[Skiboot] [PATCH] FSP/CONSOLE: Workaround for unresponsive ipmi daemon

[Vasant Hegde]

We use TCE mapped area to write data to console. Console header
(fsp_serbuf_hdr) is modified by both FSP and OPAL (OPAL updates
next_in pointer in fsp_serbuf_hdr and FSP updates next_out pointer).

Kernel makes opal_console_write() OPAL call to write data to console.
OPAL write data to TCE mapped area and sends MBOX command to FSP.
If our console becomes full and we have data to write to console,
we keep on waiting until FSP reads data.

In some corner cases, where FSP is active but not responding to
console MBOX message (due to buggy IPMI) and we have heavy console
write happening from kernel, then eventually our console buffer
becomes full. At this point OPAL starts sending OPAL_BUSY_EVENT to
kernel. Kernel will keep on retrying. This is creating kernel soft
lockups. In some extreme case when every CPU is trying to write to
console, user will not be able to ssh and thinks system is hang.

If we reset FSP or restart IPMI daemon on FSP, system recovers and
everything becomes normal.

This patch adds workaround to above issue by returning OPAL_HARDWARE
when cosole is full. Side effect of this patch is, we may endup dropping
latest console data. But better to drop console data than system hang.

Alternative approach is to drop old data from console buffer, make space
for new data. But in normal condition only FSP can update 'next_out'
pointer and if we touch that pointer, it may introduce some other
race conditions. Hence we decided to just new console write request.

Signed-off-by: Vasant Hegde <hegdevasant at linux.vnet.ibm.com>
---
 hw/fsp/fsp-console.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/fsp/fsp-console.c b/hw/fsp/fsp-console.c
index fd67b20..a7b0281 100644
--- a/hw/fsp/fsp-console.c
+++ b/hw/fsp/fsp-console.c
@@ -610,7 +610,7 @@ static int64_t fsp_console_write(int64_t term_number, int64_t *length,
 	*length = written;
 	unlock(&fsp_con_lock);
 
-	return written ? OPAL_SUCCESS : OPAL_BUSY_EVENT;
+	return written ? OPAL_SUCCESS : OPAL_HARDWARE;
 }
 
 static int64_t fsp_console_write_buffer_space(int64_t term_number,
-- 
2.9.3