I would use the read() and write() for the /dev/spiB.C which is created by the user mode spi device driver (spidev.c). Now, the SPI ransaction message follows a certain format (e.g., 24 bits, with some bits for address and some bits for data) which is defined by the chip-vendor-specific spi controller driver. How is the message format fit into the read() and write() transaction? Where and how should I define the format in the code before or after the write() or read()?
Thanks!
You need to call spidev_ioctl() mentioned in spidev.c
E.g. Just check switch case :SPI_IOC_RD_BITS_PER_WORD which sets the bits/word (line 410).
Then finally it writes to the bits_per_word member in the spi device structure (line 415).
This spi pointer is the pointer of the spi device which you are communicating to and has been allocated during spidev_probe().
Definitely you need to set the configuration before r/w.You also need to set speed and mode of SPI.
I have refered to the below link for spidev.c file:
http://lxr.free-electrons.com/source/drivers/spi/spidev.c
Related
I'm developing a custom application on the BeagleBone Black that has to use the UART capabilities of the device via the Debian Linux that runs on the BBB, for reading big chunks of data. For reading from the UART I'm opening one of the /dev/ttyO0, /dev/ttyO1, etc. devices with the open() function in nonblocking mode. And then I'm trying to read from this port with the read(2) function:
ssize_t read(int fd, void *buf, size_t count);
I would like to know what is the biggest reasonable number for the parameter count and how is it related to the UART's FIFO buffer?
In the AM335x technical reference manual (TI document spruh73p, page 4328, section 19.3.6) I can see that the HW buffer is 64 bytes long. But, as I suspect by using the read() function my program is not communicating directly with the hardware fifo buffer but it is reading from Linux' serial driver's software buffer (if there is such). Is this true? If yes, what is the size of the software fifo? Could somebody please enlighten this field for me?
And then I'm trying to read from this port with the read(2) function
Rather than the "port", you are actually several layers removed from the hardware device, and reading from the system buffer of the serial terminal.
I would like to know what is the biggest reasonable number for the parameter count and how is it related to the UART's FIFO buffer?
First of all the count must be no larger than the user buffer provided.
For a blocking read(), you could probably make this count as large as any buffer you could allocate.
For a nonblocking read(), a count larger than the terminal receive buffer makes little sense.
Note that count is merely a request, and the read() syscall can return with less than the requested number of bytes.
The sizes of the UART FIFO and serial port driver buffers are irrelevant to any read() requests from userspace.
See Linux serial drivers.
... it is reading from Linux' serial driver's software buffer (if there is such). Is this true?
Almost.
The read() syscall from userspace fetches data from the terminal buffer.
The terminal driver is a higher-level driver than the serial port driver.
The terminal buffer has no direct connection to the UART FIFO.
If DMA is employed, then data is transferred from the UART FIFO to a DMA buffer.
If PIO is employed, then data is transferred from the UART FIFO to a driver buffer.
In either case the serial port driver eventually transfers the data to a tty flip buffer.
In non-interrupt mode, data from the tty flip buffer is transferred to the terminal/line-discipline buffer.
Again refer to Linux serial drivers.
If yes, what is the size of the software fifo?
The terminal receive buffer is typically 4096 bytes, although you could write your own line discipline with a different size, or redefine the macro.
From include/linux/tty.h in Linux kernel source:
#define N_TTY_BUF_SIZE 4096
The terminal driver is common to all architectures supported by Linux, so your BBB should have a terminal buffer of 4096 bytes.
My question regards <termios.h>. As I understand, two buffers exist in reading something over a UART - a hardware buffer where received bytes are stored, and a software buffer where we load the stuff that has been stored in the hardware buffer. This software buffer is the second argument in read(uart_channel, BUFFER, length) as I understand.
Please explain: how long is the hardware buffer? Do I have control over how long it is? For me it is critical to read the 12 most recent bytes sent over UART by a device - how can I ensure this?
I had a similar situation once and what I did is to create a thread that kept on reading the UART (blocking read) and I used a FIFO between the threads.
If you cannot use threading, you might just use select.
Most uC I've seen have a hardware FIFO that can be set to interrupt after, say [1,2,4,8,16] bytes. If the FIFO is left 'partially full' for some small multiple of the character interval for the currently configured baud rate, the UART interrupts anyway. If you really must have access to bytes ASAP, then you need to set the FIFO 'length' to 1. Of course, your driver should do that when initializing the UART.
Failing that, I guess you could poll it:(
How will I map register addresses specifically UART registers to kernel for writing device drivers for UART?
