RAM -Zone Tutorial

RAM -Zone TutorialThis topic describes how to design the use of RAM zones on a phone, -and how to specify the RAM zones in the set up functions in the ASSP/Variant. -

A base port can improve the power performance of a phone through the use -of RAM Zones.

Specifying RAM zones

RAM zones are specified only -once during the boot process by the base port. This takes place within the -overridden pure virtual method Asic::Init1() by Epoc::SetRamZoneConfig(). Epoc::SetRamZoneConfig() takes an array of SRamZone objects and an optional pointer -to the base port implementation of the TRamZoneCallback function. -This is described in detail in RAM -zone call back function.

Each SRamZone object -contains the physical base address, size in bytes, unique ID, preference value -and a set of flags for each RAM zone.

Restrictions on RAM zone -properties

There are some restrictions on how the base port can -specify the RAM zones:

each RAM zone's address -space must be distinct and not overlap with any other RAM zone's address space
the value of SRamZone.iBase must -be aligned to the ASIC's MMU small page size
the value of SRamZone.iSize must -be a multiple of the ASIC's MMU small page size, usually 4KB on an ARM MMU
when added together -all of the RAM zones must cover the whole of the physical RAM address space -as specified by the bootstrap in the SuperPage member SSuperPageBase::iTotalRamSize
the value of each RAM -zone’s SRamZone.iId must be unique for that RAM zone
there can be no more -RAM zones than specified by KMaxRamZones.

Restriction on RAM zone preference ordering

The bootstrap -always allocates the RAM it requires from the lowest RAM address upwards. -This is during the system boot process and before the variant has specified -the RAM zone properties. This means that the RAM zone(s) located at the lowest -RAM addresses, used by the bootstrap, are never empty. Therefore, these RAM -zone(s) should always be assigned to be the most preferable. A low preference -value maps to a high logical preference, therefore the most preferable RAM -zones have a preference value that is lower than the preference value of all -the other RAM zones in the device.

Designing RAM -zones to reduce power consumption

Partitioning RAM into zones

For -power saving purposes, each RAM zone should represent the lowest division -of the device’s RAM IC(s) that can be either powered down or refreshed independently -of other divisions of the RAM IC(s). For example, it should not be possible -for less than an entire RAM zone to be powered or refreshed. This allows the -system to achieve lower power consumption when the level of RAM usage is reduced.

For -example, if a device contains two RAM ICs and each IC is further divided into -4 banks that can be refreshed independently, then the device’s RAM should -be divided into 8 RAM zones with each zone being mapped to one of the 8 individually -refreshable RAM IC banks.

Assigning preferences to zones

To -save power, the RAM zone preferences should be implemented in such a way that -the minimum number of RAM ICs or RAM IC refresh banks are used.

For -example, suppose a device has 2 RAM ICs, the RAM zones in the lowest addressed -RAM IC (IC0) should be more preferable than the RAM zones located in the second -RAM IC (IC1). For all RAM zones in IC0 the preference should be less than -the preference of the RAM zones in IC1. A zone with a lower preference being -the preferred RAM zone. This results in RAM IC1 only being powered on once -RAM IC0 is too full. This is determined by the RAM zone thresholds.

RAM -zone power state on system boot

When the system boots, all the -RAM specified by the bootstrap must be enabled and ready to use. Devices that -wish to reduce power consumption must only adjust the power modes(s) of the -RAM IC(s) after the RAM zone call back function has been invoked with aOp=ERamZoneOp_Init.

Designing RAM -zones for physically contiguous allocation

Some devices have particular -applications or hardware peripherals that require a block of physically contiguous -RAM. In such cases, a RAM zone should be defined that is the size of the physically -contiguous block required, aligned to the nearest ASIC’s MMU small page size. -The device then attempts to use this RAM zone whenever the particular application -or hardware peripheral is activated.

For example, a device may contain -a camera that requires a buffer of 4MB of physically contiguous memory to -operate. A RAM zone with a size of 4MB must be defined and this RAM zone should -have a greater preference value than other RAM zones in the device. This results -in the camera RAM zone:

only being used for -non-camera purposes once all of the other RAM zones in the device become too -full.
having a greater chance -of being emptied of any pages allocated for non-camera purposes when required -by the camera.

KRamZoneDiscardOnly

On devices that use demand -paging, an additional way to increase the chances of a RAM zone being -available is to set the KRamZoneDiscardOnly bit of SRamZone s' iFlags member. -This stops the system allocating pages that are not discardable in that zone. -However, setting KRamZoneDiscardOnly also has the effect -of reducing the amount of RAM available to the device for general purpose -(non-demand paging) use.

