linux-kernel - Re: [PATCH v2 08/13] ACPICA: Hardware: Add optimized access bit width support

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Message-Id: <574731DD02000078000E92FC@prv-mh.provo.novell.com>
Date:	Thu, 26 May 2016 10:26:53 -0600
From:	"Jan Beulich" <jbeulich@...e.com>
To:	<boris.ostrovsky@...cle.com>
Cc:	<ian.jackson@...citrix.com>, <zetalog@...il.com>,
	<len.brown@...el.com>, <lv.zheng@...el.com>,
	<rafael.j.wysocki@...el.com>, <robert.moore@...el.com>,
	<xen-devel@...ts.xen.org>, <rjw@...ysocki.net>,
	<linux-acpi@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 08/13] ACPICA: Hardware: Add optimized access
 bit width support

>>> Boris Ostrovsky <boris.ostrovsky@...cle.com> 05/25/16 9:17 PM >>>
>On 05/05/2016 12:58 AM, Lv Zheng wrote:
>> +static u8
>> +acpi_hw_get_access_bit_width(struct acpi_generic_address *reg, u8 max_bit_width)
>> +{
>> +    u64 address;
>> +
>> +    if (!reg->access_width) {
>> +        /*
>> +         * Detect old register descriptors where only the bit_width field
>> +         * makes senses. The target address is copied to handle possible
>> +         * alignment issues.
>> +         */
>> +        ACPI_MOVE_64_TO_64(&address, &reg->address);
>> +        if (!reg->bit_offset && reg->bit_width &&
>> +            ACPI_IS_POWER_OF_TWO(reg->bit_width) &&
>> +            ACPI_IS_ALIGNED(reg->bit_width, 8) &&
>> +            ACPI_IS_ALIGNED(address, reg->bit_width)) {
>> +            return (reg->bit_width);
>> +        } else {
>> +            if (reg->space_id == ACPI_ADR_SPACE_SYSTEM_IO) {
>> +                return (32);
>
>This (together with "... Add access_width/bit_offset support in
>acpi_hw_write") breaks Xen guests using older QEMU which doesn't support
>4-byte IO accesses.
>
>Why not return "reg->bit_width?:max_bit_width" ? This will preserve
>original behavior.

Did you figure out why we get here in the first place, instead of taking the
first "return"? I.e. isn't the issue the apparently wrong use of the second
ACPI_IS_ALIGNED() above? Afaict it ought to be
ACPI_IS_ALIGNED(address, reg->bit_width / 8)...

Jan