Re: [PATCH] i2c: i801: Add module parameter to inhibit BIOS access

[Alex Henrie <alexh@vpitech.com>]

On Thu, 20 Jan 2022 02:32:02 +0900
Hector Martin <marcan@marcan.st> wrote:

> On 20/01/2022 01.49, Alex Henrie wrote:
> > I think Hector's patch to share the SMBus is a great idea; my patch was
> > meant to complement his, not replace it. The problem is that when I
> > tried Hector's patch, I got the message "BIOS left SMBus locked". So I
> > didn't want to let the BIOS touch the SMBus at all because once it had
> > the lock, it seemed to never let go. However, today I tried v2 of
> > Hector's patch [1] instead of v3 [2], and v2 worked perfectly! My guess
> > is that despite the text of the error message, it's Linux that's
> > leaving the SMBus locked, not the BIOS.
> > 
> > The only difference between v2 and v3 is that v2 called
> > outb_p(SMBHSTSTS_INUSE_STS, SMBHSTSTS(priv)) at the end of i801_access.
> > Hector, can you clarify why you removed that call in v3?
> 
> Well that solves the mystery.
> 
> That line was split off and submitted separately in another patch, as it
> fixes the immediate breakage that I was trying to address with my patch,
> namely that *linux* left the SMBus locked and that broke BIOS accesses.
>  The full patch implements proper sharing logic; that line alone is
> enough to unbork the backlight on iMacs (among other things) which was
> the immediate regression I had encountered. The rest of the patch breaks
> horribly without that line, as you'd expect, since it's trying to work
> around a broken BIOS when Linux itself is broken.
> 
> That patch was posted in June 2021 and CC'd stable [1], and was merged
> into the stable 5.10 and 5.12 trees as well as released in 5.13. So, you
> must be running an old kernel :-).
> 
> [1]
> https://lore.kernel.org/linux-i2c/cefbeb76-5f7f-036b-fa0e-1e339d261c35@gmail.com/

Excellent! Thank you so much Hector. Indeed, this system is currently
using a custom-built 5.4 kernel.

Jean, please feel free to disregard my patch and to commit Hector's
with "Tested-by: Alex Henrie <alexh@vpitech.com>".

-Alex