I have gone through the omap-serial.c.But I did not find the mapping of the registers defined in it.
Is it different from the mapping of standalone UART driver?
As a device driver writer, it is your job to read the hardware documentation. The serial port documentation will specify the bits in the control and status registers and provide guidance on how to determine their addresses. Usually that guidance is in a system integrator's document.
Let's say your research determines that the UART's registers are at 0x31080220. Your code would then have:
struct resource *uart_res; // resource handle
uint *uart; // pointer to actual control/status registers
uart_res = request_mem_region (0x31080220, 4*4, "my_uart"); // map 16 bytes
if (!uart_res)
{
error ("unable to map memory region");
return -ENOMEM;
}
uart = ioremap (0x31080220, 4*4);
if (!uart)
{
release_mem_region (0x31080220, 4*4);
error ("unable to map");
return -ENOMEM;
}
Then you can use the uart pointer to access the registers.
status = ioread32 (uart + 0); // read the status register
iowrite32 (0xf0f0, uart + 4); // foo foo to control register
Give precise target information for the manufacturer, model, and options—just like an automobile—and someone will help you find the specifics.
Mapping uart in kernel may be definded as uart device (not driver) in the some place: kernel/arch/arm/'machine'/(devices | serial or some else).
Usualy there is no neet for mapping. When uart driver probe, it connects to device and create tty character driver. To operate tty from kernel, you may add your own line discipline to tty. Then user space progam can open needed ttySX port and attach it to your line discipline. Then your code in kernel will operates communication thrue the uart port (tty->driver).
I am working with serial port programming and i have following doubt please clear it
Does Read/Write system call reads and writes data directly from/to UART buffer?
Which buffer does Rx and Tx of "/proc/tty/driver/serial" refers to.
Please clarify.
read()and write() access data via a so called file-descriptor return by a call to open().
open() gets passed in the name of a serial device.
Serial devices could be named "/dev/ttySX" with X={0..N}.
Why are you worried about:
Does Read/Write system call reads and writes data directly from/to
UART buffer?
The kernel takes care of the actual read/write calls to a file regardless if its a device or a data file, in the case of device, IOCTL calls will be made to the device in question and handle it behind the scenes.
Which buffer does Rx and Tx of "/proc/tty/driver/serial" refers to.
Again, that is handled by the kernel and related IOCTL calls to the serial driver by way of device descriptor to the actual serial port.
If still in doubt, a refresher on serial port programming under Un*x environments such as Linux/FreeBSD would be recommended.
Ist there any possibility to read values from the pins of the COM Port? Any solution in C under Linux is appreciated!
Yes, see for instance this guide.
You use the ioctl() function, to read the various control pins. Data is, of course, best read through the normal read() handling, you don't want to be polling asynchronuous serial data.
I don't think your assumption (expressed in a comment) that the driver must check the pin-states to handle data is correct, normally a serial port is "backed" by an UART and that typically handles the RX/TX pins in hardware.
Am pretty sure , you can't read/write pins of UART.
Even at the hardware level , you have to read/write an entire byte.There is no bit access or read/write pin access.
The Byte is read/written in the Receive/Transmit UART buffer .
In either ways you can't just access the buffer directly , on your behalf the linux driver will do the job. You just have to make use of the driver in your application , to work with the UART , the linux driver for UART provides , standard API's like open(),read(),write(),ioctl() through which you interact the UART device.
If you want to work with drivers , and new to this field , the best place to start will be
this book.
The exact answer to this question depends on the precise hardware in question. I know of a piece of code where I worked, based on receiving the letter 'a' as the indication of bitrate, and it would poll the RX pin to detect the transitions between 0 and 1 to detect the "width" of the bits, and it would then calculate the correct clock-rate for the serial port and configure the serial port to match the bitrate of the other end.
A "PC" type hardware solution will not be able to read the RX/TX pins. In other hardware, it may be possible to do so. Many embedded systems allow various pins to be configured as inputs, outputs or "have a function" (in our case, RX, TX, CTS, RTS, etc) - so for example, you could configure the RX pin to be a input, and thus read the state of it. Of course, the normal serial port drivers will probably set these pins to "have a function" [or expect the boot code running before the kernel is started to have configure it this way]. So you would have to reconfigure the pins in some kernel code of your own, most likely. Beware that this may cause unexpected side-effects with the driver for the actual serial port - it may "get upset" when it tries to do things to the serial port and it's "not working as expected" because it's been "misconfigured".
You can almost certainly read (and/or write) the state of the control pins, such as CTS, RTS via IOCTL calls.