Message-ID: <54D4D085.9020808@winischhofer.net>
Date:	Fri, 06 Feb 2015 15:32:37 +0100
From:	Thomas Winischhofer <thomas@...ischhofer.net>
To:	Nicholas Mc Guire <der.herr@...r.at>
CC:	Tormod Volden <lists.tormod@...il.com>,
	Scot Doyle <lkml14@...tdoyle.com>,
	Nicholas Mc Guire <hofrat@...dl.org>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] video: fbdev: sis: condition with no effect

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Nicholas Mc Guire wrote:
> On Fri, 06 Feb 2015, Thomas Winischhofer wrote:
> 
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>> Tormod Volden wrote:
>>> On Thu, Feb 5, 2015 at 9:45 PM, Scot Doyle wrote:
>>>> On Wed, 4 Feb 2015, Nicholas Mc Guire wrote:
>>>>> The if and the else branch code are identical - so the condition has no
>>>>> effect on the effective code - this patch removes the condition and the
>>>>> duplicated code.
>>>>>
>>>>> Signed-off-by: Nicholas Mc Guire <hofrat@...dl.org>
>>>>> ---
>>>>>
>>>>> This code has been in here since commit 544393fe584d ("sisfb update") so I guess it is
>>>>> safe to simply remove the duplicated code if nobody noticed for 10 years.
>>>>>
>>>>> Note that the code is not really CodingStyle compliant - the lines inserted were formatted
>>>>> to satisfy the coding style but I'm unsure if it is not better to leave it in the
>>>>> old format.
>>>>>
>>>>> Patch was only compile tested with x86_64_defconfig +
>>>>> CONFIG_FB_SIS=m, CONFIG_FB_SIS_300=y, CONFIG_FB_SIS_315=y
>>>>>
>>>>> Patch is against 3.19.0-rc7 (localversion-next is -next-20150204)
>>>>>
>>>>>  drivers/video/fbdev/sis/init301.c |    9 ++-------
>>>>>  1 file changed, 2 insertions(+), 7 deletions(-)
>>>>>
>>>>> diff --git a/drivers/video/fbdev/sis/init301.c b/drivers/video/fbdev/sis/init301.c
>>>>> index 295e0de..9533a8ab 100644
>>>>> --- a/drivers/video/fbdev/sis/init301.c
>>>>> +++ b/drivers/video/fbdev/sis/init301.c
>>>>> @@ -7971,13 +7971,8 @@ SiS_SetCHTVReg(struct SiS_Private *SiS_Pr, unsigned short ModeNo, unsigned short
>>>>>           }
>>>>>        } else {                                               /* ---- PAL ---- */
>>>>>           /* We don't play around with FSCI in PAL mode */
>>>>> -         if(resindex == 0x04) {
>>>>> -            SiS_SetCH70xxANDOR(SiS_Pr,0x20,0x00,0xEF);       /* loop filter off */
>>>>> -            SiS_SetCH70xxANDOR(SiS_Pr,0x21,0x01,0xFE);       /* ACIV on */
>>>>> -         } else {
>>>>> -            SiS_SetCH70xxANDOR(SiS_Pr,0x20,0x00,0xEF);       /* loop filter off */
>>>>> -            SiS_SetCH70xxANDOR(SiS_Pr,0x21,0x01,0xFE);       /* ACIV on */
>>>>> -         }
>>>>> +       SiS_SetCH70xxANDOR(SiS_Pr, 0x20, 0x00, 0xEF); /* loop filter off */
>>>>> +       SiS_SetCH70xxANDOR(SiS_Pr, 0x21, 0x01, 0xFE); /* ACIV on */
>>>>>        }
>>>>>
>>>>>  #endif  /* 300 */
>>>> The code covering the PAL case had this redundancy when it was introduced
>>>> in Linux 2.4.19.
>>>>
>>>> Lines 7934-7981 consider three variables: PAL, overscan, and resindex.
>>>> Given the "#ifdef 0" block, couldn't the current six sections collapse
>>>> into two? One for (!PAL && overscan && resindex==5) and another for the
>>>> rest?
>>> Are we sure there isn't a typo in one of the duplicate clauses? Or
>>> wrong copy-pasting? Generally I am skeptical to "fixing" code without
>>> understanding what is behind or testing it, and just cosmetically
>>> brush over it. For now at least it is obvious that there is something
>>> wrong. In case (although an unlikely one) someone who understands the
>>> code and knows this chip comes along, he would quickly spot this.
>>> After your "fixups" this will be all forgotten. Additionally it adds
>>> to the impression that this code is being maintained, which is wrong.
>>>
>>> I would understand an argument about annoying compiler warnings and
>>> the like, but in that case I would prefer to #if 0 it instead of
>>> "prettifying" it.
>>>
>>> 0.02
>>> Tormod
>>>
>> The code is partly unfinished due to a lack of hardware to test this
>> with. SiS announced SiS+Chrontel 7019 combos at some point but I have
>> never seen one. The code was written based on the Chrontel datasheets,
>> which weren't clear to some extent, and there wasn't ever any test
>> hardware. I don't recall this one exactly, but identical if-else
>> statements mean that one alternative is (assumingly) correct, while the
>> other is uncertain and/or untested. I left such redunant if-statements
>> in the code to remember the conditions and the fact that there is a
>> second alternative.
>>
>> Considering the long time I'd say it's safe to simplify this.
>>
>> A word on other changes I monitored recently: Please bear in mind that
>> with video hardware reading and writing registers is not simple like
>> reading and writing to memory. Sometimes reading causes an effect in the
>> hardware as well (latches, etc), so removing seemingly redundant
>> GetReg/SetReg sequences might actually have an effect.
>>
> thanks for that note - will add that to my checklist of sideffects
> for future patches.
> 
> thx!
> hofrat 
> 

PS: Correction: This code is for the SiS+Chrontel 7005 (not 7019) case,
and there actually WAS hardware. Therefore I also probably tested this,
or this is the remains of a test, and as a consequence it is safe to
simplify/remove.

- --
Thomas Winischhofer
thomas AT winischhofer DOT net
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