KRamZoneFlagDiscardOnly only -increases the chances of a RAM zone’s availability if the minimum buffer size -used by demand paging and/or file caching, is small enough that the RAM zone -may be cleared without reducing the paging or caching buffer beyond their -specified minimum size.

RAM zone call -back function

If TRamZoneCallback has been initialised -by Epoc::SetRamZoneConfig(), then it is invoked each time -the system requires the base port to perform an operation on a RAM zone. When -the call back function is invoked the calling thread is:

in a critical section
holding the Symbian -platform RAM allocator mutex
not holding any fast -mutex
interrupt enabled and -the kernel is unlocked.

This means that the base port’s implementation of the call back function -is:

not able to be suspended -or killed by another thread
not able to acquire -a Symbian platform mutex of an order greater than or equal to the RAM allocator -mutex this is all other Symbian platform mutexes. This is a deadlock prevention -mechanism.
able to acquire a fast -mutex. The call back must release it before returning.
able to be pre-empted -by other threads/processes.

Parameters

A choice of three RAM zone operations -can be requested with TRamZoneCallback, as defined by TRamZoneOp:

ERamZoneOp_PowerUp is -used each time a RAM zone transitions from being empty to having pages allocated -into it. When this operation is invoked, all RAM zones that have their bit -position set in the mask pointed to by aParam2 MUST be -fully enabled and ready for use before returning from the call back function. -Failure to fully enable any of these RAM zones results in the device becoming -unstable and its behaviour unpredictable.

For example, a device could -have two RAM ICs, where IC1 is mapped to RAM zones 1 - 4 and IC2 is mapped -to RAM zones 5 - 8.

If while the second RAM IC is powered down, RAM -zone 5 is required, a ERamZoneOp_PowerUp operation is requested. -The IC2 must be fully enabled before returning from the call back function. -On return from the call back function, the memory in IC2 is used by the system.
ERamZoneOp_PowerDown is -used each time a RAM zone transitions from having pages allocated in it to -being empty. Devices that wish to reduce power consumption can adjust a RAM -IC’s partial array self refresh mode or power down a RAM IC if all of the -other RAM zones in that RAM IC are also empty.

For example, an ERamZoneOp_PowerDown operation -is requested when all the RAM zones mapped to the top half of the RAM IC Partial -Array Self Refresh (PASR) regions are empty. The RAM IC can be set to use -half array PASR when it next enters PASR mode, assuming the RAM IC has the -option of using half array PASR mode.
ERAMZoneOp_Init is -used only once during the initial stages of the system boot process. When -this operation is invoked all the RAM zones that have their bit positions -cleared in the mask pointed to by aParam2 are not currently -in use. The system assumes that during boot all of RAM is enabled and ready -to use. Devices that wish to reduce power consumption can then adjust each -of the RAM IC's partial array self refresh modes or power down modes, appropriately.

For -example, if a device has two RAM IC's, IC1 is mapped to zones1-4 and IC2 is -mapped to RAM zones 5-8. Then if the ERAMZoneOp_Init operation -is requested and all of the RAM zones in RAM IC2 are currently not in use, -then IC2 could be placed into power down mode by the base port at the next -appropriate moment.

The two parameters, aParam1 =TAny* and aParam2 =const -TAny* allows the call back function to perform multiple operations with the -same interface. When TRamZoneCallback is called with TRamZoneOp defined -as either ERamZoneOp_PowerDown or ERamZoneOp_PowerUp, -the last two parameters are defined as:

aParam1 is -the ID of the RAM zone that changed state
aParam2 points -to an array of bit masks that represent the new power state of the RAM zones. -The bit mask is organised in ascending RAM zone address order with the LSB -of the bit mask representing the power state of the lowest addressed zone. -A zone’s bit is set if the RAM zone is in use and cleared if the RAM zone -is not in use. The value of the bit mask MUST not be modified by the -call back function.

See also:

When to reduce -power consumption of the RAM

Depending on the RAM IC configuration -for the device, entering and exiting Partial Array Self Refresh (PASR) or -power down modes may introduce a decrease in a device’s overall performance. -The overhead incurred when transitioning a RAM IC from power off to power -on mode may have significant latency due to the amount of initialisation required. -It is recommended that a delay is used between a ERamZoneOp_PowerDown operation -being invoked and a RAM IC either entering PASR mode or being powered down. -One approach to achieving this is to only enter PASR mode or power down a -RAM IC in the device’s implementation of the DPowerController::CpuIdle() and DPowerController::PowerDown() methods. -This way while the device is active its performance is not affected by the -RAM zone operations.

