On Friday, March 13, 2015 at 07:39:08 PM, Stephen Warren wrote: > On 03/13/2015 12:13 PM, Marek Vasut wrote: > > On Friday, March 13, 2015 at 05:35:53 PM, Stephen Warren wrote: > >> On 03/13/2015 08:30 AM, Marek Vasut wrote: > >>> On Friday, March 13, 2015 at 07:13:09 AM, Stephen Warren wrote: > >>>> BCM2835 bus addresses use the top 2 bits to determine whether > >>>> peripherals use or bypass the GPU L1 and L2 cache. > >>>> BCM2835-ARM-Peripherals.pdf states that: > >>>> > >>>> 0: L1 & L2 cached > >>>> 4: L2 cache coherent (non allocaing) > >>>> 8: L2 cached only > >>>> c: Direct uncached. > >>> > >>> Caches aren't working on BCM2xxx or what's the reason for this hack ? > >>> Or are these different (not on-CPU) caches we're talking about (yes, > >>> I did notice the GPU Lx cache stuff)? > >> > >> Yes, the "GPU" has its own caches, entirely separate from the ARM core > >> and at a different location in the system bus structure, and it seems as > >> if at least some other peripherals other than GPU/graphics/VideoCore > >> access DRAM via those caches too. > >> > >> There are some brief details in BCM2835-ARM-Peripherals.pdf, although it > >> isn't terribly clear. > > > > Thanks for clearing this up. I suspect there's no way to turn those > > caches off altogether, right ? But uh ... ew :( > > There may be, Search for disable_l2cache at http://elinux.org/RPiconfig. > That option is read by the SoC's binary bootloader (which I believe > 99%-100% runs on the VideoCore not ARM) and programmed before the ARM > bootloader (U-Boot) is started. > > The disadvantages of the option are: > > * According to all descriptions of the option I've seen, it requires > that SW that wishes to run with that option enabled must pass a > different upper 2 bits of physical address to DMA engines. See for > example the elinux.org link above and: > > https://github.com/raspberrypi/linux/blob/rpi-3.18.y/arch/arm/mach-bcm2708/ > include/mach/memory.h#L38 > > https://github.com/raspberrypi/linux/blob/rpi-3.18.y/arch/arm/mach-bcm2708/ > Kconfig#L43 > > * It's a system-wide option without any runtime control that I'm aware > of, and so would affect anything U-Boot boots such as Linux, so Linux > would need to be modified too. I assume it would reduce graphics > performance at least. > > As such, I don't think we want to require that option.
Agreed. Best regards, Marek Vasut _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot
