[PATCH 1/1] video: hyperv_fb: Add ratelimit on error message

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 <redacted>
---
 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;
 }

-- 
1.8.3.1

On Tue, Apr 20, 2021 at 08:44:19AM -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 <redacted>

Applied to hyperv-next. Thanks.

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 <redacted>

None of the callers of this function ever check the return status.
Why is important/useful to emit this message at all?

From: Joe Perches <joe@perches.com> Sent: Tuesday, April 20, 2021 12:59 PM

On Tue, 2021-04-20 at 08:44 -0700, Michael Kelley wrote:

quoted

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 <redacted>

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

 

quoted

---
 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;
 }