- -Asic::Init1() -Power Management - -RAM Zones - + + + + + +RAM +Zone TutorialThis topic describes how to design the use of RAM zones on a phone, +and how to specify the RAM zones in the set up functions in the ASSP/Variant. +

A base port can improve the power performance of a phone through the use +of RAM Zones.

Specifying RAM zones

RAM zones are specified only +once during the boot process by the base port. This takes place within the +overridden pure virtual method Asic::Init1() by Epoc::SetRamZoneConfig(). Epoc::SetRamZoneConfig() takes an array of SRamZone objects and an optional pointer +to the base port implementation of the TRamZoneCallback function. +This is described in detail in RAM +zone call back function.

Each SRamZone object +contains the physical base address, size in bytes, unique ID, preference value +and a set of flags for each RAM zone.

Restrictions on RAM zone +properties

There are some restrictions on how the base port can +specify the RAM zones:

each RAM zone's address +space must be distinct and not overlap with any other RAM zone's address space
the value of SRamZone.iBase must +be aligned to the ASIC's MMU small page size
the value of SRamZone.iSize must +be a multiple of the ASIC's MMU small page size, usually 4KB on an ARM MMU
when added together +all of the RAM zones must cover the whole of the physical RAM address space +as specified by the bootstrap in the SuperPage member SSuperPageBase::iTotalRamSize
the value of each RAM +zone’s SRamZone.iId must be unique for that RAM zone
there can be no more +RAM zones than specified by KMaxRamZones.

Restriction on RAM zone preference ordering

The bootstrap +always allocates the RAM it requires from the lowest RAM address upwards. +This is during the system boot process and before the variant has specified +the RAM zone properties. This means that the RAM zone(s) located at the lowest +RAM addresses, used by the bootstrap, are never empty. Therefore, these RAM +zone(s) should always be assigned to be the most preferable. A low preference +value maps to a high logical preference, therefore the most preferable RAM +zones have a preference value that is lower than the preference value of all +the other RAM zones in the device.

Designing RAM +zones to reduce power consumption

Partitioning RAM into zones

For +power saving purposes, each RAM zone should represent the lowest division +of the device’s RAM IC(s) that can be either powered down or refreshed independently +of other divisions of the RAM IC(s). For example, it should not be possible +for less than an entire RAM zone to be powered or refreshed. This allows the +system to achieve lower power consumption when the level of RAM usage is reduced.

For +example, if a device contains two RAM ICs and each IC is further divided into +4 banks that can be refreshed independently, then the device’s RAM should +be divided into 8 RAM zones with each zone being mapped to one of the 8 individually +refreshable RAM IC banks.

Assigning preferences to zones

To +save power, the RAM zone preferences should be implemented in such a way that +the minimum number of RAM ICs or RAM IC refresh banks are used.

For +example, suppose a device has 2 RAM ICs, the RAM zones in the lowest addressed +RAM IC (IC0) should be more preferable than the RAM zones located in the second +RAM IC (IC1). For all RAM zones in IC0 the preference should be less than +the preference of the RAM zones in IC1. A zone with a lower preference being +the preferred RAM zone. This results in RAM IC1 only being powered on once +RAM IC0 is too full. This is determined by the RAM zone thresholds.

RAM +zone power state on system boot

When the system boots, all the +RAM specified by the bootstrap must be enabled and ready to use. Devices that +wish to reduce power consumption must only adjust the power modes(s) of the +RAM IC(s) after the RAM zone call back function has been invoked with aOp=ERamZoneOp_Init.

Designing RAM +zones for physically contiguous allocation

Some devices have particular +applications or hardware peripherals that require a block of physically contiguous +RAM. In such cases, a RAM zone should be defined that is the size of the physically +contiguous block required, aligned to the nearest ASIC’s MMU small page size. +The device then attempts to use this RAM zone whenever the particular application +or hardware peripheral is activated.

For example, a device may contain +a camera that requires a buffer of 4MB of physically contiguous memory to +operate. A RAM zone with a size of 4MB must be defined and this RAM zone should +have a greater preference value than other RAM zones in the device. This results +in the camera RAM zone:

only being used for +non-camera purposes once all of the other RAM zones in the device become too +full.
having a greater chance +of being emptied of any pages allocated for non-camera purposes when required +by the camera.

