[libre-riscv-dev] libresoc memory architecture

[Jacob Lifshay]

On Tue, Jun 23, 2020, 11:27 Luke Kenneth Casson Leighton <lkcl at lkcl.net>
wrote:

> On Tue, Jun 23, 2020 at 7:13 PM Michael Nolan <mtnolan2640 at gmail.com>
> wrote:
> >
> > On 6/23/20 2:04 PM, Luke Kenneth Casson Leighton wrote:
> > > there is slightly more to it than that however that is quite enough
> for now.
> > >
> > > any questions?
> > Great, that and the discussion on bug 393 helps quite a bit
>
> this one covers the "contract of sale" API.
> https://bugs.libre-soc.org/show_bug.cgi?id=397
>
> there *is* a potential way to do this without modifying Wishbone -
> however it means we have to use our own Bus protocol (PortInterface)
> right the way through to talking to Minerva LoadStoreInterface.
>
> however when it comes to using nmigen-soc Wishbone InterconnectShared:
>
> https://git.libre-soc.org/?p=nmigen-soc.git;a=blob;f=nmigen_soc/wishbone/bus.py;h=HEAD#l493
>
> we will need an "address-aware" system that tells us - in hardware
> terms not in python source code terms - in *hardware* - whether the
> address being requested will succeed/fail or not.
>
> this means that we need a *hardware* function which respects / mirrors
> the nmigen_soc WB Decoder "memory_map":
>
> https://git.libre-soc.org/?p=nmigen-soc.git;a=blob;f=nmigen_soc/wishbone/bus.py;hb=HEAD#l236
>
> that hardware function will tell us - *at the offer phase* of the
> Contract-of-Sale - whether the request WOULD succeed IF allowed to
> proceed to exchange-and-complete.
>
> this is not as straightforward as a "simple range of addresses"
> because in the case of DRAM (connected over LiteDRAM) the range of
> physical addresses that will succeed will *depend on available
> external RAM chip size*.
>

Note that the PowerISA page tables contain bits that specify if a memory
page is normal ram, read/write-combining I/O, or non-combining I/O, so we
don't need to build special hardware decoders to determine the properties
of a physical address.

See PowerISA v3.1 section 1.6 and section 6.7.10.2

The spec also specifies how memory should be accessed when using unusual
methods, such as when the page tables are disabled.

Jacob