Re: [PATCH 4/7] gpio: dwapb: Convert driver to using the GPIO-lib-based IRQ-chip

[Serge Semin <Sergey.Semin@baikalelectronics.ru>]

On Wed, Jul 29, 2020 at 06:10:24PM +0300, Andy Shevchenko wrote:
> On Wed, Jul 29, 2020 at 3:58 PM Serge Semin
> <Sergey.Semin@baikalelectronics.ru> wrote:
> > On Mon, Jul 27, 2020 at 12:22:28AM +0200, Linus Walleij wrote:
> 
> ...
> 
> > Sorry for a delay with a response to this issue. I had to give it a more thorough
> > thought since the problem is a bit more complex than it seemed originally. As I
> > see it now It might be wrong to implement the cases 2) and 3), but 1) is more
> > appropriate.
> >
> > First of all we need to note that GPIOlib framework provides the next parameters
> > to describe the IRQ-chip:
> > gc->irq.num_parents - number of parental IRQ numbers.
> > gc->irq.parents[] - array of parental IRQ numbers.
> > *gc->irq.valid_mask - a mask of IRQ/GPIO lines describing a valid IRQ.
> > *gc->irq.map - mapping of hw IRQ/GPIO ID -> parental IRQ numbers.
> >
> > Using that set we can handle any case of linear and sparse parental IRQs. Here
> > is how it can be implemented in the framework of DW APB GPIO controller.
> >
> > DW APB GPIO can be synthesized with two configs:
> > 1) Combined IRQ line (GPIO_INTR_IO == True),
> > 2) Multiple interrupt signals for each GPIO (GPIO_INTR_IO == False).
> >
> > Obviously the former case is trivial:
> >
> >      IRQ_combined
> >     ______^________
> >    /_ _ _ _ _ ___ _\
> >    |_|_|_|_|_|...|_| - GPIOs
> >
> > In that case
> > gc->irq.num_parents = 1;
> > gc->irq.parents[0] = IRQ_combined;
> > *gc->irq.valid_mask = GENMASK(ngpio - 1, 0); // This is done by the GPIOlib core itself.
> >
> > The later one (when multiple interrupt signals are involved) can be a bit more
> > complicated. It can be also split up into two cases:
> > 2a) One-on-one GPIO-IRQ mapping.
> > 2b) Sparse GPIO-IRQ mapping.
> >
> > It's straightforward to implement 2a):
> >
> >    i1i2i3i4i5 ... iN
> >     _ _ _ _ _ ___ _
> >    |_|_|_|_|_|...|_| - GPIOs
> >

> > In that case
> > gc->irq.num_parents = ngpio;
> > gc->irq.parents[] = {i1, i2, i3, i4, i5, ... iN};
> > gc->irq.map = {i1, i2, i3, i4, i5, ... iN};
> > *gc->irq.valid_mask = GENMASK(ngpio - 1, 0);
> >
> 
> This case puzzles me. Why is it not NULL and 0 and actually you handle
> everything as a nested case?

The code provided above is a sketch. Don't consider it literally. In reality
of course valid_mask will be not NULL, memory for which will be allocated by
the GPIOlib itself. The mask will be initialized by means of the
gc->irq.init_valid_mask() callback.

> 
> > The complication starts when we get to implementing 2b):
> >
> >    i1 xi3i4 x ... iN
> >     _ _ _ _ _ ___ _
> >    |_|_|_|_|_|...|_| - GPIOs
> 
> So does this.
> 
> Valid mask will define exactly GPIOs that are IRQs. So, we will handle
> only nested IRQs which are valid.

Right.

> 
> > In order to cover this case we need to answer on two question.
> > Firstly how to get such platform config? I am not sure about ACPI, but
> > aside from straightforward platform_data-based setup such configuration
> > can be reached by setting up the "interrupts-extended" DT-property with
> > zeroed phandle.
> >
> > Ok, since it's possible to meet such platform config, we need to think
> > how to handle it and here is the second question. How to describe such
> > case in the framework of GPIOlib-IRQchip?
> >
> > So from my side it was wrong to set the sparse IRQs array to
> > gc->irq.parents. Instead I should have scanned the sparse IRQs array,
> > calculated the number of non-empty parental IRQs, created an array of linear
> > (non-sparse) IRQs, initialized *gc->irq.valid_mask in accordance with the
> > sparse parental IRQs array. In other words it was wrong to assume, that
> > each gc->irq.parents entry corresponds to the IRQ/GPIO line. The gc->irq.parents
> > array just describes the parental IRQs and nothing else.
> >
> > Shortly speaking here is how the GPIOlib IRQchip parameters should be
> > initialized in this case:
> > gc->irq.num_parents - number of valid parental IRQs.
> > gc->irq.parents - non-sparse, linear array of valid IRQs.
> > *gc->irq.valid_mask - mask initialized by means of the gc->irq.init_valid_mask()
> > callback, which indicates valid IRQ/GPIO IDs.
> > *gc->irq.map - sparse array of parental IRQ numbers (which I mistakenly tried to
> > pass through the gc->irq.parents pointer).
> >
> > After that GPIOlib IRQchip should work just fine without need to be patched
> > in order to check whether the passed parental IRQs are valid or not.
> >
> > Please correct me if I am wrong in some aspects of the solution described above.
> > I'll send a fix of the problem shortly.
> 

> Maybe I'm missing something, but looks like you are solving the issue
> which is not so complex / doesn't exist.

As I see it now the problem was to provide a suitable config for GPIO-lib IRQ-chip
so one would correctly perceive our DW APB GPIO-IRQ setup with single/sparse/linear
IRQs. My point in the message before was to explain how that problem could be solved
without patching GPIO-lib or IRQ-chip implementations. I shared my thoughts to make
sure the suggested solution is correct, to make sure I didn't miss something in
my considerations.

-Sergey

> 
> -- 
> With Best Regards,
> Andy Shevchenko