linux-kernel - Re: [PATCH 1/3] ASoC: mediatek: mt8188-mt6359: support dynamic pinctrl

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Date:   Thu, 27 Jul 2023 02:03:01 +0000
From:   Trevor Wu (吳文良) <Trevor.Wu@...iatek.com>
To:     "lgirdwood@...il.com" <lgirdwood@...il.com>,
        "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
        "angelogioacchino.delregno@...labora.com" 
        <angelogioacchino.delregno@...labora.com>,
        "broonie@...nel.org" <broonie@...nel.org>,
        "tiwai@...e.com" <tiwai@...e.com>,
        "perex@...ex.cz" <perex@...ex.cz>
CC:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        "alsa-devel@...a-project.org" <alsa-devel@...a-project.org>
Subject: Re: [PATCH 1/3] ASoC: mediatek: mt8188-mt6359: support dynamic
 pinctrl

On Wed, 2023-07-26 at 13:54 +0200, AngeloGioacchino Del Regno wrote:

..snip..

> > > > > > 
> > > > > > @@ -306,6 +312,18 @@ static int
> > > > > > mt8188_mt6359_mtkaif_calibration(struct snd_soc_pcm_runtime
> > > > > > *rtd)
> > > > > >     		return 0;
> > > > > >     	}
> > > > > >     
> > > > > > +	for_each_card_widgets(rtd->card, w) {
> > > > > > +		if (!strcmp(w->name, "MTKAIF_PIN")) {
> > > > > 
> > > > > if (strncmp(w->name, "MTKAIF_PIN", strlen(w->name) == 0) {
> > > > > 	pin_w = w;
> > > > > 	break;
> > > > > }
> > > > > 
> > > > > That's safer.
> > > > > 
> > > > 
> > > > If w->name is MTKAIF, the strncmp expression will return 0.
> > > > However,
> > > > the result is not expected. I prefer to keep strcmp here.
> > > > 
> > > 
> > > You could also do, instead
> > > 
> > > if (strncmp(w->name, "MTKAIF_PIN", strlen("MTKAIF_PIN") == 0))
> > > 
> > > ...solving your concern.
> > > 
> > > 
> > 
> >  From my understanding, strncmp is utilized to determine a string
> > begins
> > with a particular prefix while strcmp is used to compare a whole
> > string. In this scenario, I wish to verify if the widget name is
> > exactly 'MTKAIF_PIN', so I believe using strcmp would be more
> > appropriate.
> > 
> > Using either strlen(w->name) or strlen("MTKAIF_PIN") may lead to
> > incorrect results when w->name is either MTKAIF or MTKAIF_PIN1.
> > 
> > Thanks,
> > Trevor
> 
> strcmp() and strncmp() are the same; except strncmp() compares *at
> most* `n` bytes,
> where `n` is my `strlen("MTKAIF_PIN")`.
> 
>  From Linux man pages....
> 
https://urldefense.com/v3/__https://www.man7.org/linux/man-pages/man3/strcmp.3.html__;!!CTRNKA9wMg0ARbw!n-X-ckbkVYv4cbrhpwb2f7NH6sQkxx1czmCU2-q5BtSOhQU0C68wj9JNcu47YfbYeTpVEzjYQVXGuQb5ulpeWwKjnVMdZpg$ 
>  
> 
> 

Hi Angelo,

My concern is that strncmp() compares at most `n` bytes, where `n` is
the length of the string 'MTKAIF_PIN'. For instance, if both
'MTKAIF_PIN' and 'MTKAIF_PINMUX' exist in the widget list, they will
both enter execute_something() function below when executing this code:
 
if (strncmp(w->name, "MTKAIF_PIN", strlen("MTKAIF_PIN")) == 0) { 	
 execute_something(); 
}. 

This is not my expected scenario. To prevent this problem, we can use
strcmp() instead of strncmp(). strcmp() compares two strings until it
finds a difference or reaches the end of one of them. Therefore, it
will compare the entire string 'MTKAIF_PIN' with w->name and make sure
that only do execute_something() when w->names is the same as
'MTKAIF_PIN'.

Thanks,
Trevor