BIT(DMA_SLAVE_BUSWIDTH_2_BYTES) |\
BIT(DMA_SLAVE_BUSWIDTH_4_BYTES))
+#define ATC_MAX_DSCR_TRIALS 10
+
/*
* Initial number of descriptors to allocate for each channel. This could
* be increased during dma usage.
*
* @current_len: the number of bytes left before reading CTRLA
* @ctrla: the value of CTRLA
- * @desc: the descriptor containing the transfer width
*/
-static inline int atc_calc_bytes_left(int current_len, u32 ctrla,
- struct at_desc *desc)
+static inline int atc_calc_bytes_left(int current_len, u32 ctrla)
{
- return current_len - ((ctrla & ATC_BTSIZE_MAX) << desc->tx_width);
-}
+ u32 btsize = (ctrla & ATC_BTSIZE_MAX);
+ u32 src_width = ATC_REG_TO_SRC_WIDTH(ctrla);
-/**
- * atc_calc_bytes_left_from_reg - calculates the number of bytes left according
- * to the current value of CTRLA.
- *
- * @current_len: the number of bytes left before reading CTRLA
- * @atchan: the channel to read CTRLA for
- * @desc: the descriptor containing the transfer width
- */
-static inline int atc_calc_bytes_left_from_reg(int current_len,
- struct at_dma_chan *atchan, struct at_desc *desc)
-{
- u32 ctrla = channel_readl(atchan, CTRLA);
-
- return atc_calc_bytes_left(current_len, ctrla, desc);
+ /*
+ * According to the datasheet, when reading the Control A Register
+ * (ctrla), the Buffer Transfer Size (btsize) bitfield refers to the
+ * number of transfers completed on the Source Interface.
+ * So btsize is always a number of source width transfers.
+ */
+ return current_len - (btsize << src_width);
}
/**
struct at_desc *desc_first = atc_first_active(atchan);
struct at_desc *desc;
int ret;
- u32 ctrla, dscr;
+ u32 ctrla, dscr, trials;
/*
* If the cookie doesn't match to the currently running transfer then
* the channel's DSCR register and compare it against the value
* of the hardware linked list structure of each child
* descriptor.
+ *
+ * The CTRLA register provides us with the amount of data
+ * already read from the source for the current child
+ * descriptor. So we can compute a more accurate residue by also
+ * removing the number of bytes corresponding to this amount of
+ * data.
+ *
+ * However, the DSCR and CTRLA registers cannot be read both
+ * atomically. Hence a race condition may occur: the first read
+ * register may refer to one child descriptor whereas the second
+ * read may refer to a later child descriptor in the list
+ * because of the DMA transfer progression inbetween the two
+ * reads.
+ *
+ * One solution could have been to pause the DMA transfer, read
+ * the DSCR and CTRLA then resume the DMA transfer. Nonetheless,
+ * this approach presents some drawbacks:
+ * - If the DMA transfer is paused, RX overruns or TX underruns
+ * are more likey to occur depending on the system latency.
+ * Taking the USART driver as an example, it uses a cyclic DMA
+ * transfer to read data from the Receive Holding Register
+ * (RHR) to avoid RX overruns since the RHR is not protected
+ * by any FIFO on most Atmel SoCs. So pausing the DMA transfer
+ * to compute the residue would break the USART driver design.
+ * - The atc_pause() function masks interrupts but we'd rather
+ * avoid to do so for system latency purpose.
+ *
+ * Then we'd rather use another solution: the DSCR is read a
+ * first time, the CTRLA is read in turn, next the DSCR is read
+ * a second time. If the two consecutive read values of the DSCR
+ * are the same then we assume both refers to the very same
+ * child descriptor as well as the CTRLA value read inbetween
+ * does. For cyclic tranfers, the assumption is that a full loop
+ * is "not so fast".
+ * If the two DSCR values are different, we read again the CTRLA
+ * then the DSCR till two consecutive read values from DSCR are
+ * equal or till the maxium trials is reach.
+ * This algorithm is very unlikely not to find a stable value for
+ * DSCR.
*/
- ctrla = channel_readl(atchan, CTRLA);
- rmb(); /* ensure CTRLA is read before DSCR */
dscr = channel_readl(atchan, DSCR);
+ rmb(); /* ensure DSCR is read before CTRLA */
+ ctrla = channel_readl(atchan, CTRLA);
+ for (trials = 0; trials < ATC_MAX_DSCR_TRIALS; ++trials) {
+ u32 new_dscr;
+
+ rmb(); /* ensure DSCR is read after CTRLA */
+ new_dscr = channel_readl(atchan, DSCR);
+
+ /*
+ * If the DSCR register value has not changed inside the
+ * DMA controller since the previous read, we assume
+ * that both the dscr and ctrla values refers to the
+ * very same descriptor.
+ */
+ if (likely(new_dscr == dscr))
+ break;
+
+ /*
+ * DSCR has changed inside the DMA controller, so the
+ * previouly read value of CTRLA may refer to an already
+ * processed descriptor hence could be outdated.
+ * We need to update ctrla to match the current
+ * descriptor.
+ */
+ dscr = new_dscr;
+ rmb(); /* ensure DSCR is read before CTRLA */
+ ctrla = channel_readl(atchan, CTRLA);
+ }
+ if (unlikely(trials >= ATC_MAX_DSCR_TRIALS))
+ return -ETIMEDOUT;
/* for the first descriptor we can be more accurate */
if (desc_first->lli.dscr == dscr)
- return atc_calc_bytes_left(ret, ctrla, desc_first);
+ return atc_calc_bytes_left(ret, ctrla);
ret -= desc_first->len;
list_for_each_entry(desc, &desc_first->tx_list, desc_node) {
}
/*
- * For the last descriptor in the chain we can calculate
+ * For the current descriptor in the chain we can calculate
* the remaining bytes using the channel's register.
- * Note that the transfer width of the first and last
- * descriptor may differ.
*/
- if (!desc->lli.dscr)
- ret = atc_calc_bytes_left_from_reg(ret, atchan, desc);
+ ret = atc_calc_bytes_left(ret, ctrla);
} else {
/* single transfer */
- ret = atc_calc_bytes_left_from_reg(ret, atchan, desc_first);
+ ctrla = channel_readl(atchan, CTRLA);
+ ret = atc_calc_bytes_left(ret, ctrla);
}
return ret;
desc->txd.cookie = -EBUSY;
desc->total_len = desc->len = len;
- desc->tx_width = dwidth;
/* set end-of-link to the last link descriptor of list*/
set_desc_eol(desc);
first->txd.cookie = -EBUSY;
first->total_len = len;
- /* set transfer width for the calculation of the residue */
- first->tx_width = src_width;
- prev->tx_width = src_width;
-
/* set end-of-link to the last link descriptor of list*/
set_desc_eol(desc);
first->txd.cookie = -EBUSY;
first->total_len = total_len;
- /* set transfer width for the calculation of the residue */
- first->tx_width = reg_width;
- prev->tx_width = reg_width;
-
/* first link descriptor of list is responsible of flags */
first->txd.flags = flags; /* client is in control of this ack */
desc->txd.cookie = 0;
desc->len = len;
- /*
- * Although we only need the transfer width for the first and
- * the last descriptor, its easier to set it to all descriptors.
- */
- desc->tx_width = src_width;
-
atc_desc_chain(&first, &prev, desc);
/* update the lengths and addresses for the next loop cycle */
/* First descriptor of the chain embedds additional information */
first->txd.cookie = -EBUSY;
first->total_len = buf_len;
- first->tx_width = reg_width;
return &first->txd;
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