Zitat von Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>:
On Mon, 18 Mar 2019, Guido Kiener wrote:
The OUT endpoint normally blocks (NAK) subsequent packets when a
short packet was received and returns an incomplete queue entry to
the gadget driver. Thereby the gadget driver can detect a short packet
when reading queue entries with a length that is not equal to a
multiple of packet size.
The start_queue() function enables receiving OUT packets regardless
of the content of the OUT FIFO. This results in a problem:
When receiving is enabled again, more OUT packets are appended to
the last short packet and the gadget driver cannot determine the end
of a short packet anymore. Furthermore the remaining data in the OUT
FIFO is not delivered to the gadget driver immediately and can produce
timeout errors.
This fix stops OUT naking only when OUT naking is enabled and when the
OUT FIFO is empty. It ensures that all received data is delivered to
the gadget driver which can detect a short packet now before new
packets overrun the last short packet.
This description should explain the race which causes the problem.
With the current code, it's possible that the "!ep->is_in &&
(readl(&ep->regs->ep_stat) & BIT(NAK_OUT_PACKETS))" test will fail,
then a short packet will be received, and then start_queue() will call
stop_out_naking(). That's what we don't want -- OUT naking gets turned
off while there is data in the FIFO. With the patch, this race can't
occur because the FIFO's state is tested after we know that OUT naking
is already turned on.
Thanks. Please feel free to change my wording/spelling. I'm not offended.
Does this description meet your request:
The OUT endpoint normally blocks (NAK) subsequent packets when a
short packet was received and returns an incomplete queue entry to
the gadget driver. Thereby the gadget driver can detect a short packet
when reading queue entries with a length that is not equal to a
multiple of packet size.
The start_queue() function enables receiving OUT packets regardless
of the content of the OUT FIFO. This results in a race:
With the current code, it's possible that the "!ep->is_in &&
(readl(&ep->regs->ep_stat) & BIT(NAK_OUT_PACKETS))" test will fail,
then a short packet will be received, and then start_queue() will call
stop_out_naking(). That's what we don't want -- OUT naking gets turned
off while there is data in the FIFO. With the patch, this race can't
occur because the FIFO's state is tested after we know that OUT naking
is already turned on.
Guido
That patch itself looks good, I just think the explanation should be
improved.
Alan Stern
Signed-off-by: Guido Kiener <guido.kiener@xxxxxxxxxxxxxxxxx>
---
drivers/usb/gadget/udc/net2280.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)
diff --git a/drivers/usb/gadget/udc/net2280.c
b/drivers/usb/gadget/udc/net2280.c
index f63f82450bf4..e0b413e9e532 100644
--- a/drivers/usb/gadget/udc/net2280.c
+++ b/drivers/usb/gadget/udc/net2280.c
@@ -866,9 +866,6 @@ static void start_queue(struct net2280_ep *ep,
u32 dmactl, u32 td_dma)
(void) readl(&ep->dev->pci->pcimstctl);
writel(BIT(DMA_START), &dma->dmastat);
-
- if (!ep->is_in)
- stop_out_naking(ep);
}
static void start_dma(struct net2280_ep *ep, struct net2280_request *req)
@@ -907,6 +904,7 @@ static void start_dma(struct net2280_ep *ep,
struct net2280_request *req)
writel(BIT(DMA_START), &dma->dmastat);
return;
}
+ stop_out_naking(ep);
}
tmp = dmactl_default;
