xhci: Stop unnecessary tracking of free trbs in a ring

Trying to keep track of free trbs in a ring by adding and subtracting
deltas each time a enqueue or dequeue is increased or moved has proven to
be buggy and complicated, especially over long periods of time.

Recently a bug in counting free trbs was fixed, now taking into account
cancelled URBs that were turned into no-ops, preventing free_trbs to
slowly wander off causing unnecessary ring expansion. See
commit fe82f16aafda ("xhci: Fix incorrect tracking of free space on
transfer rings")

Turns out its a lot easier to just calculate the numer of free TRB
based on ring size and the current enqueue and dequeue pointer values.
This is currently only needed for the command ring as multi segment
transfer rings already ensures there is enough room the ring during
the ring expansion check.

We could get rid of the ring->num_trbs_free entry completely, but as
the xhci DbC code also uses it we don't clean that up in this patch.

Reported-by: Miller Hunter <MillerH@hearthnhome.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217242
Tested-by: Miller Hunter <MillerH@hearthnhome.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Message-ID: <20230602144009.1225632-8-mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 1 insertions, 2 deletions

diff --git a/drivers/usb/host/xhci.h b/drivers/usb/host/xhci.h
index 56e318c384ff..456e1c8ca005 100644
--- a/drivers/usb/host/xhci.h
+++ b/drivers/usb/host/xhci.h
@@ -1633,8 +1633,7 @@ struct xhci_ring {
 	u32			cycle_state;
 	unsigned int		stream_id;
 	unsigned int		num_segs;
-	unsigned int		num_trbs_free;
-	unsigned int		num_trbs_free_temp;
+	unsigned int		num_trbs_free; /* used only by xhci DbC */
 	unsigned int		bounce_buf_len;
 	enum xhci_ring_type	type;
 	bool			last_td_was_short;