Re: [PATCH 1/2] i2c: i801: Switch to iomapped register access

[Heiner Kallweit <hkallweit1@gmail.com>]

On 19.03.2025 09:23, Andy Shevchenko wrote:
> On Wed, Mar 19, 2025 at 9:17 AM Heiner Kallweit <hkallweit1@gmail.com> wrote:
>> On 19.03.2025 00:22, Andi Shyti wrote:
>>> On Wed, Mar 19, 2025 at 12:18:47AM +0200, Andy Shevchenko wrote:
>>>> Wed, Mar 12, 2025 at 08:07:23PM +0100, Heiner Kallweit kirjoitti:
> 
> 
>>>>> Switch to iomapped register access as a prerequisite for adding
>>>>> support for MMIO register access.
>>>>
>>>> I believe that I at least discussed the similar change a few years ago or even
>>>> proposed a one. The problem here is that *_p() variants of IO port accessors
>>>> are not the same as non-_p ones. And commit message is kept silent about
>>>> possible consequences of this change.
>>>>
>>>> So, at bare minumum it would be good to test for some period of time before
>>>> going for it.
>>>
>>> How would you do it?
>>
>> Documentation/driver-api/device-io.rst states that the artificially delayed
>> _p versions were needed on ISA devices. And in general I didn't find any hint
>> that the non-delayed versions ever caused issues on PCI devices.
> 
> At least put this in the commit message. It will show that you were aware of _p.
> 
>> On my system using the non-delayed version works fine, but I can't say 100%
>> that it's the same for the very first (> 25 yrs ago) chipsets supported by i801.
>>
>> Likely users with old systems don't run -next kernels, therefore leaving
>> this change a full cycle in -next may not really help. We can argue that
>> we have the -rc period for testing (and reverting if needed).
> 
> My main concern is to make no regressions for most currently used
> cases, that's why one cycle in Linux Next is better than none.
> 

Even ICH7 datasheet from 2012 mentions that SMBus register space is also
memory-mapped. So all systems from at least the last 10 yrs should use MMIO
instead of PMIO now, and therefore not be affected by switching to non-delayed
PMIO access. This should significantly reduce the risk you're referring to.