Re: [PATCH 2/3] ACPI: fix acpi_parse_entries_array() so it traverses all subtables

[Al Stone]

On 07/01/2016 04:01 PM, Rafael J. Wysocki wrote:
> On Fri, Jul 1, 2016 at 11:55 PM, Al Stone <ahs3@xxxxxxxxxx> wrote:
>> On 07/01/2016 03:46 PM, Rafael J. Wysocki wrote:
>>> On Fri, Jul 1, 2016 at 11:41 PM, Al Stone <ahs3@xxxxxxxxxx> wrote:
>>>> On 07/01/2016 03:32 PM, Rafael J. Wysocki wrote:
>>>>> On Fri, Jul 1, 2016 at 11:21 PM, Al Stone <ahs3@xxxxxxxxxx> wrote:
>>>>>> Without this patch, the acpi_parse_entries_array() function will return
>>>>>> the very first time there is any error found in either the array of
>>>>>> callback functions or if one of the callbacks returns an non-zero value.
>>>>>> However, the array of callbacks could still have valid entries further
>>>>>> on in the array, or the callbacks may be able to process subsequent
>>>>>> subtables without error.  The change here makes the function consistent
>>>>>> with its description so that it will properly return the sum of all
>>>>>> matching entries for all proc handlers, instead of stopping abruptly
>>>>>> as it does today.
>>>>>
>>>>> I'm not sure I follow.
>>>>>
>>>>> You seem to be saying that the function should process all of the
>>>>> subtables etc even though errors have been found for some of them, but
>>>>> it still will return an error in the end if there are any errors.  How
>>>>> exactly does it help to continue processing in case of an error, then?
>>>>
>>>> The use case I have in mind is to simply count all of the subtables of
>>>> a certain type.  If for some reason, the callback -- or any other callback
>>>> -- fails, the traversal of all the subtables stops immediately.  So, I
>>>> could have two callbacks, and if the first one fails on the first subtable
>>>> of its type, traversal stops.  The count for the second callback will be
>>>> zero which may or may not be correct.
>>>
>>> It will be zero, because the callback has not been invoked at all.
>>> Why is this incorrect?
>>>
>>
>> Because there could be additional subtables after the one causing a failure
>> that the second callback could have counted; e.g., if the failure is on the
>> first subtable of 20 in the MADT, the following 19 would be ignored, even if
>> they were all the right subtype for the second callback.
> 
> Let me rephrase: Is there any practical value of invoking any more
> callbacks if one of them has failed?  If so, what is it?
> 
> You are changing semantics from "abort on the first failure" to
> "process everything and count errors".  That's quite a bit different
> and I'm trying to understand why the latter is better.
> 

Agreed, it is a shift in semantics.

The practical value to me is being able to use acpi_parse_entries_array() to
solve a broader range of problems.  The situation I have is that I need to
count three different subtable types in the MADT.  I could call
acpi_table_parse_madt() three different times, or I could call
acpi_parse_entries_array() once -- it seemed to me the second makes for cleaner
code and will be slightly more efficient (one map/unmap of the table, vs
three), but that only works if all of the subtables are traversed.

-- 
ciao,
al
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Al Stone
Software Engineer
Red Hat, Inc.
ahs3@xxxxxxxxxx
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