diff options
| -rw-r--r-- | drivers/dma/at_hdmac.c | 221 |
1 files changed, 110 insertions, 111 deletions
diff --git a/drivers/dma/at_hdmac.c b/drivers/dma/at_hdmac.c index 6c328cd16983..6bd9e35db8f9 100644 --- a/drivers/dma/at_hdmac.c +++ b/drivers/dma/at_hdmac.c @@ -308,6 +308,109 @@ static inline u32 atc_calc_bytes_left(u32 current_len, u32 ctrla) } /** + * atc_get_llis_residue - Get residue for a hardware linked list transfer + * + * Calculate the residue by removing the length of the child descriptors already + * transferred from the total length. To get the current child descriptor we can + * use the value of 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 + * maximum trials is reach. This algorithm is very unlikely not to find a stable + * value for DSCR. + * @atchan: pointer to an atmel hdmac channel. + * @desc: pointer to the descriptor for which the residue is calculated. + * @residue: residue to be set to dma_tx_state. + * Returns 0 on success, -errno otherwise. + */ +static int atc_get_llis_residue(struct at_dma_chan *atchan, + struct at_desc *desc, u32 *residue) +{ + struct at_desc *child; + u32 len, ctrla, dscr; + unsigned int i; + + len = desc->total_len; + dscr = channel_readl(atchan, DSCR); + rmb(); /* ensure DSCR is read before CTRLA */ + ctrla = channel_readl(atchan, CTRLA); + for (i = 0; i < ATC_MAX_DSCR_TRIALS; ++i) { + 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(i == ATC_MAX_DSCR_TRIALS)) + return -ETIMEDOUT; + + /* For the first descriptor we can be more accurate. */ + if (desc->lli.dscr == dscr) { + *residue = atc_calc_bytes_left(len, ctrla); + return 0; + } + + len -= desc->len; + list_for_each_entry(child, &desc->tx_list, desc_node) { + if (child->lli.dscr == dscr) + break; + len -= child->len; + } + + /* + * For the current descriptor in the chain we can calculate the + * remaining bytes using the channel's register. + */ + *residue = atc_calc_bytes_left(len, ctrla); + return 0; +} + +/** * atc_get_residue - get the number of bytes residue for a cookie. * The residue is passed by address and updated on success. * @chan: DMA channel @@ -321,8 +424,7 @@ static int atc_get_residue(struct dma_chan *chan, dma_cookie_t cookie, struct at_dma_chan *atchan = to_at_dma_chan(chan); struct at_desc *desc_first = atc_first_active(atchan); struct at_desc *desc; - u32 len, ctrla, dscr; - unsigned int i; + u32 len, ctrla; /* * If the cookie doesn't match to the currently running transfer then @@ -335,117 +437,14 @@ static int atc_get_residue(struct dma_chan *chan, dma_cookie_t cookie, else if (desc != desc_first) return desc->total_len; - /* cookie matches to the currently running transfer */ - len = desc_first->total_len; - - if (desc_first->lli.dscr) { + if (desc_first->lli.dscr) /* hardware linked list transfer */ + return atc_get_llis_residue(atchan, desc_first, residue); - /* - * Calculate the residue by removing the length of the child - * descriptors already transferred from the total length. - * To get the current child descriptor we can use the value of - * 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. - */ - - dscr = channel_readl(atchan, DSCR); - rmb(); /* ensure DSCR is read before CTRLA */ - ctrla = channel_readl(atchan, CTRLA); - for (i = 0; i < ATC_MAX_DSCR_TRIALS; ++i) { - 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(i == ATC_MAX_DSCR_TRIALS)) - return -ETIMEDOUT; - - /* for the first descriptor we can be more accurate */ - if (desc_first->lli.dscr == dscr) { - *residue = atc_calc_bytes_left(len, ctrla); - return 0; - } - - len -= desc_first->len; - list_for_each_entry(desc, &desc_first->tx_list, desc_node) { - if (desc->lli.dscr == dscr) - break; - - len -= desc->len; - } - - /* - * For the current descriptor in the chain we can calculate - * the remaining bytes using the channel's register. - */ - *residue = atc_calc_bytes_left(len, ctrla); - } else { - /* single transfer */ - ctrla = channel_readl(atchan, CTRLA); - *residue = atc_calc_bytes_left(len, ctrla); - } - + /* single transfer */ + len = desc_first->total_len; + ctrla = channel_readl(atchan, CTRLA); + *residue = atc_calc_bytes_left(len, ctrla); return 0; } |