21. Chrom-GRC (GFXMMU)

21.1 Introduction

The graphic MMU is a graphical oriented memory management unit aimed to:

• • Optimize memory usage according to the display shape
• • Cache linear accesses to the frame buffer
• • Prefetch data

21.2 GFXMMU main features

• • Fully programmable display shape to physically store only the visible pixel
• • Up to 4 virtual buffers
• • Each virtual buffer have 3072 or 4096 bytes per line and 1024 lines
• • Each virtual buffer can be physically mapped to any system memory
• • Optional cache for linear accesses
• • Cache can be locked to a virtual buffer
• • Cache prefetch mechanism for linear accesses anticipation
• • Interrupt in case of buffer overflow (1 per buffer)
• • Interrupt in case of memory transfer error

21.3 GFXMMU functional and architectural description

The GFXMMU is composed of two main blocks:

• • The MMU, responsible of address resolution
• • The cache, responsible of data/address caching, prefetching and packing/unpacking

Figure 101. GFXMMU block diagram

21.3.1 Virtual memory

The GFXMMU provides a virtual memory space seen by the system masters ^(a) . This virtual memory space is divided into four virtual buffers.

Virtual buffer

A virtual buffer is seen by any system master as a continuous memory space representing a virtual frame buffer of 1024 lines.

Each line is divided into 192 or 256 16-byte blocks depending on the 192BM bit of the GFXMMU configuration register (GFXMMU_CR).

Depending on the display shape and size, only the necessary blocks are mapped to a physical memory location. This mapping is done programming the LUT entry for each line:

• • The enable of the line
• • The number of the first “visible” block
• • The number of the last “visible” block
• • The address offset of the line within the physical buffer

The “visible” blocks can be arranged in the physical buffer in a continuous way programming the address offset of each line.

a. Refer to section “System architecture” for system masters accessing GFXMMU.

The LUT is common to all the buffers i.e. all the buffers have the same “shape”.

Virtual buffer overview

For a frame buffer coded in 32 bpp or 16 bpp, the virtual buffer can be configured to have 192 or 256 blocks. This results in a virtual frame buffer of 768 x 1024 or 1024 x 1024 pixels for 32 bpp and 1536 x 1024 or 2048 x 1024 for 16 bpp.

For a frame buffer coded in 24 bpp, the virtual buffer must be configured to have 192 blocks to have an integer number of pixel per lines. This results in a virtual frame buffer of 1024 x 1024 pixels for 24 bpp.

Each buffer can be physically mapped anywhere in the physical memory thanks to:

• • The physical buffer base address (PBBA) field of the GFXMMU buffer x configuration register (GFXMMU_BxCR). It configures the physical location of the 8-Mbyte area where the buffer is mapped.
• • The physical buffer location respective to the physical buffer base address is defined by the physical buffer offset (PBO) field of the GFXMMU buffer x configuration register (GFXMMU_BxCR).

Figure 103. Virtual buffer and physical buffer memory map

The buffer can not overflow the 8-Mbyte boundary of the zone defined by its base address. In case of overflow, the buffer x overflow flag (BxOF) of the GFXMMU status register (GFXMMU_SR) is set and an interrupt is generated if the buffer x overflow interrupt enable (BxOIE) bit of the GFXMMU configuration register (GFXMMU_CR) is set.

Virtual buffer application use case

As the physical locations are independently configurable, the four virtual buffers can be physically mapped to non continuous locations. This would allow for example to have the four buffers mapped on to four different SDRAM banks and avoid extra precharge cycles accessing the SDRAM.

As a consequence, one buffer must be used by the CPU/Chrom ART for frame buffer calculation while an other one must be used by the LTDC.

The two remaining buffers can be used as a graphical library requiring extra drawing buffers.

21.3.2 MMU architecture

The MMU block is responsible of the address resolution. It receives the 24-bit address and returns the physical 23-bit address and a valid signals to indicate the address is physically mapped or not. The MMU also checks overflow of a area boundary.

The MMU LUT is implemented as a 1024 x 35-bit RAM

Figure 104. MMU block diagram

Line block decoder

The line block decoder is generating the block number and the line number according the address.

