add dma

dma.md

@@ -0,0 +1,81 @@
+---
+layout: page
+title: Direct memory access
+permalink: /dma/
+---
+
+Introduction
+-----
+
+Two possible approaches to sending data from the FPGA to the CPU:
+- use the GP bus to read data from block RAM (BRAM)
+- use the HP or ACP bus to write data to DDR3 RAM
+
+The GP bus approach is fast enough for the following applications:
+- record relatively short (less than 100k) series of ADC samples with high sample rates (10-125 MSPS)
+- continuous transfer of ADC samples to the CPU after some preprocessing (for example, decimation) done on the FPGA and resulting in relatively low sample rates (less than 10 MSPS)
+
+The HP or ACP bus approach can be used for the following applications:
+- record long series of ADC samples with high sample rates
+- continuous transfer of ADC samples with high sample rates to the CPU
+
+There are several options for controlling the transfer of data from the ADC to the BRAM or DDR3 RAM buffers and accessing the data stored in these buffers.
+
+Options for controlling data transfer from FPGA:
+- Xilinx DMA IP cores
+- custom DMA IP cores
+- custom HDL modules
+
+Options for controlling IP cores and HDL modules from a Linux application running on the CPU:
+- `/dev/mem` driver
+- UIO driver
+- custom Linux driver
+
+Options for accessing data stored in BRAM or DDR3 RAM buffers from a Linux application running on the CPU:
+- `/dev/mem` driver
+- UIO driver
+- custom Linux driver
+
+The applications in this repository use the following combination:
+- custom DMA IP cores
+- `/dev/mem` driver for controlling IP cores
+- custom Linux driver for allocating DDR3 RAM buffers
+
+Custom IP cores
+-----
+
+The `axis_ram_writer` module implements the following logic:
+- wait until there are 16 entries in the FIFO buffer
+- start burst write transfer to send 16 entries from FIFO buffer to RAM
+- increment the address counter or reset it when it reaches `cfg_data`
+
+The `axis_ram_reader` module implements the following logic:
+- wait until there is space for 16 entries in the FIFO buffer
+- start burst read transfer to send 16 entries from RAM to FIFO buffer
+- increment the address counter or reset it when it reaches `cfg_data`
+
+The modules have two input ports for dynamically configurable parameters:
+- `min_addr` - starting address of the memory buffer
+- `cfg_data` - maximum value of the address counter that counts burst transfers (128 bytes per transfer)
+
+The `sts_data` port outputs the current value of the address counter. It can be used to check what memory addresses the modules have already accessed.
+
+The Verilog code of these IP cores can be found in [cores/axis_ram_writer.v](https://github.com/pavel-demin/red-pitaya-notes/tree/master/cores/axis_ram_writer.v) and [cores/axis_ram_reader.v](https://github.com/pavel-demin/red-pitaya-notes/tree/master/cores/axis_ram_reader.v).
+
+Custom Linux driver
+-----
+
+The custom Linux driver is used to allocate a memory buffer using contiguous memory allocator (CMA). The `ioctl` function is used to allocate a memory buffer and obtain its physical address. The `mmap` function is used to obtain the virtual address of the memory buffer.
+
+The source code of the custom Linux driver can be found in [patches/cma.c](https://github.com/pavel-demin/red-pitaya-notes/tree/master/patches/cma.c).
+
+Usage examples
+-----
+
+The source code of projects using direct memory access can be found at the following links:
+- [projects/adc_recorder](https://github.com/pavel-demin/red-pitaya-notes/tree/master/projects/adc_recorder)
+- [projects/adc_recorder_trigger](https://github.com/pavel-demin/red-pitaya-notes/tree/master/projects/adc_recorder_trigger)
+- [projects/adc_test](https://github.com/pavel-demin/red-pitaya-notes/tree/master/projects/adc_test)
+- [projects/dac_player](https://github.com/pavel-demin/red-pitaya-notes/tree/master/projects/dac_player)
+- [projects/sdr_receiver_wide](https://github.com/pavel-demin/red-pitaya-notes/tree/master/projects/sdr_receiver_wide)
+- [projects/sdr_receiver_wide_122_88](https://github.com/pavel-demin/red-pitaya-notes/tree/master/projects/sdr_receiver_wide_122_88)

index.md

@@ -10,6 +10,7 @@ layout: default
 1. [LED blinker]({{ "/led-blinker/" | prepend: site.baseurl }})
 1. [Buffers for AXI4, AXI4-Lite and AXI4-Stream interfaces]({{ "/axi-interface-buffers/" | prepend: site.baseurl }})
 1. [AXI4 hub]({{ "/axi-hub/" | prepend: site.baseurl }})
+1. [Direct memory access]({{ "/dma/" | prepend: site.baseurl }})
 1. [Alpine with pre-built applications]({{ "/alpine/" | prepend: site.baseurl }})
 1. Applications for Red Pitaya STEMlab 125-14:
    1. [Playground]({{ "/playground/" | prepend: site.baseurl }})