[PATCH v4 3/7] char: xillybus: Avoid possible bandwidth inefficiency

[Eli Billauer]

The host flow controls the payload data flow from the FPGA by sending
OPCODE_SET_CHECKPOINT messages. Fix the condition for sending such a
message, to correctly handle the case where leap < 0.

The previous expression leap > (fifo->size >> 3) was not intended to
evaluate true when leap is negative. However, due to C's integer
promotion rules, leap (of s32 type) is promoted to unsigned int when
compared with the unsigned fifo->size >> 3 expression. As a result,
negative leap values are interpreted as large positive numbers, causing
the condition to evaluate true unintentionally.

Consequently, the device receives correctly formed checkpoint messages
that encourage it to send data, but too frequently. This may cause the
device to send short data chunks, wasting USB bandwidth.

Assisted-by: Deepseek:v4-pro Kimi:K2.6 ChatGPT:GPT-5.5 Claude:Sonnet-4.6
Signed-off-by: Eli Billauer <eli.billauer@xxxxxxxxx>
---

Notes:
    Changelog:
    =========
    
    No change on v3->v4.
    
    Changes v2->v3:
     -- Add Assisted-by tag to description
    
    No change on v1->v2.

 drivers/char/xillybus/xillyusb.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/char/xillybus/xillyusb.c b/drivers/char/xillybus/xillyusb.c
index ee819e2e3f82..ef5b1816b277 100644
--- a/drivers/char/xillybus/xillyusb.c
+++ b/drivers/char/xillybus/xillyusb.c
@@ -1512,8 +1512,8 @@ static ssize_t xillyusb_read(struct file *filp, char __user *userbuf,
 		 */
 
 		if (chan->read_data_ok &&
-		    (leap > (fifo->size >> 3) ||
-		     (checkpoint_for_complete && leap > 0))) {
+		    (leap > 0 && (leap > (fifo->size >> 3) ||
+				  checkpoint_for_complete))) {
 			chan->in_current_checkpoint = checkpoint;
 			rc = xillyusb_send_opcode(xdev, chan_num,
 						  OPCODE_SET_CHECKPOINT,
-- 
2.34.1