KRamZoneDiscardOnly

On devices that use demand +paging, an additional way to increase the chances of a RAM zone being +available is to set the KRamZoneDiscardOnly bit of SRamZone s' iFlags member. +This stops the system allocating pages that are not discardable in that zone. +However, setting KRamZoneDiscardOnly also has the effect +of reducing the amount of RAM available to the device for general purpose +(non-demand paging) use.

KRamZoneFlagDiscardOnly only +increases the chances of a RAM zone’s availability if the minimum buffer size +used by demand paging and/or file caching, is small enough that the RAM zone +may be cleared without reducing the paging or caching buffer beyond their +specified minimum size.

RAM zone call +back function

If TRamZoneCallback has been initialised +by Epoc::SetRamZoneConfig(), then it is invoked each time +the system requires the base port to perform an operation on a RAM zone. When +the call back function is invoked the calling thread is:

in a critical section
holding the Symbian +platform RAM allocator mutex
not holding any fast +mutex
interrupt enabled and +the kernel is unlocked.

This means that the base port’s implementation of the call back function +is:

not able to be suspended +or killed by another thread
not able to acquire +a Symbian platform mutex of an order greater than or equal to the RAM allocator +mutex this is all other Symbian platform mutexes. This is a deadlock prevention +mechanism.
able to acquire a fast +mutex. The call back must release it before returning.
able to be pre-empted +by other threads/processes.

Parameters

A choice of three RAM zone operations +can be requested with TRamZoneCallback, as defined by TRamZoneOp:

ERamZoneOp_PowerUp is +used each time a RAM zone transitions from being empty to having pages allocated +into it. When this operation is invoked, all RAM zones that have their bit +position set in the mask pointed to by aParam2 MUST be +fully enabled and ready for use before returning from the call back function. +Failure to fully enable any of these RAM zones results in the device becoming +unstable and its behaviour unpredictable.

For example, a device could +have two RAM ICs, where IC1 is mapped to RAM zones 1 - 4 and IC2 is mapped +to RAM zones 5 - 8.

If while the second RAM IC is powered down, RAM +zone 5 is required, a ERamZoneOp_PowerUp operation is requested. +The IC2 must be fully enabled before returning from the call back function. +On return from the call back function, the memory in IC2 is used by the system.
ERamZoneOp_PowerDown is +used each time a RAM zone transitions from having pages allocated in it to +being empty. Devices that wish to reduce power consumption can adjust a RAM +IC’s partial array self refresh mode or power down a RAM IC if all of the +other RAM zones in that RAM IC are also empty.

For example, an ERamZoneOp_PowerDown operation +is requested when all the RAM zones mapped to the top half of the RAM IC Partial +Array Self Refresh (PASR) regions are empty. The RAM IC can be set to use +half array PASR when it next enters PASR mode, assuming the RAM IC has the +option of using half array PASR mode.
ERAMZoneOp_Init is +used only once during the initial stages of the system boot process. When +this operation is invoked all the RAM zones that have their bit positions +cleared in the mask pointed to by aParam2 are not currently +in use. The system assumes that during boot all of RAM is enabled and ready +to use. Devices that wish to reduce power consumption can then adjust each +of the RAM IC's partial array self refresh modes or power down modes, appropriately.

For +example, if a device has two RAM IC's, IC1 is mapped to zones1-4 and IC2 is +mapped to RAM zones 5-8. Then if the ERAMZoneOp_Init operation +is requested and all of the RAM zones in RAM IC2 are currently not in use, +then IC2 could be placed into power down mode by the base port at the next +appropriate moment.

The two parameters, aParam1 =TAny* and aParam2 =const +TAny* allows the call back function to perform multiple operations with the +same interface. When TRamZoneCallback is called with TRamZoneOp defined +as either ERamZoneOp_PowerDown or ERamZoneOp_PowerUp, +the last two parameters are defined as:

aParam1 is +the ID of the RAM zone that changed state
aParam2 points +to an array of bit masks that represent the new power state of the RAM zones. +The bit mask is organised in ascending RAM zone address order with the LSB +of the bit mask representing the power state of the lowest addressed zone. +A zone’s bit is set if the RAM zone is in use and cleared if the RAM zone +is not in use. The value of the bit mask MUST not be modified by the +call back function.