Re: [PATCH] ACPICA / hwreg: Use acpi_gbl_reduced_hardware to prevent accessing PM registers

[Hanjun Guo <hanjun.guo@linaro.org>]

Hi Bob,

Sorry for the late reply, I had being on holiday last few days.

On 2013-9-18 23:09, Moore, Robert wrote:
> 
> 
>> -----Original Message-----
>> From: Hanjun Guo [mailto:hanjun.guo@linaro.org]
>> Sent: Wednesday, September 18, 2013 2:32 AM
>> To: Moore, Robert
>> Cc: 'Rafael J. Wysocki'; 'Len Brown'; Box, David E; Zheng, Lv; 'linux-
>> acpi@vger.kernel.org'; 'patches@linaro.org'; 'linaro-
>> kernel@lists.linaro.org'; 'linaro-acpi@lists.linaro.org'
>> Subject: Re: [PATCH] ACPICA / hwreg: Use acpi_gbl_reduced_hardware to
>> prevent accessing PM registers
>>
>> On 2013-9-17 1:26, Moore, Robert wrote:
>>> + #define ACPI_REDUCED_HARDWARE           TRUE
>>>
>>> The intent of this feature is of course, to remove all code that is not
>> needed -- specifically for hardware-reduced machines where the size of the
>> kernel is important.
>>>
>>> On a larger machine, the hardware-reduced flag should be sufficient.
>> However, I would think that the host OS would look at this flag and
>> realize that it should not be doing certain ACPI hardware-related things
>> up front, rather than later when it finds out that a write to some ACPI
>> hardware fails because the hardware isn't there.
>>
>> Do you mean we should change the ACPI device driver instead of changing
>> the ACPICA code? that would be a hard job, because hardware ACPI is used
>> everywhere.
>>
> 
> 
> I don't really know the answer to this, but something tells me that bad things 
> may happen when a driver expects the ACPI hardware to be there, and it finds 
> out that it isn't, simply by calling one of the ACPI hardware interfaces.
> 
> Or, we could word it this way: if a driver is expecting the ACPI hardware to 
> exist, and we are running on a hardware-reduced platform, why is the driver
> being loaded in the first place?

ok, that would be a reasonable solution.

Oh, things get complicated now, could some linux experts have comments here
please?

> 
> BTW, hardware-reduced is not restricted to ARM platforms.

Thanks for reminding, may be some other platforms (not only x86, IA64, ARM)
will use ACPI in the future.

Thanks
Hanjun

>