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path: root/drivers/usb/gadget/udc/aspeed-vhub/vhub.h
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Diffstat (limited to 'drivers/usb/gadget/udc/aspeed-vhub/vhub.h')
-rw-r--r--drivers/usb/gadget/udc/aspeed-vhub/vhub.h33
1 files changed, 33 insertions, 0 deletions
diff --git a/drivers/usb/gadget/udc/aspeed-vhub/vhub.h b/drivers/usb/gadget/udc/aspeed-vhub/vhub.h
index 2b040257bc1f..4ed03d33a5a9 100644
--- a/drivers/usb/gadget/udc/aspeed-vhub/vhub.h
+++ b/drivers/usb/gadget/udc/aspeed-vhub/vhub.h
@@ -462,6 +462,39 @@ enum std_req_rc {
#define DDBG(d, fmt, ...) do { } while(0)
#endif
+static inline void vhub_dma_workaround(void *addr)
+{
+ /*
+ * This works around a confirmed HW issue with the Aspeed chip.
+ *
+ * The core uses a different bus to memory than the AHB going to
+ * the USB device controller. Due to the latter having a higher
+ * priority than the core for arbitration on that bus, it's
+ * possible for an MMIO to the device, followed by a DMA by the
+ * device from memory to all be performed and services before
+ * a previous store to memory gets completed.
+ *
+ * This the following scenario can happen:
+ *
+ * - Driver writes to a DMA descriptor (Mbus)
+ * - Driver writes to the MMIO register to start the DMA (AHB)
+ * - The gadget sees the second write and sends a read of the
+ * descriptor to the memory controller (Mbus)
+ * - The gadget hits memory before the descriptor write
+ * causing it to read an obsolete value.
+ *
+ * Thankfully the problem is limited to the USB gadget device, other
+ * masters in the SoC all have a lower priority than the core, thus
+ * ensuring that the store by the core arrives first.
+ *
+ * The workaround consists of using a dummy read of the memory before
+ * doing the MMIO writes. This will ensure that the previous writes
+ * have been "pushed out".
+ */
+ mb();
+ (void)__raw_readl((void __iomem *)addr);
+}
+
/* core.c */
void ast_vhub_done(struct ast_vhub_ep *ep, struct ast_vhub_req *req,
int status);