[U-Boot] [RFC PATCH] usb: dwc2: handle bcm2835 phys->virt address translations

[Marek Vasut]

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