Look up RAM

The look up RAM is a 1024 x 35-bit RAM with the following fields:

• • 1-bit line enable
• • 8-bit first valid block
• • 8-bit last valid block
• • 18-bit for line offset

As the RAM is bigger than a word, each entry is split into two words on the memory map. The write access are done in two steps:

1. 1. Write the first word with enable/first valid block/last valid block in the GFXMMU_LUTxL memory location (internally buffered).
2. 2. Write the second word with line offset in the GFXMMU_LUTxH memory location (effective write into the memory together with the internally buffered value).

A write in the LUT can happen any time but it can lead to inconsistencies if a master is using the MMU at the same time. As the CPU has the priority during LUT programming, this may slow down MMU calculation.

There is no restriction during read operations, but this may slow down CPU as the MMU has the priority on LUT accesses.

Block validation/comparator

This block is checking if the block is valid.

A block is considered as valid (physically mapped) when:

• • Line is enable.
• • The block number is greater or equal to the first valid block.
• • The block number is lower or equal to the last valid block.

When the block is valid, the physical address generated is considered as correct.

If the result of the MMU evaluation is not valid, the write operations are ignored, and read operations return the default 32-bit value stored in the default value (DV) field of the GFXMMU default value register (GFXMMU_DVR).

Block offset address calculation within the buffer

The block number is added to the line offset to get the offset of the block within the physical buffer.

As a consequence, the line offset stored in the LUT is given by the following formula:

with:

• • The maximum value for the line offset is when all the block of all the line are used. As the consequence the line offset for the last line can be maximum:
  \( 1023 \times 256 \times 16 = 0x3F:F00x \)
• • The minimum value for the line offset is when the last block of the first line is the first valid block: \( -255 \times 16 = -0xFFx \) i.e \( 0x3F:F01x \)

As the consequence the full range of the line offset entry of the LUT is used.

Carry is not taken into account as this stage to be able to perform negative offset calculations (values from \( 0x3F:F01x \) to \( 0x3F:FFFx \) )

As the block offset is within a 4-Mbyte buffer, the address generated is 22-bit wide.

Block offset address calculation

Once the offset of the block within the buffer has been calculated, this value is added to the offset of the block respective to the physical buffer base address.

The offset of the blocks are defined in registers as shown in Figure 105 :

Figure 105. Block validation/comparator implementation

The resulting address and the buffer offset address must be on 23-bit.

The carry is taken into account to trigger address overflow. The carry is propagated to the GFXMMU status register (GFXMMU_SR) to set the buffer x overflow flag (BxOF).

Example of calculation

Consider the following configuration for virtual buffer 0:

• • First visible block of line 0: block 7
• • Number of visible block in line 0: 10
• • First visible block of line 1: block 6
• • Number of visible block in line 1: 12
• • Address of the physical buffer: 0xC020:0000

The configuration must be:

• • The base address of the physical buffer 0: 0xC000:0000
• • The offset of buffer 0: 0x20:0000
• • First visible block of line 0: block 7
• • Last visible block of line 0: block 16
• • Block 0 offset of line 0: \( (0 - 7) \times 0x10 = -0x70 = 0x3F:FF90 \)
• • First visible block of line 1: block 6
• • Last visible block of line 1: block 17
• • Block 0 offset of line 1: \( (10 - 6) \times 0x10 = (0xA - 0x6) \times 0x10 = 0x40 \)

As a consequence:

• • the physical address of block 7 of line 0 is:

• • the physical address of block 16 of line 0 is:

• • the physical address of block 6 of line 1 is:

• • the physical address of block 17 of line 1 is:

21.3.3 Cache and prefetch mechanism

The GFXMMU integrated cache targets internal or external RAM devices storing the graphical frame buffer.

