Re: [PATCH v6 08/11] iio: adc: ad7768-1: add support for Synchronization over SPI

[Jonathan Santos <jonath4nns@gmail.com>]

On 04/28, Andy Shevchenko wrote:
> On Mon, Apr 28, 2025 at 3:14 AM Jonathan Santos
> <Jonathan.Santos@analog.com> wrote:
> >
> > The synchronization method using GPIO requires the generated pulse to be
> > truly synchronous with the base MCLK signal. When it is not possible to
> > do that in hardware, the datasheet recommends using synchronization over
> > SPI, where the generated pulse is already synchronous with MCLK. This
> > requires the SYNC_OUT pin to be connected to SYNC_IN pin.
> 
> to the SYNC_IN
> 
> > Use trigger-sources property to enable device synchronization over SPI
> > and multi-device synchronization, as an alternative to adi,sync-in-gpios
> > property.
> 
> ...
> 
> > +static int ad7768_send_sync_pulse(struct ad7768_state *st)
> > +{
> > +       if (st->en_spi_sync)
> > +               return regmap_write(st->regmap, AD7768_REG_SYNC_RESET, 0x00);
> 
> > +       if (st->gpio_sync_in) {
> 
> Dup check, the following have already it.
> 
> > +               gpiod_set_value_cansleep(st->gpio_sync_in, 1);
> 
> Yes, I see the original code, but still the Q is why no delay. Perhaps
> a comment explaining that the GPIO op is slow enough (?) to add.
> 

Datasheet specifies a minimum of 1.5*Tmclk pulse width. For the
recommended mclk of 16.384 MHz, it usually takes 4 times the minimum
pulse width. If it can be less than that for other plataforms I can add
this delay.

> > +               gpiod_set_value_cansleep(st->gpio_sync_in, 0);
> > +       }
> > +
> > +       return 0;
> > +}
> 
> ...
> 
> > +static struct gpio_desc *ad7768_trigger_source_get_gpio(struct device *dev,
> > +                                                       struct fwnode_handle *fwnode)
> > +{
> > +       const char *value;
> > +       int ret;
> > +
> > +       ret = fwnode_property_read_string(fwnode, "compatible", &value);
> > +       if (ret)
> > +               return ERR_PTR(ret);
> > +
> > +       if (strcmp("gpio-trigger", value))
> > +               return ERR_PTR(-EINVAL);
> 
> Reinvention of fwnode_device_is_compatible().
> 

Thanks!

> > +       return devm_fwnode_gpiod_get_index(dev, fwnode, NULL, 0,
> > +                                          GPIOD_OUT_LOW, "sync-in");
> > +}
> 
> ...
> 
> > +static int ad7768_trigger_sources_get_sync(struct device *dev,
> > +                                          struct ad7768_state *st)
> > +{
> > +       struct fwnode_reference_args args;
> > +       struct fwnode_handle *fwnode = NULL;
> 
> Redundant assignment AFAICS.
> 
> > +       int ret;
> > +
> > +       /*
> > +        * The AD7768-1 allows two primary methods for driving the SYNC_IN pin
> > +        * to synchronize one or more devices:
> > +        * 1. Using an external GPIO.
> > +        * 2. Using a SPI command, where the SYNC_OUT pin generates a
> > +        *    synchronization pulse that drives the SYNC_IN pin.
> > +        */
> > +       if (!device_property_present(dev, "trigger-sources")) {
> > +               /*
> > +                * In the absence of trigger-sources property, enable self
> > +                * synchronization over SPI (SYNC_OUT).
> > +                */
> > +               st->en_spi_sync = true;
> > +               return 0;
> > +       }
> > +
> > +       ret = fwnode_property_get_reference_args(dev_fwnode(dev),
> 
> In this case the above is better to be also fwnode_property_present().
> You save a double call to dev_fwnode().
> 
> > +                                                "trigger-sources",
> > +                                                "#trigger-source-cells",
> > +                                                0,
> > +                                                AD7768_TRIGGER_SOURCE_SYNC_IDX,
> > +                                                &args);
> > +       if (ret)
> > +               return ret;
> > +
> > +       fwnode = args.fwnode;
> > +       /*
> > +        * First, try getting the GPIO trigger source and fallback to
> > +        * synchronization over SPI in case of failure.
> > +        */
> > +       st->gpio_sync_in = ad7768_trigger_source_get_gpio(dev, fwnode);
> > +       if (IS_ERR(st->gpio_sync_in)) {
> > +               /*
> > +                * For this case, it requires one argument, which indicates the
> > +                * output pin referenced.
> > +                */
> > +               if (args.nargs < 1)
> > +                       goto err_not_supp;
> > +
> > +               if (args.args[0] != AD7768_TRIGGER_SOURCE_SYNC_OUT)
> > +                       goto err_not_supp;
> > +
> > +               /*
> > +                * Only self trigger is supported for now, i.e.,
> > +                * external SYNC_OUT is not allowed.
> > +                */
> > +               if (fwnode->dev == dev) {
> 
> ?!?! What is this?!
> 
> For the reference:
> https://elixir.bootlin.com/linux/v6.15-rc3/source/include/linux/fwnode.h#L51
> 

I see, my bad. I will follow David's suggestion, so we will avoid this.

> > +                       st->en_spi_sync = true;
> > +                       goto out_put_node;
> > +               }
> > +
> > +               goto err_not_supp;
> > +       }
> > +
> > +       goto out_put_node;
> > +
> > +err_not_supp:
> > +       ret = dev_err_probe(dev, -EOPNOTSUPP,
> > +                           "Invalid synchronization trigger source");
> 
> Missing \n, and can be one line anyway (we don't complain about long
> strings ending with string literals for ages, way before the 100
> character limit).
> 
> > +out_put_node:
> > +       fwnode_handle_put(args.fwnode);
> > +       return ret;
> > +}
> 
> -- 
> With Best Regards,
> Andy Shevchenko