Re: [PATCH 01/28] mfd: Add Microchip ZL3073x support

[Ivan Vecera <ivecera@redhat.com>]

On 14. 04. 25 4:16 odp., Andy Shevchenko wrote:
> On Mon, Apr 14, 2025 at 5:13 PM Andy Shevchenko
> <andy.shevchenko@gmail.com> wrote:
>> On Mon, Apr 14, 2025 at 5:10 PM Andy Shevchenko
>> <andy.shevchenko@gmail.com> wrote:
>>> On Mon, Apr 14, 2025 at 5:07 PM Ivan Vecera <ivecera@redhat.com> wrote:
>>>> On 14. 04. 25 1:52 odp., Ivan Vecera wrote:
> 
> ...
> 
>>>> Long story short, I have to move virtual range outside real address
>>>> range and apply this offset in the driver code.
>>>>
>>>> Is this correct?
>>>
>>> Bingo!
>>>
>>> And for the offsets, you form them as "page number * page offset +
>>> offset inside the page".
>>
>> Note, for easier reference you may still map page 0 to the virtual
>> space, but make sure that page 0 (or main page) is available outside
>> of the ranges, or i.o.w. ranges do not overlap the main page, even if
>> they include page 0.
> 
> So, you will have the following layout
> 
> 0x00 - 0xnn - real registers of page 0.
> 
> 0x100 - 0xppp -- pages 0 ... N
> 
> Register access either direct for when direct is required, or as
> 0x100 + PageSize * Index + RegOffset

Now, get it...

I was a little bit confused by code of _regmap_select_page() that takes 
care of selector_reg.

Btw, why is this needed? why they cannot overlap?

Let's say I have virtual range <0, 0xfff>, window <0, 0xff> and window 
selector 0xff>.
1. I'm calling regmap_read(regmap, 0x8f, ...)
2. The regmap looks for the range and it finds it (0..0xfff)
3. Then it calls _regmap_select_page() that computes:
    window_offset = (0x8f - 0x000) % 0x100 = 0x8f
    window_page = (0x8f - 0x000) / 0x100 = 0
4. _regmap_select_page() set window selector to 0 and reg is updated to
    reg = window_start + window_offset = 0x8f

And for window_selector value: regmap_read(regmap, 0xff, ...) is the 
same except _regmap_select_page() checks that the given address is 
selector_reg and won't perform page switching.

When I think about it, in my case there is no normal page, there is only 
volatile register window <0x00-0x7e> and only single direct register 
that is page selector at 0x7f.

Thanks,
Ivan