Master accessing the GFXMMU

Several masters are supposed to access memories through the GFXMMU:

• • When the CPU is accessing the framebuffer, it is because it is performing a read/modify/write of a single pixel in the frame buffer. As a consequence the granularity of the R/M/W is less or equal to a word and a data cache is necessary. As the frame buffer is scanned linearly, it is realistic to say that the next data that is requested by the CPU within a buffer, is the next pixel. A prefetch mechanism can anticipate this efficiently.
• • When Chrom ART or the LTDC are accessing the framebuffer, they are generating long accesses up to 128 bytes. A cache may not be necessary in this case as successive operation is done only once.

As a consequence the cache is aimed for CPU and must not be used together with DMA2D or LTDC. It is recommended to use the cache only with the CPU and manage coherency by software, flushing the cache when the CPU operations are finished.

Cache enabling

The cache is enabled setting the CE (cache enable) bit of the GFXMMU control register (GFXMMU_CR).

Cache lock mechanism

It is possible to lock the cache by setting the cache lock bit (CL) and configuring the cache lock buffer (CLB) field of the GFXMMU configuration register (GFXMMU_CR).

When the cache is locked on a given buffer, all the other buffer can not be cached.

An operation on the locked buffered can be cachable according to attribute of the master request, or can be always cachable setting the force caching (FC) bit of the GFXMMU configuration register (GFXMMU_CR). The force caching mechanism is only available when the cache is locked to a buffer. The force caching (FC) bit of the GFXMMU configuration register (GFXMMU_CR) is automatically reset when the cache lock (CL) bit of the GFXMMU configuration register (GFXMMU_CR) is reset.

Cache line size

A cache line size is 32 bytes.

Number of cache lines

The number of line is reduced as much as possible taking into account that CPU is performing linear accesses.

As a consequence three lines are needed:

• • one line for the current access
• • one line for the previous access (as pixels can be split into two 16-byte blocks)
• • one line for the prefetched access

Prefetch mechanism

Because the CPU performs most of the time, linear accesses to the frame buffer, a prefetch mechanism is provided in order to automatically retrieve the next cache line from the memory.

The prefetch mechanism can be disabled setting the prefetch disable (PD) bit of the GFXMMU configuration register (GFXMMU_CR).

When the prefetch mechanism is disabled, only the TAG of the line dedicated for prefetching is updated but the data are retrieved from the memory.

Cache maintenance operation

When the cache is working in non force caching mode, this is the case when the force caching (FC) bit of the GFXMMU configuration register (GFXMMU_CR) is reset, then line eviction is done:

• • Each time a non cachable access is performed on the buffer having data cached
• • Each time a miss occurs

As a consequence the cache maintenance operation can be naturally automatic.

Nevertheless, it is possible to force a flush if the cache setting the force flush (FF) bit of the GFXMMU cache control register (GFXMMU_CCR). When flushing the cache, all the dirty entries are sent to the write buffer, and all the dirty bit of the TAGs are reset. But the entries are not invalidate.

It is also possible to invalidate the cache entries setting the force invalidate (FI) bit of the GFXMMU cache control register (GFXMMU_CCR). This does not send the dirty entries to the write buffer (modification done in the cache are lost). This just resets the ID field of the TAGs (unused state)

Setting the two force flush (FF) bits and forcing invalidate (FI) of the GFXMMU cache control register (GFXMMU_CCR) trigs the following operations:

• • flush the cache, all the dirty entries are sent to the write buffer
• • reset the dirty bit of the TAGs
• • reset the ID field of the TAGs

When any of these two operations is done while the flash is having transaction already in the write buffer, the write buffer continues its operations.

The force flush (FF) and force invalidate (FI) are reset automatically when all the operations are finished (write buffer empty) creating a synchronization barrier.

21.4 GFXMMU interrupts

An interrupt can be produced on the following events:

• • Buffer 0 overflow
• • Buffer 1 overflow
• • Buffer 2 overflow
• • Buffer 3 overflow
• • AHB master error

Separate interrupt enable bits are available for flexibility.

Table 131. GFXMMU interrupt requests

21.5 GFXMMU registers

21.5.1 GFXMMU configuration register (GFXMMU_CR)

Address offset: 0x0000

Reset value: 0x0000 0000

Bits 31:23 Reserved, must be kept at reset value.

Bits 22:20 Reserved, must be kept at reset value.

