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spiTransfer() only transmits 16 bytes at maximum #3

gamer-cndg opened this issue Oct 15, 2017 · 0 comments


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commented Oct 15, 2017

I'm currently working on a SPI display driver and I have hit a roadblock with your SPI implementation. In my case, I'd like to transmit my whole framebuffer (176*220*2 bytes) at once.

However, once I go over 16 bytes of length in the spiTransfer() function, the function just returns an error.

[+] Trying to write length 16.
onion-spi:: Trasferring 0xff, 16 bytes
   Received: 0x00, ioctl status: 16
[+] Write 16 bytes okay
[+] Trying to write length 17.
onion-spi:: Trasferring 0xff, 17 bytes
ERROR: SPI transfer failed

Test code:

	//test max size
	int err = EXIT_SUCCESS;
	int len = 1;
        static uint8_t dummyRXBuf[176*220*2];
	while(err == EXIT_SUCCESS) {
		printf("[+] Trying to write length %d.\n", len);
		uint8_t* testBuf = malloc(len);
		memset(testBuf, 0xff, len);
		err = spiTransfer(&params, testBuf, dummyRXBuf, len);
		if(err == EXIT_SUCCESS) {
			printf("[+] Write %d bytes okay\n", len);

SPI params initialization:

	params.mosiGpio = initParams->mosi;
	params.sckGpio = initParams->sclk;
	params.csGpio = initParams->cs;
	params.speedInHz = initParams->speedInHz; //20 mhz
	params.modeBits = SPI_MODE_0; //only SPIMODE_0, no dual TX or RX as "default"..
	params.busNum = 1; //this comes from my local machine. no idea if it works elsewhere.. (ls /dev/spi*)
	params.deviceId = 32766;

All I can see in ( is a call to ioctl, so I think the fault must be with the kernel module.

Can you please correct this error? With your implementation, transmitting in blocks of 16 bytes with a bit-banged driver yields a display FPS of about 2.5 FPS. Also, what's the maximum speed with this implementation? Is this implementation backed by DMA?

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