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

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Message-ID: <00cf9bc9-073c-4a6b-8024-2103942fe5b1@redhat.com>
Date: Mon, 14 Apr 2025 19:42:37 +0200
From: Ivan Vecera <ivecera@...hat.com>
To: Andy Shevchenko <andy.shevchenko@...il.com>
Cc: Andy Shevchenko <andy@...nel.org>, netdev@...r.kernel.org,
 Michal Schmidt <mschmidt@...hat.com>,
 Vadim Fedorenko <vadim.fedorenko@...ux.dev>,
 Arkadiusz Kubalewski <arkadiusz.kubalewski@...el.com>,
 Jiri Pirko <jiri@...nulli.us>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Prathosh Satish <Prathosh.Satish@...rochip.com>,
 Lee Jones <lee@...nel.org>, Kees Cook <kees@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 01/28] mfd: Add Microchip ZL3073x support

On 14. 04. 25 7:09 odp., Andy Shevchenko wrote:
>> 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.
> Because you are effectively messing up with cache. Also it's not
> recommended in general to do such due to some registers that might
> need to be accessed directly. And also note, that each time you access
> this you will call a selector write_each_ time you want to write the
> register in the map (if there is no cache, or if cache is messed up).

Get it fully now...

Thank you, Andy, for the explanation and for the patience.

Will send v3 soon.

Ivan