Bits 19:18 Reserved, must be kept at reset value.

Bit 17 OB : Outter bufferability

This bit configure the bufferability of an access generated by the GFXMMU cache.

0: No bufferable

1: Bufferable

Bit 16 OC : Outter cachability

This bit configure the cachability of an access generated by the GFXMMU cache.

0: No cachable

1: Cachable

Bits 15:13 Reserved, must be kept at reset value.

Bit 12 PD : Prefetch disable

This bit disables the prefetch of the cache.

0: Prefetch enable

1: Prefetch disable

Bit 11 FC : Force caching

This bit force the caching into the cache regardless of the MPU attributes.

The cache must be enable (CE bit set).

0: Caching not forced

1: Caching forced

Bits 10:9 CLB[1:0] : Cache lock buffer

This field select the buffer on which the cache is locked.

00: Cache locked on buffer 0

01: Cache locked on buffer 1

10: Cache locked on buffer 2

11: Cache locked on buffer 3

Bit 8 CL : Cache lock

This bit lock the cache onto the buffer defined in the CLB field.

0: Cache not locked

1: Cache locked to a buffer

Bit 7 CE : Cache enable

This bit enables the cache unit.

0: Cache disable

1: Cache enable

Bit 6 192BM : 192 Block mode

This bit defines the number of blocks per line

0: 256 blocks per line

1: 192 blocks per line

Bit 5 Reserved, must be kept at reset value.

Bit 4 AMEIE : AHB master error interrupt enable

This bit enables the AHB master error interrupt.

0: Interrupt disable

1: Interrupt enabled

Bit 3 B3OIE : Buffer 3 overflow interrupt enable

This bit enables the buffer 3 overflow interrupt.

0: Interrupt disable

1: Interrupt enabled

Bit 2 B2OIE : Buffer 2 overflow interrupt enable

This bit enables the buffer 2 overflow interrupt.

0: Interrupt disable

1: Interrupt enabled

Bit 1 B1OIE : Buffer 1 overflow interrupt enable

This bit enables the buffer 1 overflow interrupt.

0: Interrupt disable

1: Interrupt enabled

Bit 0 B0OIE : Buffer 0 overflow interrupt enable

This bit enables the buffer 0 overflow interrupt.

0: Interrupt disable

1: Interrupt enabled

21.5.2 GFXMMU status register (GFXMMU_SR)

Address offset: 0x0004

Reset value: 0x0000 0000

Bits 31:5 Reserved, must be kept at reset value.

Bit 4 AMEF : AHB master error flag

This bit is set when an AHB error happens during a transaction. It is cleared by writing 1 to CAMEF.

Bit 3 B3OF : Buffer 3 overflow flag

This bit is set when an overflow occurs during the offset calculation of the buffer 3. It is cleared by writing 1 to CB3OF.

Bit 2 B2OF : Buffer 2 overflow flag

This bit is set when an overflow occurs during the offset calculation of the buffer 2. It is cleared by writing 1 to CB2OF.

Bit 1 B1OF : Buffer 1 overflow flag

This bit is set when an overflow occurs during the offset calculation of the buffer 1. It is cleared by writing 1 to CB1OF.

Bit 0 B0OF : Buffer 0 overflow flag

This bit is set when an overflow occurs during the offset calculation of the buffer 0. It is cleared by writing 1 to CB0OF.

21.5.3 GFXMMU flag clear register (GFXMMU_FCR)

Address offset: 0x0008

Reset value: 0x0000 0000

Bits 31:5 Reserved, must be kept at reset value.

Bit 4 CAMEF : Clear AHB master error flag

Writing 1 clears the AHB master error flag in the GFXMMU_SR register.

Bit 3 CB3OF : Clear buffer 3 overflow flag

Writing 1 clears the buffer 3 overflow flag in the GFXMMU_SR register.

Bit 2 CB2OF : Clear buffer 2 overflow flag

Writing 1 clears the buffer 2 overflow flag in the GFXMMU_SR register.

Bit 1 CB1OF : Clear buffer 1 overflow flag

Writing 1 clears the buffer 1 overflow flag in the GFXMMU_SR register.

