Day39 - IIO Capability Exposure¶

🎯 Goal¶

Enhance IIO driver usability by exposing supported configuration values to user-space.

Focus on:

sampling_frequency_available
Table-based validation
Clean separation of:
user-visible values (Hz)
hardware encoding (register values)

🧠 Design Overview¶

Control Plane Evolution¶

Stage	Description
Day38	User can configure parameters
Day39	Driver exposes supported capabilities

📊 Sampling Frequency Design¶

Key Decision¶

sampling_frequency:
  > 0 → periodic sampling (Hz)
  = 0 → disable periodic sampling

Note:

0 does NOT imply one-shot conversion
one-shot will be handled separately in future design

🧩 Mapping Table (Core Concept)¶

Problem¶

Hardware uses register encoding:

reg_val: 0,1,2,3,...

User-space expects:

Hz: 0,1,2,4,8,...

Solution¶

Use a mapping table:

struct myadc_sampling_freq {
    u8 reg_val;
    unsigned int hz;
};

static const struct myadc_sampling_freq myadc_sampling_freqs[] = {
    { MYADC_SR_DISABLE, 0   },
    { MYADC_SR_1_HZ,     1   },
    { MYADC_SR_2_HZ,     2   },
    { MYADC_SR_4_HZ,     4   },
    { MYADC_SR_8_HZ,     8   },
    { MYADC_SR_16_HZ,   16  },
    { MYADC_SR_32_HZ,   32  },
    { MYADC_SR_64_HZ,   64  },
    { MYADC_SR_128_HZ, 128  },
};

🔍 Lookup Helpers¶

static const struct myadc_sampling_freq *
myadc_find_sample_freq_by_hz(unsigned int hz);

static const struct myadc_sampling_freq *
myadc_find_sample_freq_by_reg(u8 reg_val);

Design Rules¶

Table is the single source of truth
No MAX-based validation
No duplicated logic

⚙️ Write Path (Validation)¶

Before (Day38)¶

if (val > MAX)
    return -EINVAL;

After (Day39)¶

if (!myadc_find_sample_freq_by_hz(val))
    return -EINVAL;

📤 Capability Exposure¶

sampling_frequency_available¶

Exposed as:

/sys/bus/iio/devices/iio:deviceX/sampling_frequency_available

Output Format¶

0 1 2 4 8 16 32 64 128

Implementation¶

static ssize_t myadc_sampling_frequency_available_show(
        struct device *dev,
        struct device_attribute *attr,
        char *buf)
{
    int i, len = 0;

    for (i = 0; i < ARRAY_SIZE(myadc_sampling_freqs); i++) {
        len += sysfs_emit_at(buf, len, "%u%s",
                             myadc_sampling_freqs[i].hz,
                             (i == ARRAY_SIZE(myadc_sampling_freqs) - 1)
                             ? "\n" : " ");
    }

    return len;
}

🔒 Concurrency Considerations¶

No lock required for _available
Uses static const table
No runtime state accessed

🧱 Design Principles¶

1. Separation of concerns¶

Layer	Representation
user-space	Hz
driver	mapping table
hardware	register encoding

2. Table as source of truth¶

Validation
Capability exposure
Register mapping

All derived from the same table.

3. Sysfs convention¶

_available attributes are read-only
values are space-separated
newline terminated

✅ Summary¶

Introduced capability exposure
Replaced MAX-based validation with table-based validation
Improved user-space usability
Established clean abstraction between hardware and API