On 10/19/06, Rahul Theraja <rahul.theraja@xxxxxxxxx> wrote:
Hello All
I have some doubts in working with block drivers. I have a "sbd_device.c"
a sample block driver file and a small application that does a read request
to this driver. I am working on linux-2.6.9.
After insmod'ing the driver i have run my application. My doubt is, though
i have given only one read() in appln.c , it does many requests to the
driver. I could not understand why the "sbd_request " is called even though
i have given only one read call from user space file.
In the below user space file, i tried changing the buf size from 1 to 512
but the number of read requests done driver are same.
int fd = -1, buf[1], count = 1, ret, i=0;
fd = open("/dev/sbd_dev0", O_RDWR);
if(fd < 0)
printf("Could not open /dev/sbd_device\n");
else
{
printf("\nOpened device file : \n");
printf("\nSending a Read.... \n");
ret = read(fd, buf, count);
printf("\n1. The value ret = %x\n", ret);
for(i=0; i<count;i++)
printf("buf[%d] = 0x%x\n",i, buf[i]);
printf("\n\n");
and the dmesg ouput is:Init module...
sbd_dev0:sbd_request
Calling sbd_transfer
Doing a read from device
unknown partition table
sbd_request
Calling sbd_transfer
Doing a read from device
Calling sbd_transfer
Doing a read from device
Calling sbd_transfer
Doing a read from device
Calling sbd_transfer
...
....Does around 20 times.
Ok, have you tried to read again just after doing first read ? I think
that time you must get less number of requests. Most likely performing
first read actually making kernel to ask for different drive
information like partition-table, size, probably file-system
information etc; so you will get multiple requests. By the way data
transfer in block device takes place in chunks/blocks.
And also in the driver, in the function sbd_request() how does the driver
get the parameters like sector, current_nr_sectors and buffer. From user
space we just give the size of data transfer but not the sector etc.
sbd_transfer(&sbd_device, req->sector, req->current_nr_sectors,
req->buffer, rq_data_dir(req));
The read from user-space is actually doing sys_call "read" in kernel
which calls VFS functions and VFS/FS provides sector, nr_sector etc to
device.
Anyone please help me in clearing this basic doubts so that i could move
furhter fastly.
I think I made some doubts clear. You can ask more if needed, and do
read book "Linux Device Driver, 3rd Ed"
Thanks in advance
Rahul
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Fawad Lateef
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