[PATCH] ofnet: Remove 200 ms timeout in get_card_packet to reduce input latency - Michael Chang via Grub-devel

From: Michael Chang via Grub-devel <grub-devel@gnu.org>
To: The development of GNU GRUB <grub-devel@gnu.org>
Cc: Michael Chang <mchang@suse.com>, Tony Jones <tonyj@suse.com>
Subject: [PATCH] ofnet: Remove 200 ms timeout in get_card_packet to reduce input latency
Date: Mon,  6 May 2024 10:34:22 +0800	[thread overview]
Message-ID: <20240506023422.6891-1-mchang@suse.com> (raw)

When grub image is netbooted on ppc64le, the keyboard input exhibits
significant latency, reports even say that characters are processed
about once per second. This issue makes interactively trying to debug a
ppc64le config very difficult.

It seems that the latency is largely caused by a 200 ms timeout in the
idle event loop, during which the network card interface is consistently
polled for incoming packets. Often, no packets arrive during this
period, so the timeout nearly always expires, which blocks the response
to key inputs.

Furthermore, this 200 ms timeout might not need to be enforced at this
basic layer, considering that grub performs synchronous reads and its
timeout management is actually handled by higher layers, not directly in
the card instance. Additionally, the idle polling, which reacts to
unsolicited packets like ICMP and SLAAC, would be fine at a less
frequent polling interval, rather than needing a timeout for receiving a
response.

For these reasons, we believe the timeout in get_card_packet should be
effectively removed. According to test results, the delay has
disappeared, and it is now much easier to use interactively.

Signed-Off-by: Michael Chang <mchang@suse.com>
Tested-by: Tony Jones <tonyj@suse.com>
---
 grub-core/net/drivers/ieee1275/ofnet.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/grub-core/net/drivers/ieee1275/ofnet.c b/grub-core/net/drivers/ieee1275/ofnet.c
index 78f03df8e..3bf48b3f0 100644
--- a/grub-core/net/drivers/ieee1275/ofnet.c
+++ b/grub-core/net/drivers/ieee1275/ofnet.c
@@ -82,15 +82,11 @@ get_card_packet (struct grub_net_card *dev)
   grub_ssize_t actual;
   int rc;
   struct grub_ofnetcard_data *data = dev->data;
-  grub_uint64_t start_time;
   struct grub_net_buff *nb;
 
-  start_time = grub_get_time_ms ();
-  do
-    rc = grub_ieee1275_read (data->handle, dev->rcvbuf, dev->rcvbufsize, &actual);
-  while ((actual <= 0 || rc < 0) && (grub_get_time_ms () - start_time < 200));
+  rc = grub_ieee1275_read (data->handle, dev->rcvbuf, dev->rcvbufsize, &actual);
 
-  if (actual <= 0)
+  if (actual <= 0 || rc < 0)
     return NULL;
 
   nb = grub_netbuff_alloc (actual + 2);
-- 
2.44.0


_______________________________________________
Grub-devel mailing list
Grub-devel@gnu.org
https://lists.gnu.org/mailman/listinfo/grub-devel
