Re: [PATCH 3/5] iio: adc: sunxi-gpadc-iio: enable iio_buffers

From: Quentin Schulz
Date: Sat Sep 24 2016 - 13:40:58 EST

Hi Jonathan,

Sorry for the (long) delay, I did not have time to work on it. I'll
mainly work in my free time now.

Keep in mind this patch was proposed based on the v2 of the ADC patches.
Since then, substantial changes have been made and I'm working on
rebasing this series of patches on the v6, so comments here might
include references to code parts added later in the ADC patch series.

On 24/07/2016 13:03, Jonathan Cameron wrote:
> On 20/07/16 09:29, Quentin Schulz wrote:
>> This enables the use of buffers on ADC channels of sunxi-gpadc-iio driver.
>> It also prepares the code which will be used by the touchscreen driver
>> named sunxi-gpadc-ts.
>> The GPADC on Allwinner SoCs (A10, A13 and A31) has a 12 bits register for
>> conversion's data. The GPADC uses the same ADC channels for the ADC and the
>> touchscreen therefore exposes these channels to the sunxi-gpadc-ts iio
>> consumer which will be in charge of reading data from these channels for
>> the input framework.
>> The temperature can only be read when in touchscreen mode. This means if
>> the buffers are being used for the ADC, the temperature sensor cannot be
>> read.
> That may be the bizarest hardware restriction I've heard of in a while! :)
>> When a FIFO_DATA_PENDING irq occurs, its handler will read the entire FIFO
>> and fill a buffer before sending it to the consumers which registered in
>> IIO for the ADC channels.
>> When a consumer starts buffering ADC channels,
>> sunxi_gpadc_buffer_postenable is called and will enable FIFO_DATA_PENDING
>> irq and select the mode in which the GPADC should run (ADC or touchscreen)
>> depending on a property of the DT ("allwinner,ts-attached").
>> When the consumer stops buffering, it disables the same irq.
> Hmm. Might be possible to distinguish which consumer caused the start.
> Thus, if the touchscreen is there we would know purely based on the
> driver being the requester that we need to be in touchscreen mode.

As of yet, can't see in which way I can retrieve the consumer in
provider code. Maybe I'm missing something, I don't know?

>> Signed-off-by: Quentin Schulz <quentin.schulz@xxxxxxxxxxxxxxxxxx>
> You are moving fast on this - I'd have been tempted to do a mega
> series with the updated version of the basic support and this on top
> rather than a new unconnected series.
> (I'd forgotten that was still under review so got confused when I
> went to look something up in the files you are modifying!).
>> ---
>> drivers/iio/adc/Kconfig | 1 +
>> drivers/iio/adc/sunxi-gpadc-iio.c | 153 ++++++++++++++++++++++++++++++++++----
>> 2 files changed, 138 insertions(+), 16 deletions(-)
>> diff --git a/drivers/iio/adc/Kconfig b/drivers/iio/adc/Kconfig
>> index 184856f..15e3b08 100644
>> --- a/drivers/iio/adc/Kconfig
>> +++ b/drivers/iio/adc/Kconfig
>> @@ -342,6 +342,7 @@ config SUNXI_ADC
>> tristate "ADC driver for sunxi platforms"
>> depends on IIO
>> depends on MFD_SUNXI_ADC
>> + depends on IIO_BUFFER_CB
>> help
>> Say yes here to build support for Allwinner (A10, A13 and A31) SoCs
>> ADC. This ADC provides 4 channels which can be used as an ADC or as a
>> diff --git a/drivers/iio/adc/sunxi-gpadc-iio.c b/drivers/iio/adc/sunxi-gpadc-iio.c
>> index 87cc913..2e44ca7 100644
>> --- a/drivers/iio/adc/sunxi-gpadc-iio.c
>> +++ b/drivers/iio/adc/sunxi-gpadc-iio.c
>> @@ -16,8 +16,9 @@
>> #include <linux/platform_device.h>
>> #include <linux/regmap.h>
>> -#include <linux/iio/iio.h>
>> +#include <linux/iio/buffer.h>
>> #include <linux/iio/driver.h>
>> +#include <linux/iio/iio.h>
> Can't say I'm a particular fan of reordering headers to be in alphabetical
> order, but I suppose it doesn't really matter if you want to do it.
> (to my mind there is a tree structure implicit in these headers with iio.h
> at the top for generic support, then the various sub elements below).
>> #include <linux/iio/machine.h>
>> #include <linux/mfd/sunxi-gpadc-mfd.h>
>> @@ -71,6 +72,7 @@
>> #define SUNXI_GPADC_TP_FIFO_TRIG_LEVEL(x) ((x) << 8) /* 5 bits */
>> +/* Be careful, flushing FIFO spawns SUNXI_GPADC_FIFO_DATA_PENDING interrupts */
> Sounds like you learned that one the hard way ;)
>> @@ -79,6 +81,7 @@
>> +#define SUNXI_GPADC_RXA_CNT GENMASK(12, 8)
>> @@ -101,19 +104,43 @@ struct sunxi_gpadc_dev {
>> unsigned int fifo_data_irq;
>> unsigned int temp_data_irq;
>> unsigned int flags;
>> + struct iio_dev *indio_dev;
> I was suprised to see this as normally it is cleaner to structure
> the whole code to go in one direction through the structures (which is
> why we don't provide a generic iio_device_from_priv bit of pointer magic).
> Anyhow, don't htink you are actually using it ;)

I'm using to push to buffers from the irq handler since I pass the local
structure (sunxi_gpadc_dev) to the irq handler when registering it. But
I guess I can pass the iio_dev instead and remove this from the local

