380.c
> > > @@ -133,6 +133,7 @@ struct ad7380_state {
> > >       struct spi_device *spi;
> > >       struct regulator *vref;
> > >       struct regmap *regmap;
> > > +     struct spi_message *msg;
> > >       /*
> > >        * DMA (thus cache coherency maintenance) requires the
> > >        * transfer buffers to live in their own cache lines.
> > > @@ -231,19 +232,55 @@ static int ad7380_debugfs_reg_access(struct iio_dev
> > > *indio_dev, u32 reg,
> > >       return ret;
> > >  }
> > > 
> > > +static int ad7380_buffer_preenable(struct iio_dev *indio_dev)
> > > +{
> > > +     struct ad7380_state *st = iio_priv(indio_dev);
> > > +     struct spi_transfer *xfer;
> > > +     int ret;
> > > +
> > > +     st->msg = spi_message_alloc(1, GFP_KERNEL);
> > > +     if (!st->msg)
> > > +             return -ENOMEM;
> > > +
> > > +     xfer = list_first_entry(&st->msg->transfers, struct spi_transfer,
> > > +                             transfer_list);
> > > +
> > > +     xfer->bits_per_word = st->chip_info->channels[0].scan_type.realbits;
> > > +     xfer->len = 4;
> > > +     xfer->rx_buf = st->scan_data.raw;
> > > +
> > > +     ret = spi_optimize_message(st->spi, st->msg);
> > > +     if (ret) {
> > > +             spi_message_free(st->msg);
> > > +             return ret;
> > > +     }
> > > +
> > > +     return 0;
> > > +}
> > > +
> > > +static int ad7380_buffer_postdisable(struct iio_dev *indio_dev)
> > > +{
> > > +     struct ad7380_state *st = iio_priv(indio_dev);
> > > +
> > > +     spi_unoptimize_message(st->msg);
> > > +     spi_message_free(st->msg);
> > > +
> > > +     return 0;
> > > +}
> > > +
> > 
> > Not such a big deal but unless I'm missing something we could have the
> > spi_message (+ the transfer) statically allocated in struct ad7380_state and
> > do
> > the optimize only once at probe (naturally with proper devm action for
> > unoptimize). Then we would not need to this for every buffer enable +
> > disable. I
> > know in terms of performance it won't matter but it would be less code I
> > guess.
> > 
> > Am I missing something?
> 
> No, your understanding is correct for the current state of everything
> in this series. So, we could do as you suggest, but I have a feeling
> that future additions to this driver might require that it gets
> changed back this way eventually.

Hmm, not really sure about that as chip_info stuff is always our friend :). And
I'm anyways of the opinion of keeping things simpler and start to evolve when
really needed (because often we never really need to evolve). But bah, as I
said... this is really not a big deal.

- Nuno Sá