linux-kernel - Re: [PATCH] ACPI, APEI: Fixup incorrect 16-bit access width firmware bug

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Date:   Wed, 26 Jul 2017 13:44:32 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Liwei Song <liwei.song@...driver.com>
Cc:     "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        linux-acpi <linux-acpi@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ACPI, APEI: Fixup incorrect 16-bit access width firmware bug

On Wed, Jul 26, 2017 at 9:16 AM, Liwei Song <liwei.song@...driver.com> wrote:
> On 07/21/2017 05:56 PM, Andy Shevchenko wrote:
>> On Fri, Jul 21, 2017 at 12:41 PM, Song liwei <liwei.song@...driver.com> wrote:

>>     /* Fixup common BIOS bug */
>>     if (bit_offset == 0) {
>>       if (bit_width == 16 && (*paddr & 0x01) == 0 && *access_bit_width < 16)
>>                *access_bit_width = 16;
>>       else if (bit_width == 32 && (*paddr & 0x03) == 0 &&
>> *access_bit_width < 32)
>>                *access_bit_width = 32;
>>       else if (bit_width == 64 && (*paddr & 0x07) == 0 &&
>> *access_bit_width < 64)
>>                *access_bit_width = 64;
>>     }

> Thanks for your suggestion, what about the condition like the following?
> The main bug in bios is bit_width is not comfortable with access_bit_width
> So check it first.
>
> if (*access_bit_width < bit_width && bit_offset == 0) {
>         if ((bit_width == 16 && (*paddr & 0x01) == 0) ||
>             (bit_width == 32 && (*paddr & 0x03) == 0) ||
>             (bit_width == 64 && (*paddr & 0x07) == 0))
>                 *access_bit_width = bit_width;
> }

Either works to me.

P.S. Less explicit case is to use bit operations to check those, i.e.
is_power_of_2(), fls().

-- 
With Best Regards,
Andy Shevchenko