Touch driver interrupt handler

The code

Embedded Linux C. It compiles. There are 3 real bugs in it: the kind that pass a bench test and fail in the field.

static irqreturn_t tp_irq(int irq, void *dev)
{
    struct tp *tp = dev;
    mutex_lock(&tp->lock);
    tp_read_points(tp);              /* I2C transfer, sleeps */
    mutex_unlock(&tp->lock);
    return IRQ_HANDLED;
}

static int tp_setup_irq(struct tp *tp)
{
    return request_irq(tp->irq, tp_irq,
                       IRQF_TRIGGER_LOW, "tp", tp);
}

static void tp_remove(struct tp *tp)
{
    kfree(tp);
    free_irq(tp->irq, tp);
}

What an interviewer expects

• Point to each line you'd stop in a code review.
• Say what actually breaks: the failure mode, not just the rule it violates.
• Fix it, then defend the fix when asked the follow-up.

The bugs, reference fixes and the follow-up question are in the practice app, after you've tried it.

Companies that test this

Matched on the skills each company's embedded roles emphasise.

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