dmaengine: at_xdmac: fix residue computation

#define AT_XDMAC_MAX_CHAN	0x20

#define AT_XDMAC_MAX_CSIZE	16	/* 16 data */

#define AT_XDMAC_MAX_DWIDTH	8	/* 64 bits */

#define AT_XDMAC_RESIDUE_MAX_RETRIES	5

#define AT_XDMAC_DMA_BUSWIDTHS\

	(BIT(DMA_SLAVE_BUSWIDTH_UNDEFINED) |\

	struct at_xdmac_desc	*desc, *_desc;

	struct list_head	*descs_list;

	enum dma_status		ret;

	int			residue;

	u32			cur_nda, mask, value;

	int			residue, retry;

	u32			cur_nda, check_nda, cur_ubc, mask, value;

	u8			dwidth = 0;

	unsigned long		flags;

			cpu_relax();

	}

	/*

	 * When processing the residue, we need to read two registers but we

	 * can't do it in an atomic way. AT_XDMAC_CNDA is used to find where

	 * we stand in the descriptor list and AT_XDMAC_CUBC is used

	 * to know how many data are remaining for the current descriptor.

	 * Since the dma channel is not paused to not loose data, between the

	 * AT_XDMAC_CNDA and AT_XDMAC_CUBC read, we may have change of

	 * descriptor.

	 * For that reason, after reading AT_XDMAC_CUBC, we check if we are

	 * still using the same descriptor by reading a second time

	 * AT_XDMAC_CNDA. If AT_XDMAC_CNDA has changed, it means we have to

	 * read again AT_XDMAC_CUBC.

	 * Memory barriers are used to ensure the read order of the registers.

	 * A max number of retries is set because unlikely it can never ends if

	 * we are transferring a lot of data with small buffers.

	 */

	cur_nda = at_xdmac_chan_read(atchan, AT_XDMAC_CNDA) & 0xfffffffc;

	rmb();

	cur_ubc = at_xdmac_chan_read(atchan, AT_XDMAC_CUBC);

	for (retry = 0; retry < AT_XDMAC_RESIDUE_MAX_RETRIES; retry++) {

		rmb();

		check_nda = at_xdmac_chan_read(atchan, AT_XDMAC_CNDA) & 0xfffffffc;

		if (likely(cur_nda == check_nda))

			break;

		cur_nda = check_nda;

		rmb();

		cur_ubc = at_xdmac_chan_read(atchan, AT_XDMAC_CUBC);

	}

	if (unlikely(retry >= AT_XDMAC_RESIDUE_MAX_RETRIES)) {

		ret = DMA_ERROR;

		goto spin_unlock;

	}

	/*

	 * Remove size of all microblocks already transferred and the current

	 * one. Then add the remaining size to transfer of the current

		if ((desc->lld.mbr_nda & 0xfffffffc) == cur_nda)

			break;

	}

	residue += at_xdmac_chan_read(atchan, AT_XDMAC_CUBC) << dwidth;

	residue += cur_ubc << dwidth;

	dma_set_residue(txstate, residue);