September 3, 2018
I am basing this around some previous work the first time I was writing software for this computer. I started making a framework called catos, but it didn't really work out. I never kept with the project. This time around, I am starting over, copy and pasting some of it into a new project called KittyOS.
So far, this is what is working: Read/write the serial port thru a chardevice structure.
Unix is based around the idea that everything is a file. Devices are just 'magic files' that when reading and writing causes I/O to happen. I am flipping this around: in KittyOS everything is a device. Part of this is the reason that perhaps I want to use KittyOS in other AVR projects. There may not be a filesystem available for use, and it doesn't make sense to emulate one just for the sake of being like Unix. Yes, there are many embedded Linux devices with no physical filesystem that run out of initrd or initramfs, but that is only because in Unix you need a filesystem because eveything is a file. The AVR has 4k of memory and 64k of flash, and I don't want to fill that up with a fake filesystem. In KittyOS, all you need are devices to read and write. There are a few different classes of devices:>
Basic device: A basic device has one function: ioctl. Ioctl passes 2 integers to a device, a command number and a value. This might mean different things to different devices. For a serial port, it might set the baud rate. For a video card, it might change the video mode. Maybe this initalizes a SPI bus, or sets the mode on a temperature sensor from degrees F to degrees C, or maybe it changes the color of an RGB LED. It's just a basic command channel.
Char device: A character device supports everything a Basic device does, but supports serialized reading and writing. These might be 'files', serial ports, sockets, keyboards, mice, anything.
- read1: reads 1 byte from a device. Blocks until there is a character ready.
- write1: writes 1 byte to a device. Blocks if the device is busy.
- kbhit: returns true if a character is ready to be read. Name is taken from the dos conio 'kbhit' function to see if reading the keyboard would block
Block device: A device that reads or writes data in chunks, such as a disk. This is a little different from a linux block device. Linux block devices are still 'files' and can be read or written single bytes at a time, and the OS uses a block caching layer to actually read and writes blocks at a time. This is not supported in KittyOS. Reading or writing to a block device is done, in random access, one whole block at a time. This maps cleanly to the sdcard read and write sector commands, and probably most disk devices.
file device: A file on a disk (in a filesystem) is just a chardevice that supports two additional functions: seek and pos. This will reposition a read/write stream, or tell you where it is. The act of opening a file on a disk creates a file chardevice for the user to interact with. Instead of treating a virtual serial port as a file, KittyOS treats a file as a virtual serial port.
Selector device: A device that finds, lists, organizes or creates other devices. Supports the call 'subdev' which given a name and some flags, returns another device to use. It will also support another call 'list' which returns a list of available subdevice names. If opening a file on disk, it will create a chardevice that represents the contents of that file. On a disk a selector device might list other selector devices. (Hint: its a directory )
The SDCard filesystem will be organized like this:
- The SPI bus is a chardevice, since it reads and writes one byte at a time.
- The SPI bus will supports the calls IOCTLs lock and unlock, which prevent more than one driver from using the SPI bus at a time.
- The SD card block device will lock the SPI bus, and use it to read and write blocks from the sdcard.
- A filedevice driver will support reading and writing 'files' on the sdcard. A file is created by finding a free block. Files grow in a linked list fashion from free blocks. Existing files can opened by specifying their block number.
- A selector device will be created that lists other devices on the filesystem. This 'root list' will start at a known block number. This will list file devices by name (plain files), or will list other selector devices (directories)
The above seems like a lot of layers, but I still think the Atmega will mostly be held up waiting for data on the bus. I do have some sdcard routines that directly read/write spi, but I want to try making them go thru a chardevice wrapper, the same way serial routines currently do. Why? This is closer to how a modern OS might be structured. It's also very powerful to abstract things like this. If reading/writing SPI is done thru the chardevice, then what keeps me from simulating SPI devices in software, or debugging what is being sent to an SPI device by sending it over the serial port to my PC? Suppose by SPI bitrate is 2 MHz. Then on a 20Mhz atmega, there's 10 instructions per bit, so 80 per byte. 80 instructions is enough to traverse the chardevice struct pointer to the function that puts the next byte on the SPI bus
But, SPI isn't a regular chardevice is it? SPI doesn't read/write seperately like a serial port, but exchanges bytes: You always read and write at the same time. In the past I did do some experiments with treating the SPI as a serial port with only a read and write function. This was easy by keeping a 1 byte buffer:
- Write byte: Writes a byte to the SPI bus. The received byte is placed in the buffer. If there are multiple writes in a row, the buffer is replaced with the newly received byte.
- Read byte: If the buffer contains a byte, returns it without writing anything to the SPI bus. If there is another read, and the buffer does not contain data, a zero is written to the SPI bus, and the received byte is returned
This fits all the use cases of SPI I've come across so far. Either you write and then immeediately read, to do a byte exchange. Or you write several bytes in a row, ignoring the return value. Or you read several bytes in a row, outputting "don't care" values. This also maps well to how the AVR hardware works.
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