Subject: Re: [PATCH 3/3] soc/tegra: pmc: Don't create fake interrupt hierarchy levels

From: Thierry Reding <[email-masked]>
Date: Mon, 5 Oct 2020 13:33:35 +0200

On Mon, Oct 05, 2020 at 12:14:43PM +0100, Marc Zyngier wrote:
> The Tegra PMC driver does ungodly things with the interrupt hierarchy,
> repeatedly corrupting it by pulling hwirq numbers out of thin air,
> overriding existing IRQ mappings and changing the handling flow
> of unsuspecting users.
> 
> All of this is done in the name of preserving the interrupt hierarchy
> even when these levels do not exist in the HW. Together with the use
> of proper IRQs for IPIs, this leads to an unbootable system as the
> rescheduling IPI gets repeatedly repurposed for random drivers...
> 
> Instead, let's allow the hierarchy to be trimmed to the level that
> actually makes sense for the HW, and not any deeper. This avoids
> having unnecessary callbacks, overriding mappings, and otherwise
> keeps the hierarchy sane.
> 
> Signed-off-by: Marc Zyngier <[email-masked]>
> ---
>  drivers/soc/tegra/pmc.c | 79 +++++++++++++++--------------------------
>  1 file changed, 29 insertions(+), 50 deletions(-)
> 
> diff --git a/drivers/soc/tegra/pmc.c b/drivers/soc/tegra/pmc.c
> index 9960f7c18431..4eea3134fb3e 100644
> --- a/drivers/soc/tegra/pmc.c
> +++ b/drivers/soc/tegra/pmc.c
> @@ -1993,6 +1993,30 @@ static int tegra_pmc_irq_translate(struct irq_domain *domain,
>  	return 0;
>  }
>  
> +/* Trim the irq hierarchy from a particular irq domain */
> +static void trim_hierarchy(unsigned int virq, struct irq_domain *domain)
> +{
> +	struct irq_data *tail, *irq_data = irq_get_irq_data(virq);
> +
> +	/* The PMC doesn't generate any interrupt by itself */
> +	if (WARN_ON(!irq_data->parent_data))
> +		return;
> +
> +	/* Skip until we find the right domain */
> +	while (irq_data->parent_data && irq_data->parent_data->domain != domain)
> +		irq_data = irq_data->parent_data;
> +
> +	/* Sever the inner part of the hierarchy...  */
> +	tail = irq_data->parent_data;
> +	irq_data->parent_data = NULL;
> +
> +	/* ... and free it */
> +	for (irq_data = tail; irq_data; irq_data = tail) {
> +		tail = irq_data->parent_data;
> +		kfree(irq_data);
> +	};
> +}

That kind of looks like what I originally wanted to do and (naively)
thought that passing the (0, NULL, NULL) triplet would achieve.

Given that this is fairly low-level stuff that deals with the inner
workings of the IRQ infrastructure, should we eventually pull this out
of the driver and make it into a core helper? I don't seriously expect
this to be widely useful, but putting it into the core might help keep
it more maintainable.

I volunteer to do that work if you think it's a good idea.

Thierry
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