Re: [PATCH] iio: at91-sama5d2_adc: fix iio_triggered_buffer_{predisable,postenable} positions

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

On Tue, 2019-12-03 at 09:49 +0000, Eugen.Hristev@microchip.com wrote:
> [External]
> 
> 
> 
> On 29.11.2019 09:02, Ardelean, Alexandru wrote:
> 
> > On Thu, 2019-11-28 at 15:19 +0000, Eugen.Hristev@microchip.com wrote:
> > 
> > Hey,
> > 
> > Sorry for the late reply.
> > I'm also juggling a few things.
> > 
> > > On 28.11.2019 10:36, Eugen.Hristev@microchip.com wrote:
> > > 
> > > > On 25.11.2019 17:03, Ardelean, Alexandru wrote:
> > > > > On Wed, 2019-10-23 at 11:25 +0300, Alexandru Ardelean wrote:
> > > > > > The iio_triggered_buffer_{predisable,postenable} functions
> > > > > > attach/detach
> > > > > > poll functions.
> > > > > > 
> > > > > > The iio_triggered_buffer_postenable() should be called first to
> > > > > > attach
> > > > > > the
> > > > > > poll function, and then the driver can init the data to be
> > > > > > triggered.
> > > > > > 
> > > > > > Similarly, iio_triggered_buffer_predisable() should be called
> > > > > > last
> > > > > > to
> > > > > > first
> > > > > > disable the data (to be triggered) and then the poll function
> > > > > > should be
> > > > > > detached.
> > > > 
> > > > Hi Alexandru,
> > > > 
> > > > Sorry for this late reply,
> > > > 
> > > > I remember that by adding specific at91_adc code for
> > > > predisable/postenable , I was replacing the existing standard
> > > > callback
> > > > with my own, and have my specific at91 code before postenable and
> > > > then
> > > > calling the subsystem postenable,
> > > > and in similar way, for predisable, first call the subsystem
> > > > predisable
> > > > then doing my predisable code (in reverse order as in postenable)
> > > > 
> > > > If you say the order should be reversed (basically have the
> > > > pollfunction
> > > > first), how is current code working ?
> > > > Should current code fail if the poll function is not attached in
> > > > time ?
> > > > Or there is a race between triggered data and the attachment of the
> > > > pollfunc ?
> > > > 
> > > > I am thinking that attaching the pollfunc later makes it work
> > > > because
> > > > the DMA is not started yet. What happens if we have the pollfunc
> > > > attached but DMA is not started (basically the trigger is not
> > > > started)
> > > > ,
> > > > can this lead to unexpected behavior ? Like the pollfunc polling
> > > > but no
> > > > trigger started/no DMA started.
> > > 
> > > I looked a bit more into the code and in DMA case, using postenable
> > > first will lead to calling attach pollfunc, which will also enable
> > > the
> > > trigger, but the DMA is not yet started.
> > > Is this the desired effect ?
> > 
> > Yes.
> 
> How is this correct ? We start the trigger but have no buffer to carry 
> to... what happens with the data ? -> I think we both have an answer to 
> that, as you state below
> 
> > > Normally when using DMA I would say we
> > > would need to enable DMA first to be ready to carry data (and
> > > coherent
> > > area etc.) and then enable the trigger.
> > 
> > So, there is a change in our tree [from some time ago].
