On Wed, Dec 27, 2017 at 10:40:00PM +0100, Cyrille Pitchen wrote: > Le 27/12/2017 à 00:23, Rob Herring a écrit : > > On Sun, Dec 24, 2017 at 05:36:05AM +0100, Cyrille Pitchen wrote: > >> +Optional properties: > >> +- dmacap,memcpy: Reserve a DMA channel to perform DMA memcpy() between the > >> + system memory and the QSPI mapped memory. > > How is this a h/w property? Why would I not want to always enable DMA if > > possible? > The number of DMA channels is limited for a given SoC. This number may be > lower than the number of enabled controllers (spi, i2c, qspi, aes, sha, > des, sdmmc, usart, ...). > So we use a DT property to explicitly tell the matching drivers to request > and reserved the DMA channels they need. This policy is not driver or even > SoC specific but board specific. It's very common to reserved DMA channels > for the most used or most performance dependent controllers, setting the > relevant properties in the device-tree then restricting remaining > controllers to their PIO mode. Why can't we just time share the DMA channels at runtime, why do we need this static allocation?
Attachment:
signature.asc
Description: PGP signature
