Re: [PATCH v6 07/11] platform: cznic: turris-omnia-mcu: Add support for MCU provided TRNG

["Marek Behún" <kabel@kernel.org>]

On Tue, 23 Apr 2024 18:58:19 +0300
Andy Shevchenko <andy@kernel.org> wrote:

> On Thu, Apr 18, 2024 at 02:11:12PM +0200, Marek Behún wrote:
> > Add support for true random number generator provided by the MCU.
> > New Omnia boards come without the Atmel SHA204-A chip. Instead the
> > crypto functionality is provided by new microcontroller, which has
> > a TRNG peripheral.  
> 
> ...
> 
> > +int omnia_mcu_register_trng(struct omnia_mcu *mcu)
> > +{
> > +	struct device *dev = &mcu->client->dev;
> > +	int irq, err;
> > +	u8 irq_idx;
> > +
> > +	if (!(mcu->features & FEAT_TRNG))
> > +		return 0;  
> 
> > +	irq_idx = omnia_int_to_gpio_idx[__bf_shf(INT_TRNG)];
> > +	irq = devm_irq_create_mapping(dev, mcu->gc.irq.domain, irq_idx);
> > +	if (irq < 0)
> > +		return dev_err_probe(dev, irq, "Cannot map TRNG IRQ\n");  
> 
> This looks like some workaround against existing gpiod_to_irq(). Why do you
> need this?

Hmmm, I thought that would not work because that line is only valid
as an IRQ, not as a GPIO (this is enforced via the valid_mask member of
gpio_chip and gpio_irq_chip).

But looking at the code of gpiolib, if I do
  irq = gpiod_to_irq(gpiochip_get_desc(gc, irq_idx));
the valid_mask is not enforced anywhere.

Is this semantically right to do even in spite of the fact that the
line is not a valid GPIO line?

Marek