diff options
| -rw-r--r-- | drivers/dma/amba-pl08x.c | 18 |
1 files changed, 17 insertions, 1 deletions
diff --git a/drivers/dma/amba-pl08x.c b/drivers/dma/amba-pl08x.c index 74fa5a0f1490..4b63fc39a6df 100644 --- a/drivers/dma/amba-pl08x.c +++ b/drivers/dma/amba-pl08x.c @@ -53,7 +53,23 @@ * * ASSUMES default (little) endianness for DMA transfers * - * Only DMAC flow control is implemented + * The PL08x has two flow control settings: + * - DMAC flow control: the transfer size defines the number of transfers + * which occur for the current LLI entry, and the DMAC raises TC at the + * end of every LLI entry. Observed behaviour shows the DMAC listening + * to both the BREQ and SREQ signals (contrary to documented), + * transferring data if either is active. The LBREQ and LSREQ signals + * are ignored. + * + * - Peripheral flow control: the transfer size is ignored (and should be + * zero). The data is transferred from the current LLI entry, until + * after the final transfer signalled by LBREQ or LSREQ. The DMAC + * will then move to the next LLI entry. + * + * Only the former works sanely with scatter lists, so we only implement + * the DMAC flow control method. However, peripherals which use the LBREQ + * and LSREQ signals (eg, MMCI) are unable to use this mode, which through + * these hardware restrictions prevents them from using scatter DMA. * * Global TODO: * - Break out common code from arch/arm/mach-s3c64xx and share |