linux-kernel - Re: [PATCH] usb: musb: remove unused variable 'devctl'

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Message-ID: <X7eKetkCN+Z3meyB@kroah.com>
Date:   Fri, 20 Nov 2020 10:20:58 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Min Guo <min.guo@...iatek.com>
Cc:     Bin Liu <b-liu@...com>, Matthias Brugger <matthias.bgg@...il.com>,
        chunfeng.yun@...iatek.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH] usb: musb: remove unused variable 'devctl'

On Fri, Nov 20, 2020 at 05:02:15PM +0800, Min Guo wrote:
> On Fri, 2020-11-20 at 09:36 +0100, Greg Kroah-Hartman wrote:
> > On Fri, Nov 20, 2020 at 03:42:06PM +0800, Min Guo wrote:
> > > On Fri, 2020-11-20 at 07:54 +0100, Greg Kroah-Hartman wrote:
> > > > On Fri, Nov 20, 2020 at 02:48:50PM +0800, Min Guo wrote:
> > > > > Hi greg k-h:
> > > > > On Wed, 2020-11-18 at 12:48 +0100, Greg Kroah-Hartman wrote:
> > > > > > On Tue, Nov 17, 2020 at 04:21:25PM +0800, min.guo@...iatek.com wrote:
> > > > > > > From: Min Guo <min.guo@...iatek.com>
> > > > > > > 
> > > > > > > Remove unused 'devctl' variable to fix compile warnings:
> > > > > > > 
> > > > > > >     drivers/usb/musb/musbhsdma.c: In function 'dma_controller_irq':
> > > > > > >     drivers/usb/musb/musbhsdma.c:324:8: warning: variable 'devctl' set
> > > > > > >     but not used [-Wunused-but-set-variable]
> > > > > > > 
> > > > > > > Signed-off-by: Min Guo <min.guo@...iatek.com>
> > > > > > > ---
> > > > > > >  drivers/usb/musb/musbhsdma.c | 4 ----
> > > > > > >  1 file changed, 4 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/drivers/usb/musb/musbhsdma.c b/drivers/usb/musb/musbhsdma.c
> > > > > > > index 0aacfc8be5a1..7acd1635850d 100644
> > > > > > > --- a/drivers/usb/musb/musbhsdma.c
> > > > > > > +++ b/drivers/usb/musb/musbhsdma.c
> > > > > > > @@ -321,8 +321,6 @@ irqreturn_t dma_controller_irq(int irq, void *private_data)
> > > > > > >  				musb_channel->channel.status =
> > > > > > >  					MUSB_DMA_STATUS_BUS_ABORT;
> > > > > > >  			} else {
> > > > > > > -				u8 devctl;
> > > > > > > -
> > > > > > >  				addr = musb_read_hsdma_addr(mbase,
> > > > > > >  						bchannel);
> > > > > > >  				channel->actual_len = addr
> > > > > > > @@ -336,8 +334,6 @@ irqreturn_t dma_controller_irq(int irq, void *private_data)
> > > > > > >  						< musb_channel->len) ?
> > > > > > >  					"=> reconfig 0" : "=> complete");
> > > > > > >  
> > > > > > > -				devctl = musb_readb(mbase, MUSB_DEVCTL);
> > > > > > 
> > > > > > Are you sure that the hardware does not require this read to complete
> > > > > > the command?  Lots of hardware is that way, so be very careful about
> > > > > > this.  Did you test it?
> > > > > 
> > > > > I have tested this patch on Mediatek's platform, and not sure if it
> > > > > will affect other vendors' platforms.
> > > > > 
> > > > > Dear Bin:
> > > > > 
> > > > > Does this patch will affect other vendors' platforms?
> > > > 
> > > > The hardware specs will answer this question, what do they say about
> > > > this read?
> > > 
> > > Sorry, I didn't seen the comment on the hardware specs indicate that
> > > devctl register needs to read once to take effect.
> > 
> > Perhaps you might want to add a comment here so that people will not
> > keep making this same mistake when they run auto-checkers on the
> > codebase?
> 
> Sorry for confused you, I searched the hardware specs, but did not find
> the special description of the register devctl indicating that it needs
> to be read out before the hardware can work.

If this is a PCI device, that is implied as that is how all PCI devices
work, right?

What is the problem that is fixed by removing this read?

thanks,

greg k-h