> > See here:
> > https://github.com/analogdevicesinc/linux/commit/eee97d12665fef8cf429a1e5035b23ae969705b8
> > 
> > Particularly, what's interesting is around line:
> > https://github.com/analogdevicesinc/linux/commit/eee97d12665fef8cf429a1e5035b23ae969705b8#diff-0a87744ce945d2c1c89ea19f21fb35bbR722
> > And you may need to expand some stuff to see more of the function-body.
> > And some things may have changed in upstream IIO since that change.
> > 
> > The change is to make the pollfunc attach/detach become part of the IIO
> > framework, because plenty of drivers just call
> > iio_triggered_buffer_postenable() & iio_triggered_buffer_predisable()
> > to
> > manually attach/detach the pollfunc for triggered buffers.
> 
> Okay, I understand this. at91-sama5d2_adc does not manually 
> attach/detach the pollfunc. So why do we need to change anything here ?
> 
> 
> > That change is from 2015, and since then, some drivers were added that
> > just
> > manually attach/detach the pollfunc [and do nothing more with the
> > postenable/predisable hooks].
> > 
> > I tried to upstream a more complete version of that patch a while ago
> > [u1].
> > https://patchwork.kernel.org/patch/10482167/
> > https://patchwork.kernel.org/patch/10737291/
> > 
> > The conclusion was to first fix the attach/detach pollfunc order in all
> > IIO
> > drivers, so that when patch [u1] is applied, there is no more
> > discussion
> > about the correct order for attach/detach pollfunc.
> 
> Allright, what is required to be fixed regarding the order, in this 
> specific case? We enable the DMA, and then we do the normal 'postenable' 
> that was called anyway if we did not override the 'postenable' in the 
> ops. Do you want to move this code to 'preenable' and keep 'postenable' 
> to the standard subsystem one ?
> 
> The same applies to the predisable, we first call the subsystem 
> 'predisable' then do the specific at91 stuff. You want to move this to 
> the 'postdisable' ?
> 
> I think reverting the order inside the functions themselves is not good 
> as we replace the order of starting trigger/DMA setup.
> So, coming to your question below...
> 
> > Coming back here [and to your question], my answer is: I don't know if
> > the
> > at91 DMA needs to be enabled/disabled before/after the pollfunc
> > attach/detach.
> > This sounds like specific stuff for at91 [which is fine].
> > 
> > It could be that some other hooks may need to used to enable DMA
> > before/after the attach/detach pollfunc. Maybe
> > preenable()/postdisable() ?
> > 
> > In any case, what I would like [with this discussion], is to resolve a
> > situation where we can get closer to moving the attach/pollfunc code to
> > IIO
> > core. So, if AT91 requires a different ordering, I think you would be
> > more
> > appropriate to tell me, and propose an alternative to this patch.
> 
> ... yes, this looks more appropriate, to move things to 
> 'preenable/postdisable', if you feel like 'postenable/predisable' is not 
> the proper place to put them.
> But the order itself, first enable DMA then trigger, and disable in 
> reverse order, I do not think there is anything wrong with that? Am I 
> misunderstanding ?

