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

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Date:	Fri, 27 May 2016 03:24:45 +0000
From:	"Zheng, Lv" <lv.zheng@...el.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Jan Beulich <jbeulich@...e.com>
CC:	"ian.jackson@...citrix.com" <ian.jackson@...citrix.com>,
	"zetalog@...il.com" <zetalog@...il.com>,
	"Brown, Len" <len.brown@...el.com>,
	"Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
	"Moore, Robert" <robert.moore@...el.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 08/13] ACPICA: Hardware: Add optimized access bit
 width support

Hi,

> From: Boris Ostrovsky [mailto:boris.ostrovsky@...cle.com]
> Sent: Friday, May 27, 2016 12:56 AM
> Subject: Re: [PATCH v2 08/13] ACPICA: Hardware: Add optimized access
> bit width support
> 
> On 05/26/2016 12:26 PM, Jan Beulich wrote:
> >>>> 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)...
> 
> We are trying to access address 0x...b004 (PM1a control) so yes, fixing
> alignment check would probably resolve the problem that we are seeing
> now.
> 
> However, for compatibility purposes we may consider not doing any
> checks
> and simply return bit_width if access_width is not available.
[Lv Zheng] 

There might be GAS defined with AccessWidth = 0.
And its BitOffset can still make senses.
That's why I put the check here, making it a tuning rather than a regression safe check.

Thanks for the information, I'll submit the fix of the address check.
To convert it to:
ACPI_IS_ALIGNED(address, reg->bit_width / 8)
This is my fault.

Thanks
-Lv

> 
> -boris
>