1 files changed, 26 insertions, 4 deletions

diff --git a/drivers/usb/musb/musb_gadget.c b/drivers/usb/musb/musb_gadget.c
index d0b87e7b4abf..b6b84dacc791 100644
--- a/drivers/usb/musb/musb_gadget.c
+++ b/drivers/usb/musb/musb_gadget.c
@@ -707,11 +707,12 @@ static void rxstate(struct musb *musb, struct musb_request *req)
                 fifo_count = musb_readw(epio, MUSB_RXCOUNT);
 
                 /*
-                 *  use mode 1 only if we expect data of at least ep packet_sz
-                 *  and have not yet received a short packet
+                 * Enable Mode 1 on RX transfers only when short_not_ok flag
+                 * is set. Currently short_not_ok flag is set only from
+                 * file_storage and f_mass_storage drivers
                  */
-                if ((request->length - request->actual >= musb_ep->packet_sz) &&
-                        (fifo_count >= musb_ep->packet_sz))
+
+                if (request->short_not_ok && fifo_count == musb_ep->packet_sz)
                         use_mode_1 = 1;
                 else
                         use_mode_1 = 0;
@@ -727,6 +728,27 @@ static void rxstate(struct musb *musb, struct musb_request *req)
                                 c = musb->dma_controller;
                                 channel = musb_ep->dma;
 
+        /* We use DMA Req mode 0 in rx_csr, and DMA controller operates in
+         * mode 0 only. So we do not get endpoint interrupts due to DMA
+         * completion. We only get interrupts from DMA controller.
+         *
+         * We could operate in DMA mode 1 if we knew the size of the tranfer
+         * in advance. For mass storage class, request->length = what the host
+         * sends, so that'd work.  But for pretty much everything else,
+         * request->length is routinely more than what the host sends. For
+         * most these gadgets, end of is signified either by a short packet,
+         * or filling the last byte of the buffer.  (Sending extra data in
+         * that last pckate should trigger an overflow fault.)  But in mode 1,
+         * we don't get DMA completion interrupt for short packets.
+         *
+         * Theoretically, we could enable DMAReq irq (MUSB_RXCSR_DMAMODE = 1),
+         * to get endpoint interrupt on every DMA req, but that didn't seem
+         * to work reliably.
+         *
+         * REVISIT an updated g_file_storage can set req->short_not_ok, which
+         * then becomes usable as a runtime "use mode 1" hint...
+         */
+
                                 /* Experimental: Mode1 works with mass storage use cases */
                                 if (use_mode_1) {
                                         csr |= MUSB_RXCSR_AUTOCLEAR;