linux-kernel - RE: [PATCH 1/1] video: hyperv_fb: Add ratelimit on error message

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Date:   Wed, 21 Apr 2021 15:27:24 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     Joe Perches <joe@...ches.com>, KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>
Subject: RE: [PATCH 1/1] video: hyperv_fb: Add ratelimit on error message

From: Joe Perches <joe@...ches.com> Sent: Tuesday, April 20, 2021 12:59 PM
> 
> On Tue, 2021-04-20 at 08:44 -0700, Michael Kelley wrote:
> > Due to a full ring buffer, the driver may be unable to send updates to
> > the Hyper-V host.  But outputing the error message can make the problem
> > worse because console output is also typically written to the frame
> > buffer.  As a result, in some circumstances the error message is output
> > continuously.
> >
> > Break the cycle by rate limiting the error message.  Also output
> > the error code for additional diagnosability.
> >
> > Signed-off-by: Michael Kelley <mikelley@...rosoft.com>
> 
> None of the callers of this function ever check the return status.
> Why is important/useful to emit this message at all?
>

Except during device initialization, the messages tell Hyper-V
about updates to the frame buffer contents.  The fbdev
framework functions (such as fillrect) that eventually call synthvid_send()
are void functions.   So the message is the only alert that something
has gone wrong in the communication with Hyper-V, which makes
it useful for diagnostic purposes.

Michael

 
> > ---
> >  drivers/video/fbdev/hyperv_fb.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/video/fbdev/hyperv_fb.c b/drivers/video/fbdev/hyperv_fb.c
> > index 4dc9077..a7e6eea 100644
> > --- a/drivers/video/fbdev/hyperv_fb.c
> > +++ b/drivers/video/fbdev/hyperv_fb.c
> > @@ -308,7 +308,7 @@ static inline int synthvid_send(struct hv_device *hdev,
> >  			       VM_PKT_DATA_INBAND, 0);
> >
> >
> >  	if (ret)
> > -		pr_err("Unable to send packet via vmbus\n");
> > +		pr_err_ratelimited("Unable to send packet via vmbus; error %d\n", ret);
> >
> >
> >  	return ret;
> >  }
>