net: altera_tse: fix msgdma_tx_completion on non-zero fill_level case

If fill_level was not zero and status was not BUSY,
result of "tx_prod - tx_cons - inuse" might be zero.
Subtracting 1 unconditionally results invalid negative return value
on this case.
Make sure not to return an negative value.

Signed-off-by: Tomonori Sakita <tomonori.sakita@sord.co.jp>
Signed-off-by: Atsushi Nemoto <atsushi.nemoto@sord.co.jp>
Reviewed-by: Dalon L Westergreen <dalon.westergreen@linux.intel.com>
Acked-by: Thor Thayer <thor.thayer@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/ethernet/altera/altera_msgdma.c b/drivers/net/ethernet/altera/altera_msgdma.c
index 0fb986b..0ae723f 100644 (file)
--- a/drivers/net/ethernet/altera/altera_msgdma.c
+++ b/drivers/net/ethernet/altera/altera_msgdma.c
@@ -145,7 +145,8 @@ u32 msgdma_tx_completions(struct altera_tse_private *priv)
                        & 0xffff;
 
        if (inuse) { /* Tx FIFO is not empty */
-               ready = priv->tx_prod - priv->tx_cons - inuse - 1;
+               ready = max_t(int,
+                             priv->tx_prod - priv->tx_cons - inuse - 1, 0);
        } else {
                /* Check for buffered last packet */
                status = csrrd32(priv->tx_dma_csr, msgdma_csroffs(status));