>> static int sunxi_gpadc_adc_read(struct iio_dev *indio_dev, int channel,
>> int *val)
>> {
>> struct sunxi_gpadc_dev *info = iio_priv(indio_dev);
>> + bool buffered = info->buffered;
> Not worth the local version...
>> int ret = 0;
>> + unsigned int reg;
>> mutex_lock(&indio_dev->mlock);
>> reinit_completion(&info->completion);
>> +
>> + regmap_update_bits(info->regmap, SUNXI_GPADC_TP_INT_FIFOC, reg, reg);
> I'd put it in directly rahter than having a reg local variable. To mind
> mind that would be slightly easier to understand.
>> +
>> if (info->flags & SUNXI_GPADC_ARCH_SUN6I)
>> regmap_write(info->regmap, SUNXI_GPADC_TP_CTRL1,
>> @@ -153,9 +185,9 @@ static int sunxi_gpadc_adc_read(struct iio_dev *indio_dev, int channel,
>> - regmap_write(info->regmap, SUNXI_GPADC_TP_INT_FIFOC,
> Whole load of infrastructure in place to lock buffered mode out and
> revent transitions when we can't have them.
> iio_claim_direct_mode etc. I think you can just use that here?
> If you need to do extra checks on it being enabled that should be
> fine too.

Yes, way better with iio_device_claim_direct_mode and iio_buffer_enabled!

> As a general rule, it makes sense to simply disable polled reads
> if in buffered mode. Leads to much simpler code and generally
> the data is already known to userspace anyway.

That's what I try to do.
However, I think the temperature of the SoC is an interesting feature to
have. Since it ("hardwarely") works while the ADC is read in touchscreen
mode (even in buffer mode), I guess it could be a good idea to allow it
in the driver. If we don't do that, boards with a touchscreen connected
to the ADC of the SoC will not get SoC temperatures and can't have
proper thermal management. We already have one board in that case: the

Therefore, I also need to know if when the buffer is enabled, if it's
for buffering ADC data or touchscreen data. If it's for ADC data, then I
should disable temperature readings since it will return senseless
values (from memory, always 0 which means something like -144°C).

> I have been meaning to do it a bit better when we have multiple
> in kernel consumers, some expecting polled readings and some
> pushed. There some core caching magic will make sense to
> keep the polled channels as available as possible when running
> the buffers.
> A bit fiddly to implement + might have some slightly suprising
> results on delays on channels when say a sysfs trigger is
> being used... (not a problem here as you have a fifo and hence
> aren't using triggers).
> Anyhow, not really relevant here :)
> Definitely use the iio_claim_direct_mode stuff here to avoid possible races
> with the buffer being enabled whilst this read is in flight.


>> + if (info->buffered)
>> + return -EBUSY;
>> +
>> ret = sunxi_gpadc_adc_read(indio_dev, chan->channel, val);
>> if (ret)
>> return ret;
>> @@ -261,7 +302,29 @@ static irqreturn_t sunxi_gpadc_temp_data_irq_handler(int irq, void *dev_id)
>> static irqreturn_t sunxi_gpadc_fifo_data_irq_handler(int irq, void *dev_id)
>> {
>> struct sunxi_gpadc_dev *info = dev_id;
>> - int ret;
>> + int ret, reg, i, fifo_count;
>> +
>> + if (info->buffered) {
>> + if (regmap_read(info->regmap, SUNXI_GPADC_TP_INT_FIFOS, &reg))
>> + return IRQ_HANDLED;
>> +
>> + fifo_count = (reg & SUNXI_GPADC_RXA_CNT) >> 8;
>> + /* Sometimes, the interrupt occurs when the FIFO is empty. */
>> + if (!fifo_count)
>> + return IRQ_HANDLED;
>> +
>> + for (i = 0; i < fifo_count; i++) {
>> + if (regmap_read(info->regmap, SUNXI_GPADC_TP_DATA,
>> + &info->buffer.buffer[i]))
>> + return IRQ_HANDLED;
>> + }
>> +
>> + info->buffer.buff_size = i;
>> +
>> + iio_push_to_buffers(info->indio_dev, &info->buffer);
> This is expecting a single 'scan' - e.g. set of channels read at one
> time. Here I think we could have repeated sets of channels?
> (at least that would be what is normally meant by a fifo in such
> a device).
> If so you need to read 'whole' scans and push them one at a time.
> We don't yet have a bulk iio_push_to_buffers, though we can add
> one if it makes sense. Care will be needed though as we'd need
> handle the case of different consumers either supporting or
> not supporting this new functionality. Not particularly hard though
> if it is worth doing.

I didn't know it was meant for only one scan. Then I need a bulk

I have a rather big problem. The whole first FIFO at each touch is
unusable so I have to drop it. I can detect the beginning of a touch
when the TP_UP irq occurs, then I know the next full FIFO the consumer
receives by callback is to be dropped. If I use push_to_buffers to send
coordinates by coordinates, the consumer has no mean to know when the
second FIFO (the first to be valid) starts and can be used. Either we
can find a way to notify the consumer of the start of a new FIFO or I
have to use a bulk iio_push_to_buffers.

The workaround would be to register the TP_UP irq in the provider (the
ADC driver) and do not send the first FIFO to the consumer. But then, we
need a way to know which consumer requests buffering to know when to
enable this irq and do all touchscreen-only logic (dropping first
frame). And I guess we don't have something like that yet. Or I could
only code a buffering in touchscreen mode and add the ADC buffering
later? But it doesn't feel right to do what I think should be handled
(TP_UP irq handler and first FIFO dropping) in the consumer, in the

So it's quiet a dead-end yet if I can't use iio_push_to_buffers with a
whole FIFO (which you told is not how it is meant to be used).



Quentin Schulz, Free Electrons
Embedded Linux and Kernel engineering