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.
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