File system in battery backed SRAM

Wath about adding a hard disk, mmc or SD ?

I've developed a little device block driver to do that.
The way i do it is very simple:

Look at some block devices, and choose one to use as a skeleton.
The request function may be something like:


#include <linux/blk.h>

// ....

#define DEVICE_REQUEST do_rh_request

static void
do_rh_request( request_queue_t * q )
{
    mm_segment_t fs;
    u_long start, len, addr, size;


    while ( TRUE )
                {
                INIT_REQUEST;

                minor = MINOR(CURRENT->rq_dev);

                start = CURRENT->sector << 9;
                len  = CURRENT->current_nr_sectors << 9;
                size = YOUR_MEMORY_SIZE;

                if( (start + len) > size )
                        {
                        printk( KERN_ERR DEVICE_NAME ": bad access:
block=%ld, count=%ld\n",
                        CURRENT->sector,
                        CURRENT->current_nr_sectors);
                        end_request( FALSE );
                        continue;
                        }

                fs = get_fs();
                set_fs(KERNEL_DS);
                addr = YOUR_MEMORY_BASE_ADDRESS + start;

                if( ( CURRENT->cmd == READ ) )
                        {
                        //  READ blocks
                        copy_to_user ( CURRENT->buffer, (char *)addr, len );
                        }
                else
                        {
                        //  WRITE blocks
                        copy_from_user ( (char*)addr, CURRENT->buffer,
len );
                        }

                set_fs(fs);
                end_request( TRUE );
                }
}


This code write or read directly to memory. Each 512 block will be saved in
the SRAM. You may improve security by reserving one sector area and save in
that point each sector before modify it.
At system startup you may check the reserved area, and if it contains a
stored record you may recover it (the system may be turned-off when writing
to a sector).

Then you may format your block device as you do for other disks, for example
with minix, and mount it.

Bye, Gabriele