On Fri, 2014-07-25 at 07:00 +0300, Siarhei Siamashka wrote: > On Mon, 21 Jul 2014 20:31:30 +0100 > Ian Campbell <[email protected]> wrote: > > > On Fri, 2014-07-18 at 19:22 +0300, Siarhei Siamashka wrote: > > > The sun5i hardware (Allwinner A13) introduced configurable MBUS clock > > > speed. > > > Allwinner A13 uses only 16-bit data bus width to connect the external > > > DRAM, > > > which is halved compared to the 32-bit data bus of sun4i (Allwinner A10), > > > so > > > it does not make much sense to clock a wider internal bus at a very high > > > speed. > > > The Allwinner A13 manual specifies 300 MHz MBUS clock speed limit and 533 > > > MHz > > > DRAM clock speed limit. Newer sun7i hardware (Allwinner A20) has a full > > > width > > > 32-bit external memory interface again, but still keeps the MBUS clock > > > speed > > > configurable. Clocking MBUS too low inhibits memory performance and one > > > has > > > to find the optimal MBUS/DRAM clock speed ratio, which may depend on many > > > factors. > > > > > > This patch introduces a new 'mbus_clock' parameter for the 'dram_para' > > > struct > > > and uses it as a desired MBUS clock speed target. If 'mbus_clock' is not > > > set, > > > 300 MHz is used by default to match the older hardcoded settings. > > > > Nothing in this series seems to set it for any board -- is that > > expected? > > Yes. Not touching the board config files avoids any extra dependencies > and merging conflicts.
Makes sense. > I could explicitly add ".mbus_clock = 300" to > the Cubietruck 'dram_para' struct, but this is the default MBUS value > anyway and makes no real difference. Sure, no need unless you feel it. > > When pll6 is viable you prefer the faster clock, even if it might happen > > to be further from the requested clock, is that right? Or does all the > > arithmetic end up with that never being the case? > > The 'pll5p_rate' and 'pll6x_rate' values are always equal to or less > than the requested 'mbus_clock'. Selecting the larger of these two will > make it closer to the requested clock. Makes sense. Can you write that in a comment please. Ian. _______________________________________________ U-Boot mailing list [email protected] http://lists.denx.de/mailman/listinfo/u-boot
