Memory System

Kermit memory system internals

This is all based on assumptions and may be completely wrong.

Terminology

• Agent

A device connected to the memory system.

• Access

An interface between an Agent and the memory

• TA

Short for Target Access?

• IA

Short for Initiator Access?

• Transaction

A data exchange performed via the memory bus system. Transactions transfer data between an IA and a TA. A transaction from X (an IA) to Y (a TA) is initiated by X and targets Y.

• Bus

Short for time-shared bus. A link between agents on which a single transaction may be in progress at a time.

• XBar

Short for Crossbar. A link between agents on which multiple transactions may be in progress at a time (as long as they do not target the same TA).

• Arbiter

This could be a single physical device of the memory system that allocates access to the bus/XBar to IAs (most plausible hypothesis). However, the PS Vita may instead be using a decentralized bus arbitration system in which there is no single arbiter.

• Bus Master

When a transaction is occurring on a bus, the agent that started the transaction is referred to as the Bus Master.

There are three revisions of the memory system layout: One for Kermit1.0 ES1, one for Kermit1.0 ES2 (not documented but close to ES3) and one for Kermit1.0 ES3, ES4, and Kermit1.5 ES1.

In the following sections, you may see register names such as BEBT and SBEBT mentioned. Unless otherwise specified, a register prefixed with S is merely the ARM Secure variant of the register (e.g. SBEBT is BEBT for ARM Secure). Everything that applies to a register should apply to its Secure variant, except that the register is updated by Secure bus transactions, instead of Non-Secure bus transactions.

Control registers

There are several MMIO registers available to alter the behavior of the memory system, or query information about the memory system's state.

IA/TA registers

A common register group associated to every IA or TA of the memory system. The register group is 0x1000 bytes sized (but maybe not all room is used).

(List of physical address for all these interfaces can be found on Physical Memory page - names start with SSX_ - will be migrated here at some point)

Known registers

Name	Offset in group	Size	Usage/Notes
STATUS	0x28	0x8	Bit 0x0100_0000 indicates an error
ERROR_LOG	0x58	0x8
ADDR	0x60	0x8

Bus Errors

When a bus error occurs, an interrupt is delivered to ARM Secure (Interrupt ID 0x21=33) or Non-Secure (Interrupt ID 0x20=32). The OS considers all bus errors to be fatal. The interrupt handlers installed for these IDs perform a register dump then stop the system.

It appears that device-initiated bus errors (e.g. DMAC) are always routed to Non-Secure interrupt. There may be other rules to consider (e.g. some register to set whether a device is Secure/Non-Secure).

It is unknown whether or not CMeP can receive bus error interrupts or how CMeP fits in the memory system.

Bus Error Attribute

The bus error attribute is a 32-bit value that can be recovered in a per-device MMIO register (along with the bus error address).

The attribute holds multiple informations: who was the bus master when the error occurred, what bus command was ongoing, and (sometimes) the reason of the bus error.

Bus error attribute meaning

Bitmask	Name	Notes
0x003F_0000	master	Bus master
0x0000_0700	cmd	Ongoing command on the bus
0x0000_0008	Burst Access Error
0x0000_0004	Register Permission Error
0x0000_0001	Address Hole Error	If set, the error occurred because the address was invalid.

Note that for some devices (Spad32K, Spad128K, Compati SRAM), the only valid attribute is 0x1, indicating an invalid address.

To decode the meaning of master and cmd, shift them by 16 and 8 respectively to obtain a value between 0-63/0-7 and use the following tables:

cmd decoding table

Value	Command
0	Idle
1	Write
2	Read
3	ReadEx
4	ReadLinked
5	WriteNonPost
6	WriteConditional
7	Broadcast

master decoding table

Value	Command
0	Reserved
1	ARM Core0
2	ARM Core1
3	ARM Core2
4	ARM Core3
5	ARM L2
6	Reserved
7	Reserved
8	Reserved
9	Reserved
10	Reserved
11	Reserved
12	Reserved
13	Reserved
14	Reserved
15	Reserved
16	DMAC0
17	DMAC1
18	DMAC2
19	DMAC3
20	DMAC4
21	DMAC5
22	Reserved
23	GPU
24	Venezia
25	VIP vdpd
26	VIP vdpm
27	VIP bap
28	Reserved
29	IFTU0a
30	IFTU0b
31	IFTU1a
32	IFTU1b
33	IFTU2
34	Reserved
35	Reserved
36	USB1 Host EHCI
37	USB1 Host OCHI
38	Reserved
39	Sensor in 0
40	Sensor in 1
41	Reserved
42	LCD DMAC
43	Performance Monitor
44	USB2 Device DMAC
45	Sub LCD
46	DMAC6
47	USB0 Host EHCI
48	USB0 Host OHCI
49	USB2 Host EHCI
50	USB2 Host OHCI
51	USB0 Device DMAC
52	Reserved
53	Reserved
54	Reserved
55	USB1 Device DMAC
56	Reserved
57	Reserved
58	Reserved
59	SD/HSMMC0
60	SD/HSMMC1
61	SD/HSMMC2
62	SD/HSMMC3
63	Memory Stick

Clear TA error

In old firmwares (e.g. 0.920 - pre-ES3, so this may no longer be valid), there exists inside the bus error module a function named BusErrorClearTA which works the following way.

First, choose a bus/XBar that will serve as start point (MainXBar for ES2). Second, recursively build a list of all TAs connected to this start point. Repeat the process for all TAs that are busses or XBars. Third, walk the obtained tree. Check the STATUS of all TAs. If an error is present, repeat the procedure recursively (if bus/XBar) then clear the error.

To detect an error, check if bit 0x01000_0000ull is present in STATUS To clear the error, simply write 0x01000_0000ull to STATUS.

NOTE: old firmwares do not have informations about IAs, only TAs. This could explain the routine's name, but also means this procedure may also work/be needed on IA side.

Misc

The memory system is able to distinguish if a transaction is originating from ARM Secure state or Non-Secure state because ARM Cortex-A9 processors with Security Extensions have an additional bit indicating whether the access is Secure or Non-Secure added to all memory system transactions.

?All devices on the Kermit bus diagram that have an IA are treated as ARM NS.? (attempting to perform a DMAC memcpy() from Secure to Non-Secure LPDDR0 region results in a NS bus error)

On ES1 hardware, the bus registers are prefixed with SMX instead of SSX. The meaning of both these acronyms is unknown.