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

From: Marc Zyngier <[email-masked]>
Date: Mon, 05 Oct 2020 14:10:13 +0100

On 2020-10-05 12:33, Thierry Reding wrote:
> 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.

That's the ultimate plan, but I wanted to give it some soaking time
on Tegra before exposing it to the outside world 
(irq_domain_free_irq_data()
could be rewritten in terms of this primitive, for example).

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

Sure, once we know we're good to go with this.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...

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