On Fri, 14 Aug 2020 at 16:03, Linus Walleij <linus.walleij@xxxxxxxxxx> wrote: > > On Thu, Jul 23, 2020 at 3:19 AM Stephen Boyd <sboyd@xxxxxxxxxx> wrote: > > > > > textofs-$(CONFIG_ARCH_IPQ40XX) := 0x00208000 > > > > textofs-$(CONFIG_ARCH_MSM8X60) := 0x00208000 > > > > textofs-$(CONFIG_ARCH_MSM8960) := 0x00208000 > > > > > > But what on earth is this? I just deleted this and the platform > > > boots just as well. > > > > We need to shift the kernel text to start 2MB beyond the start of memory > > because there is the shared memory region used to communicate with other > > processors in the SoC there. It took a while for us to convince other OS > > folks in the company to put shared memory somewhere else besides the > > start of RAM, but eventually we won that battle. > > > > Does your booted kernel have its text section at the start of RAM or is > > it offset by 2MB without this change? Check out /proc/iomem to see where > > the kernel text is in relation to the start of RAM. > > The memory on this machine starts at 0x40200000 since the effect > of the current code is to take pc &= 0xf8000000 and that results in > 0x40000000 and then this adds textofs 0x00208000 to that > resulting in 0x40208000 for the kernel physical RAM. Which > is what we want to achieve since the RAM starts at > 0x40200000. > > But TEXT_OFFSET is also used inside the kernel to offset the > virtual memory. This means that when we set up the virtual > memory split, the kernel virtual memory is also bumped by > these 2 MB so the virtual memory starts at 0xC0208000 > instead of 0xC0008000 as is normal. > > It looks weird to me but maybe someone can explain how > logical that is? > The ARM mm code assumes that the relative alignment between PA and VA is 16 MB, so if we skip 2 MB in the physical space, we must do the same in the virtual space.
