stm32/usb: Patching framework for USB descriptors

In some cases, we want to be able to dynamically modify a few bytes
in the USB descriptor (in our case, length of referenced items),
but it could also be other things like flags.

These 2 new functions allow to keep all the USB descriptor in flash, and
modify these few bytes before writing them in the USB buffer.

BRANCH=none
BUG=b:37447752
TEST=Flash hammer, USB descriptors are valid.

Change-Id: I8624255fa43f52a0aaa21d20e963f3974f236912
Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/771057
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>

3 files changed, 57 insertions, 3 deletions

diff --git a/chip/stm32/usb.c b/chip/stm32/usb.c
index 27ea117de5..c97c68f03c 100644
--- a/chip/stm32/usb.c
+++ b/chip/stm32/usb.c
@@ -119,6 +119,40 @@ void usb_read_setup_packet(usb_uint *buffer, struct usb_setup_packet *packet)
 	packet->wLength       = buffer[3];
 }
 
+struct usb_descriptor_patch {
+	const void *address;
+	uint16_t data;
+};
+
+static struct usb_descriptor_patch desc_patches[USB_DESC_PATCH_COUNT];
+
+void set_descriptor_patch(enum usb_desc_patch_type type,
+			  const void *address, uint16_t data)
+{
+	desc_patches[type].address = address;
+	desc_patches[type].data = data;
+}
+
+void *memcpy_to_usbram_ep0_patch(const void *src, size_t n)
+{
+	int i;
+	void *ret;
+
+	ret = memcpy_to_usbram((void *)usb_sram_addr(ep0_buf_tx), src, n);
+
+	for (i = 0; i < USB_DESC_PATCH_COUNT; i++) {
+		unsigned int offset = desc_patches[i].address - src;
+
+		if (offset >= n)
+			continue;
+
+		memcpy_to_usbram((void *)(usb_sram_addr(ep0_buf_tx) + offset),
+			&desc_patches[i].data, sizeof(desc_patches[i].data));
+	}
+
+	return ret;
+}
+
 static void ep0_send_descriptor(const uint8_t *desc, int len,
 				uint16_t fixup_size)
 {
@@ -133,7 +167,7 @@ static void ep0_send_descriptor(const uint8_t *desc, int len,
 		desc_ptr = desc + USB_MAX_PACKET_SIZE;
 		len = USB_MAX_PACKET_SIZE;
 	}
-	memcpy_to_usbram(EP0_BUF_TX_SRAM_ADDR, desc, len);
+	memcpy_to_usbram_ep0_patch(desc, len);
 	if (fixup_size) /* set the real descriptor size */
 		ep0_buf_tx[1] = fixup_size;
 	btable_ep[0].tx_count = len;
diff --git a/chip/stm32/usb_hid.c b/chip/stm32/usb_hid.c
index f24bdd5982..873d80009a 100644
--- a/chip/stm32/usb_hid.c
+++ b/chip/stm32/usb_hid.c
@@ -107,8 +107,7 @@ int hid_iface_request(usb_uint *ep0_buf_rx, usb_uint *ep0_buf_tx,
 			return report_left ? 1 : 0;
 		} else if (ep0_buf_rx[1] == (USB_HID_DT_HID << 8)) {
 			/* Setup : HID specific : Get HID descriptor */
-			memcpy_to_usbram((void *) usb_sram_addr(ep0_buf_tx),
-					hid_desc, sizeof(*hid_desc));
+			memcpy_to_usbram_ep0_patch(hid_desc, sizeof(*hid_desc));
 			btable_ep[0].tx_count = sizeof(*hid_desc);
 			STM32_TOGGLE_EP(0, EP_TX_RX_MASK, EP_TX_RX_VALID,
 					EP_STATUS_OUT);
diff --git a/chip/stm32/usb_hw.h b/chip/stm32/usb_hw.h
index 89c8e9d6f0..94734d8569 100644
--- a/chip/stm32/usb_hw.h
+++ b/chip/stm32/usb_hw.h
@@ -63,6 +63,27 @@ void usb_read_setup_packet(usb_uint *buffer, struct usb_setup_packet *packet);
 void *memcpy_to_usbram(void *dest, const void *src, size_t n);
 void *memcpy_from_usbram(void *dest, const void *src, size_t n);
 
+/*
+ * Descriptor patching support, useful to change a few values in the descriptor
+ * (typically, length or bitfields) without having to move descriptors to RAM.
+ */
+
+enum usb_desc_patch_type {
+	USB_DESC_PATCH_COUNT,
+};
+
+/*
+ * Set patch in table: replace uint16_t at address (STM32 flash) with data.
+ *
+ * The patches need to be setup before _before_ usb_init is executed (or, at
+ * least, before the first call to memcpy_to_usbram_ep0_patch).
+ */
+void set_descriptor_patch(enum usb_desc_patch_type type,
+			  const void *address, uint16_t data);
+
+/* Copy to USB ram, applying patches to src as required. */
+void *memcpy_to_usbram_ep0_patch(const void *src, size_t n);
+
 /* Compute the address inside dedicate SRAM for the USB controller */
 #define usb_sram_addr(x) ((x - __usb_ram_start) * sizeof(uint16_t))