Should be good.

> 
> If Jonathan or Ludovic have a different idea, please let me know.

There is an alternative here [to this].
Maybe using the IIO Buffer DMA[Engine] integration that Lars wrote [1].
This would avoid calling dmaengine_terminate_sync() and similar hooks in
the AT91 driver. That also preserves the correct order (start DMA first,
then attach pollfunc ; and reverse on disable).
But that is more work; not on the patch itself, but more on the testing.

[1] Upstreaming more parts for the IIO Buffer DMA[Engine] integration is on
my to-do-list as well. I think there are still some patches that we use,
but are not upstreamed yet.

I'll come-up a with a V2 for this with preenable()/postdisable()
alternative here.

Thanks
Alex

> 
> Also, I can test your patch to see if everything is fine.
> 
> Thanks,
> Eugen
> 
> > Thanks :)
> > Alex
> > 
> > > > > > For this driver, the predisable & postenable hooks are also
> > > > > > need to
> > > > > > take
> > > > > > into consideration the touchscreen, so the hooks need to be put
> > > > > > in
> > > > > > places
> > > > > > that avoid the code for that cares about it.
> > > > > > 
> > > > > 
> > > > > ping here
> > > > > 
> > > > > > Signed-off-by: Alexandru Ardelean <
> > > > > > alexandru.ardelean@analog.com>
> > > > > > ---
> > > > > >     drivers/iio/adc/at91-sama5d2_adc.c | 19 ++++++++++---------
> > > > > >     1 file changed, 10 insertions(+), 9 deletions(-)
> > > > > > 
> > > > > > diff --git a/drivers/iio/adc/at91-sama5d2_adc.c
> > > > > > b/drivers/iio/adc/at91-
> > > > > > sama5d2_adc.c
> > > > > > index e1850f3d5cf3..ac3e5c4c9840 100644
> > > > > > --- a/drivers/iio/adc/at91-sama5d2_adc.c
> > > > > > +++ b/drivers/iio/adc/at91-sama5d2_adc.c
> > > > > > @@ -889,20 +889,24 @@ static int
> > > > > > at91_adc_buffer_postenable(struct
> > > > > > iio_dev *indio_dev)
> > > > > >          if (!(indio_dev->currentmode &
> > > > > > INDIO_ALL_TRIGGERED_MODES))
> > > > > >                  return -EINVAL;
> > > > > > 
> > > > > > +     ret = iio_triggered_buffer_postenable(indio_dev);
> > > > > > +     if (ret)
> > > > > > +             return ret;
> > > > > > +
> > > > > >          /* we continue with the triggered buffer */
> > > > > >          ret = at91_adc_dma_start(indio_dev);
> > > > > >          if (ret) {
> > > > > >                  dev_err(&indio_dev->dev, "buffer postenable
> > > > > > failed\n");
> > > > > > +             iio_triggered_buffer_predisable(indio_dev);
> > > > > >                  return ret;
> > > > > >          }
> > > > > > 
> > > > > > -     return iio_triggered_buffer_postenable(indio_dev);
> > > > > > +     return 0;
> > > > > >     }
> > > > > > 
> > > > > >     static int at91_adc_buffer_predisable(struct iio_dev
> > > > > > *indio_dev)
> > > > > >     {
> > > > > >          struct at91_adc_state *st = iio_priv(indio_dev);
> > > > > > -     int ret;
> > > > > >          u8 bit;
> > > > > > 
> > > > > >          /* check if we are disabling triggered buffer or the
> > > > > > touchscreen */
> > > > > > @@ -916,13 +920,8 @@ static int
> > > > > > at91_adc_buffer_predisable(struct
> > > > > > iio_dev
> > > > > > *indio_dev)
> > > > > >          if (!(indio_dev->currentmode &
> > > > > > INDIO_ALL_TRIGGERED_MODES))
> > > > > >                  return -EINVAL;
> > > > > > 
> > > > > > -     /* continue with the triggered buffer */
> > > > > > -     ret = iio_triggered_buffer_predisable(indio_dev);
> > > > > > -     if (ret < 0)
> > > > > > -             dev_err(&indio_dev->dev, "buffer predisable
> > > > > > failed\n");
> > > > > > -
> > > > > >          if (!st->dma_st.dma_chan)
> > > > > > -             return ret;
> > > > > > +             goto out;
> > > > > > 
> > > > > >          /* if we are using DMA we must clear registers and end
> > > > > > DMA
> > > > > > */
> > > > > >          dmaengine_terminate_sync(st->dma_st.dma_chan);
> > > > > > @@ -949,7 +948,9 @@ static int
> > > > > > at91_adc_buffer_predisable(struct
> > > > > > iio_dev
> > > > > > *indio_dev)
> > > > > > 
> > > > > >          /* read overflow register to clear possible overflow
> > > > > > status
> > > > > > */
> > > > > >          at91_adc_readl(st, AT91_SAMA5D2_OVER);
> > > > > > -     return ret;
> > > > > > +
> > > > > > +out:
> > > > 
> > > > I would prefer if this label is named with a function name prefix,
> > > > otherwise 'out' is pretty generic and can collide with other things
> > > > in
> > > > the file... I want to avoid having an out2 , out3 later if code
> > > > changes.
> > > > 
> > 
> > Sure.
> > Will do that.
> > 
> > I did not bother much with these labels, because after applying [u1],
> > some
> > of them [maybe all] should go away.
> > 
> > 
> > > > Thanks for the patch,
> > > > Eugen
> > > > 
> > > > > > +     return iio_triggered_buffer_predisable(indio_dev);
> > > > > >     }
> > > > > > 
> > > > > >     static const struct iio_buffer_setup_ops
> > > > > > at91_buffer_setup_ops =
> > > > > > {
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