diff options
| -rw-r--r-- | drivers/dma/at_hdmac.c | 132 | ||||
| -rw-r--r-- | drivers/dma/at_hdmac_regs.h | 3 |
2 files changed, 88 insertions, 47 deletions
diff --git a/drivers/dma/at_hdmac.c b/drivers/dma/at_hdmac.c index 59892126d175..d3629b7482dd 100644 --- a/drivers/dma/at_hdmac.c +++ b/drivers/dma/at_hdmac.c @@ -48,6 +48,8 @@ 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. @@ -285,28 +287,19 @@ static struct at_desc *atc_get_desc_by_cookie(struct at_dma_chan *atchan, * * @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); } /** @@ -320,7 +313,7 @@ static int atc_get_bytes_left(struct dma_chan *chan, dma_cookie_t cookie) 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 @@ -346,15 +339,82 @@ static int atc_get_bytes_left(struct dma_chan *chan, dma_cookie_t cookie) * 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) { @@ -365,16 +425,14 @@ static int atc_get_bytes_left(struct dma_chan *chan, dma_cookie_t cookie) } /* - * 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; @@ -726,7 +784,6 @@ atc_prep_dma_interleaved(struct dma_chan *chan, 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); @@ -804,10 +861,6 @@ atc_prep_dma_memcpy(struct dma_chan *chan, dma_addr_t dest, dma_addr_t src, 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); @@ -956,10 +1009,6 @@ atc_prep_slave_sg(struct dma_chan *chan, struct scatterlist *sgl, 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 */ @@ -1077,12 +1126,6 @@ atc_prep_dma_sg(struct dma_chan *chan, 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 */ @@ -1256,7 +1299,6 @@ atc_prep_dma_cyclic(struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len, /* 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; diff --git a/drivers/dma/at_hdmac_regs.h b/drivers/dma/at_hdmac_regs.h index bc8d5ebedd19..7f5a08230f76 100644 --- a/drivers/dma/at_hdmac_regs.h +++ b/drivers/dma/at_hdmac_regs.h @@ -112,6 +112,7 @@ #define ATC_SRC_WIDTH_BYTE (0x0 << 24) #define ATC_SRC_WIDTH_HALFWORD (0x1 << 24) #define ATC_SRC_WIDTH_WORD (0x2 << 24) +#define ATC_REG_TO_SRC_WIDTH(r) (((r) >> 24) & 0x3) #define ATC_DST_WIDTH_MASK (0x3 << 28) /* Destination Single Transfer Size */ #define ATC_DST_WIDTH(x) ((x) << 28) #define ATC_DST_WIDTH_BYTE (0x0 << 28) @@ -182,7 +183,6 @@ struct at_lli { * @txd: support for the async_tx api * @desc_node: node on the channed descriptors list * @len: descriptor byte count - * @tx_width: transfer width * @total_len: total transaction byte count */ struct at_desc { @@ -194,7 +194,6 @@ struct at_desc { struct dma_async_tx_descriptor txd; struct list_head desc_node; size_t len; - u32 tx_width; size_t total_len; /* Interleaved data */ |