Getting EINVAL when trying to write to mtd device - c

Question

I am referring to the code in this answer.
I added error handling though.
The open() erase and the read() all perform without error and the 20 bytes that I read all are 0xff.
However, when trying to write the 20 bytes from the data[] array, I get an EINVAL errorcode from the write() function.
What could be the cause of the problem? I did erase the memory before trying to write...

Answer 1

I have seen your original post.
I have the same problem recently, and I found that the write size is important.
mtd_info_t(struct mtd_info_user) have a variable named writesize
(reference: https://elixir.bootlin.com/linux/v3.2/source/include/mtd/mtd-abi.h#L125)
struct mtd_info_user {
__u8 type;
__u32 flags;
__u32 size; /* Total size of the MTD */
__u32 erasesize;
__u32 writesize;
__u32 oobsize; /* Amount of OOB data per block (e.g. 16) */
__u64 padding; /* Old obsolete field; do not use */
};
when write to the mtd, should notice writesize
#include <fcntl.h>
#include <mtd/mtd-user.h>
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <sys/ioctl.h>
int main(){
mtd_info_t mtd_info; // the MTD structure
erase_info_t ei; // the erase block structure
int i;
unsigned char read_buf[20] = {0x00}; // empty array for reading
int fd = open("/dev/mtd0", O_RDWR); // open the mtd device for reading and
// writing. Note you want mtd0 not mtdblock0
// also you probably need to open permissions
// to the dev (sudo chmod 777 /dev/mtd0)
ioctl(fd, MEMGETINFO, &mtd_info); // get the device info
// dump it for a sanity check, should match what's in /proc/mtd
printf("MTD Type: %x\nMTD total size: %x(hex) bytes\nMTD erase size: %x(hex) bytes\nMTD write size: %x(hex) bytes\n",
mtd_info.type, mtd_info.size, mtd_info.erasesize, mtd_info.writesize);
ei.length = mtd_info.erasesize; //set the erase block size
for(ei.start = 0; ei.start < mtd_info.size; ei.start += ei.length)
{
ioctl(fd, MEMUNLOCK, &ei);
// printf("Eraseing Block %#x\n", ei.start); // show the blocks erasing
// warning, this prints a lot!
ioctl(fd, MEMERASE, &ei);
}
lseek(fd, 0, SEEK_SET); // go to the first block
read(fd, read_buf, sizeof(read_buf)); // read 20 bytes
// sanity check, should be all 0xFF if erase worked
for(i = 0; i<20; i++)
printf("buf[%d] = 0x%02x\n", i, (unsigned int)read_buf[i]);
/**********************************************************
* important part! *
* notice the size of data array is mtd_info.writesize *
**********************************************************/
uint32_t write_size = mtd_info.writesize;
unsigned char data[write_size];//write 0
bzero(data, write_size);
lseek(fd, 0, SEEK_SET); // go back to first block's start
write(fd, data, sizeof(data)); // write our message
lseek(fd, 0, SEEK_SET); // go back to first block's start
read(fd, read_buf, sizeof(read_buf));// read the data
// sanity check, now you see the message we wrote!
for(i = 0; i<20; i++)
printf("buf[%d] = 0x%02x\n", i, (unsigned int)read_buf[i]);
close(fd);
return 0;
}
Hope it can help

Answer 2

From ./drivers/mtd/nand/nand_base.c
#define NOTALIGNED(x) ((x & (chip->subpagesize - 1)) != 0)
This is the check performed by driver:
/* Reject writes, which are not page aligned */
if (NOTALIGNED(to) || NOTALIGNED(ops->len)) {
pr_notice("%s: attempt to write non page aligned data\n",__func__);
return -EINVAL;
}
Both the address you are starting the write and and length of the buffer you are writing must satisfy the macro condition(to be multiple of the subpage size).