+#define PKT_ALIGN 512
+ /*
+ * The USB transfer is split into qTD transfers. Eeach qTD transfer is
+ * described by a transfer descriptor (the qTD). The qTDs form a linked
+ * list with a queue head (QH).
+ *
+ * Each qTD transfer starts with a new USB packet, i.e. a packet cannot
+ * have its beginning in a qTD transfer and its end in the following
+ * one, so the qTD transfer lengths have to be chosen accordingly.
+ *
+ * Each qTD transfer uses up to QT_BUFFER_CNT data buffers, mapped to
+ * single pages. The first data buffer can start at any offset within a
+ * page (not considering the cache-line alignment issues), while the
+ * following buffers must be page-aligned. There is no alignment
+ * constraint on the size of a qTD transfer.
+ */
+ if (req != NULL)
+ /* 1 qTD will be needed for SETUP, and 1 for ACK. */
+ qtd_count += 1 + 1;
+ if (length > 0 || req == NULL) {
+ /*
+ * Determine the qTD transfer size that will be used for the
+ * data payload (not considering the first qTD transfer, which
+ * may be longer or shorter, and the final one, which may be
+ * shorter).
+ *
+ * In order to keep each packet within a qTD transfer, the qTD
+ * transfer size is aligned to PKT_ALIGN, which is a multiple of
+ * wMaxPacketSize (except in some cases for interrupt transfers,
+ * see comment in submit_int_msg()).
+ *
+ * By default, i.e. if the input buffer is aligned to PKT_ALIGN,
+ * QT_BUFFER_CNT full pages will be used.
+ */
+ int xfr_sz = QT_BUFFER_CNT;
+ /*
+ * However, if the input buffer is not aligned to PKT_ALIGN, the
+ * qTD transfer size will be one page shorter, and the first qTD
+ * data buffer of each transfer will be page-unaligned.
+ */
+ if ((uint32_t)buffer & (PKT_ALIGN - 1))
+ xfr_sz--;
+ /* Convert the qTD transfer size to bytes. */
+ xfr_sz *= EHCI_PAGE_SIZE;
+ /*
+ * Approximate by excess the number of qTDs that will be
+ * required for the data payload. The exact formula is way more
+ * complicated and saves at most 2 qTDs, i.e. a total of 128
+ * bytes.
+ */
+ qtd_count += 2 + length / xfr_sz;
+ }
+/*
+ * Threshold value based on the worst-case total size of the allocated qTDs for
+ * a mass-storage transfer of 65535 blocks of 512 bytes.
+ */
+#if CONFIG_SYS_MALLOC_LEN <= 64 + 128 * 1024
+#warning CONFIG_SYS_MALLOC_LEN may be too small for EHCI
+#endif
+ qtd = memalign(USB_DMA_MINALIGN, qtd_count * sizeof(struct qTD));
+ if (qtd == NULL) {
+ printf("unable to allocate TDs\n");