When cable is unplugged, dbc_ensure_running() correctly detects this
situation (DBC_CTRL_DCR flag is clear), and prevent sending data
immediately to the device. It gets only queued in work ring buffers.
When cable is plugged in again, subsequent dbc_flush() will send the
buffered data.
But there is a corner case, where no subsequent data was buffered in the
work buffer, but a TRB was still pending. Ring the doorbell to let the
controller re-send them. For console output it is rare corner case (TRB
is pending for a very short time), but for console input it is very
normal case (there is always one pending TRB for input).

Extract doorbell ringing into separate function to avoid duplication.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
---
Changes in v6:
 - keep barriers consistent
---
 xen/drivers/char/xhci-dbc.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/xen/drivers/char/xhci-dbc.c b/xen/drivers/char/xhci-dbc.c
index 9f7e1dd60a78..829f1d1d910f 100644
--- a/xen/drivers/char/xhci-dbc.c
+++ b/xen/drivers/char/xhci-dbc.c
@@ -554,6 +554,15 @@ static unsigned int dbc_work_ring_space_to_end(const struct dbc_work_ring *ring)
     return ring->deq - ring->enq;
 }
 
+static void dbc_ring_doorbell(struct dbc *dbc, int doorbell)
+{
+    uint32_t __iomem *db_reg = &dbc->dbc_reg->db;
+    uint32_t db = (readl(db_reg) & ~DBC_DOORBELL_TARGET_MASK) |
+                  (doorbell << DBC_DOORBELL_TARGET_SHIFT);
+
+    writel(db, db_reg);
+}
+
 static void dbc_push_trb(struct dbc *dbc, struct xhci_trb_ring *ring,
                          uint64_t dma, uint64_t len)
 {
@@ -1024,6 +1033,8 @@ static bool dbc_ensure_running(struct dbc *dbc)
         writel(ctrl | (1U << DBC_CTRL_DRC), &reg->ctrl);
         writel(readl(&reg->portsc) | (1U << DBC_PSC_PED), &reg->portsc);
         wmb();
+        dbc_ring_doorbell(dbc, dbc->dbc_iring.db);
+        dbc_ring_doorbell(dbc, dbc->dbc_oring.db);
     }
 
     return true;
@@ -1041,10 +1052,6 @@ static bool dbc_ensure_running(struct dbc *dbc)
 static void dbc_flush(struct dbc *dbc, struct xhci_trb_ring *trb,
                       struct dbc_work_ring *wrk)
 {
-    struct dbc_reg *reg = dbc->dbc_reg;
-    uint32_t db = (readl(&reg->db) & ~DBC_DOORBELL_TARGET_MASK) |
-                  (trb->db << DBC_DOORBELL_TARGET_SHIFT);
-
     if ( xhci_trb_ring_full(trb) )
         return;
 
@@ -1068,7 +1075,7 @@ static void dbc_flush(struct dbc *dbc, struct xhci_trb_ring *trb,
     }
 
     wmb();
-    writel(db, &reg->db);
+    dbc_ring_doorbell(dbc, trb->db);
 }
 
 /**
-- 
git-series 0.9.1

On 02.09.2022 15:17, Marek Marczykowski-Górecki wrote:
> When cable is unplugged, dbc_ensure_running() correctly detects this
> situation (DBC_CTRL_DCR flag is clear), and prevent sending data
> immediately to the device. It gets only queued in work ring buffers.
> When cable is plugged in again, subsequent dbc_flush() will send the
> buffered data.
> But there is a corner case, where no subsequent data was buffered in the
> work buffer, but a TRB was still pending. Ring the doorbell to let the
> controller re-send them. For console output it is rare corner case (TRB
> is pending for a very short time), but for console input it is very
> normal case (there is always one pending TRB for input).
> 
> Extract doorbell ringing into separate function to avoid duplication.
> 
> Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>

Acked-by: Jan Beulich <jbeulich@suse.com>