Bit 0 CB0OF : Clear buffer 0 overflow flag

Writing 1 clears the buffer 0 overflow flag in the GFXMMU_SR register.

21.5.4 GFXMMU cache control register (GFXMMU_CCR)

Address offset: 0x000C

Reset value: 0x0000 0000

Bits 31:2 Reserved, must be kept at reset value.

Bit 1 FI : Force invalidate

When set, the cache entries are invalidated. This bit is reset by hardware when the invalidation is complete. Write 0 has no effect.

0: Invalidation process complete

1: Force invalidation/invalidation process on going

Bit 0 FF : Force flush

When set, the cache entries are flushed. This bit is reset by hardware when the flushing is complete. Write 0 has no effect.

0: Flushing process complete

1: Force flush/flushing process on going

21.5.5 GFXMMU default value register (GFXMMU_DVR)

Address offset: 0x0010

Reset value: 0x0000 0000

Bits 31:0 DV[31:0] : Default value

This field indicates the default 32-bit value which is returned when a master accesses a virtual memory location not physically mapped.

21.5.6 GFXMMU buffer 0 configuration register (GFXMMU_B0CR)

Address offset: 0x0020

Reset value: 0x0000 0000

Bits 31:23 PBBA[31:23] : Physical buffer base address
Base address MSB of the physical buffer.

Bits 22:4 PBO[22:4] : Physical buffer offset
Offset of the physical buffer.

Bits 3:0 Reserved, must be kept at reset value.

21.5.7 GFXMMU buffer 1 configuration register (GFXMMU_B1CR)

Address offset: 0x0024

Reset value: 0x0000 0000

Bits 31:23 PBBA[31:23] : Physical buffer base address
Base address MSB of the physical buffer.

Bits 22:4 PBO[22:4] : Physical buffer offset
Offset of the physical buffer.

Bits 3:0 Reserved, must be kept at reset value.

21.5.8 GFXMMU buffer 2 configuration register (GFXMMU_B2CR)

Address offset: 0x0028

Reset value: 0x0000 0000

Bits 31:23 PBBA[31:23] : Physical buffer base address
Base address MSB of the physical buffer.

Bits 22:4 PBO[22:4] : Physical buffer offset
Offset of the physical buffer.

Bits 3:0 Reserved, must be kept at reset value.

21.5.9 GFXMMU buffer 3 configuration register (GFXMMU_B3CR)

Address offset: 0x002C

Reset value: 0x0000 0000

Bits 31:23 PBBA[31:23] : Physical buffer base address
Base address MSB of the physical buffer.

Bits 22:4 PBO[22:4] : Physical buffer offset
Offset of the physical buffer.

Bits 3:0 Reserved, must be kept at reset value.

21.5.10 GFXMMU LUT entry x low (GFXMMU_LUTxL)

Address offset: 0x1000 + 0x8 * x, (x = 0 to 1023)

Reset value: 0x0000 0000

Bits 31:24 Reserved, must be kept at reset value.

Bits 23:16 LVB[7:0] : Last Valid Block

Number of the last valid block of line number X.

Bits 15:8 FVB[7:0] : First Valid Block

Number of the first valid block of line number x.

Bits 7:1 Reserved, must be kept at reset value.

Bit 0 EN : Enable

Line enable.

0: Line is disabled (no MMU evaluation is performed)

1: Line is enabled (MMU evaluation is performed)

21.5.11 GFXMMU LUT entry x high (GFXMMU_LUTxH)

Address offset: 0x1004 + 0x8 * x, (x = 0 to 1023)

Reset value: 0x0000 0000

Bits 31:22 Reserved, must be kept at reset value.

Bits 21:4 LO[21:4] : Line offset

Line offset of line number x (i.e. offset of block 0 of line x)

Bits 3:0 Reserved, must be kept at reset value.

21.5.12 GFXMMU register map

The following table summarizes the graphic MMU registers. Refer to the register boundary addresses table for the graphic MMU register base address.

Table 132. GFXMMU register map and reset values

Refer to Section 2.3 on page 131 for the register boundary addresses.