Re: [PATCH 3/4] iio: ad7949: fix SPI xfer delays

["Ardelean, Alexandru" <alexandru.Ardelean@analog.com>]

On Fri, 2019-09-13 at 10:23 +0200, Andrea Merello wrote:
> Il giorno ven 13 set 2019 alle ore 09:00 Ardelean, Alexandru
> <alexandru.Ardelean@analog.com> ha scritto:
> > On Thu, 2019-09-12 at 16:43 +0200, Andrea Merello wrote:
> > > [External]
> > > 
> > > The driver calls udelay(2) after each SPI xfer. However, according to
> > > the specifications, the SPI timing should be as follows:
> > > 
> > > 1- The end of SPI xfer (CNV/CS rising edge) causes the device to initiate
> > >    the conversion phase, which takes up to 2.2uS.
> > 
> > Yes, but there does not seem to be a minimum time for conversion.
> > ( 2.2 uS is the max value )
> > 
> > This can be confusing a bit (I know).
> > If you do see issues with 2 uS, we can probably try 3 uS (+1 uS which is roughly half the max value).
> > Though, we are already gaining min 200 nS from the fact that the acquisition time is 1.8 uS and the delay is 2 uS.
> > 
> > But if there aren't any visible issues, I would leave 2 uS.
> > Increasing this delay can annoy people that would like to have some speed when reading samples.
> 
> I admit that I got some hard time trying to fully understand the
> timing specifications; so it's perfectly possible that I've got
> something wrong here.. My interpretation was that the HW takes up to
> 2.2uS (thus it's a max value, as you said) and, since we are not using
> the busy indication to check when the HW really finished, I stayed on
> the safe side.
> 
> I've done this change while I was searching for the cause of some
> reading issues that turned out to be actually fixed with the last
> patch of the series, so I have no real evidence of  issues caused by
> the 2uS delay. However, if I understood correctly the datasheet, the
> effect of performing an early SPI xfer during the conversion phase -
> specifically if it happens after tDATA and before EndOfConversion -
> might be mild, so not it might be not obvious to notice (maybe affects
> just LSBs) it with my basic testing. Quoting the datasheet: "The time
> between tDATA and tCONV is a safe time when digital activity should
> not occur, or sensitive bit decisions may be corrupted. "..
> 
> > I know 1-2 uS isn't much of a performance killer, but if there aren't reasons to change it, I wouldn't.
> 
> I guess you know the HW by far better than me, so if you say that 2uS
> is OK then I will not insist anymore here :)

Well, yes & no.
To get the best possible answer, we would need to ask more directly to the business-unit that has released the part,
which is a bit disconnected from the group that writes & upstreams Linux drivers.
Depending on the effort required to go through this, or depending how important this item is, we can do that.

But, if testing suggests that 2 uS works, then it's also fine to use it.

> 
> > > 2- At the end of the conversion phase, the device starts the acquisition
> > >    phase for the next conversion automatically (regardless to the state of
> > >    CNV pin); the conversion phase should last at least 1.8 uS
> > > 
> > > The whole cycle timing is thus 4uS long. The SPI data is read during the
> > > acquisition phase (RAC mode, no need to worry about "Tdata").
> > > 
> > > In order to be compliant wrt these timing specifications we should wait
> > > 4uS after each SPI xfer (that is conservative, because there is also the
> > > SPI xfer duration itself - which at the maximum supported clock should be
> > > about 320nS).
> > > 
> > > This patch enlarges the delay up to 4uS and it also removes the explicit
> > > calls to udelay(), relying on spi_transfer->delay_usecs.
> > > 
> > 
> > I like the switch from explicit udelay() to spi_transfer->delay_usecs.
> > The code looks cleaner.
> > 
> > > Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
> > > ---
> > >  drivers/iio/adc/ad7949.c | 13 ++-----------
> > >  1 file changed, 2 insertions(+), 11 deletions(-)
> > > 
> > > diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c
> > > index 5c2b3446fa4a..25d1e1b24257 100644
> > > --- a/drivers/iio/adc/ad7949.c
> > > +++ b/drivers/iio/adc/ad7949.c
> > > @@ -69,6 +69,7 @@ static int ad7949_spi_write_cfg(struct ad7949_adc_chip *ad7949_adc, u16 val,
> > >                       .tx_buf = &ad7949_adc->buffer,
> > >                       .len = 2,
> > >                       .bits_per_word = bits_per_word,
> > > +                     .delay_usecs = 4,
> > >               },
> > >       };
> > > 
> > > @@ -77,11 +78,6 @@ static int ad7949_spi_write_cfg(struct ad7949_adc_chip *ad7949_adc, u16 val,
> > >       spi_message_init_with_transfers(&msg, tx, 1);
> > >       ret = spi_sync(ad7949_adc->spi, &msg);
> > > 
> > > -     /*
> > > -      * This delay is to avoid a new request before the required time to
> > > -      * send a new command to the device
> > > -      */
> > > -     udelay(2);
> > >       return ret;
> > >  }
> > > 
> > > @@ -97,6 +93,7 @@ static int ad7949_spi_read_channel(struct ad7949_adc_chip *ad7949_adc, int *val,
> > >                       .rx_buf = &ad7949_adc->buffer,
> > >                       .len = 2,
> > >                       .bits_per_word = bits_per_word,
> > > +                     .delay_usecs = 4,
> > >               },
> > >       };
> > > 
> > > @@ -112,12 +109,6 @@ static int ad7949_spi_read_channel(struct ad7949_adc_chip *ad7949_adc, int *val,
> > >       if (ret)
> > >               return ret;
> > > 
> > > -     /*
> > > -      * This delay is to avoid a new request before the required time to
> > > -      * send a new command to the device
> > > -      */
> > > -     udelay(2);
> > > -
> > >       ad7949_adc->current_channel = channel;
> > > 
> > >       *val = ad7949_adc->buffer & mask;