Hi Nidhi
Please use, plain text for writing mails to newbies, it does not
look good.
> hi all
> i was practising writing block device driver from linux device driver 3rd
> edition
> but was confused
> as
> it gives two implementation to transfer data from device
> 1.without using bios
> 2.using bios
> in ist implementation ---- cut short version of book code is -----
> while ((req = elv_next_request(q)) != NULL) {
> struct sbull_dev *dev = req->rq_disk->private_data;
> sbull_transfer(dev, req->sector, req->current_nr_sectors,req->buffer,
> rq_data_dir(req));
> end_request(req, 1);
> }
> static void sbull_transfer(struct sbull_dev *dev, unsigned long sector,
> unsigned long nsect, char *buffer, int write)
> {
> unsigned long offset = sector*KERNEL_SECTOR_SIZE;
> unsigned long nbytes = nsect*KERNEL_SECTOR_SIZE;
> if ((offset + nbytes) > dev->size) {
> printk (KERN_NOTICE "Beyond-end write (%ld %ld)\n", offset,
> nbytes);
> return;
> }
> if (write)
> memcpy(dev->data + offset, buffer, nbytes);
> else
> memcpy(buffer, dev->data + offset, nbytes);
> }
> ****************************************************************************************************************************
dont put stars like this it does not look good.
> Now my question is '
> when current_nr_sectors ==== is the number of sectors left to be
> transferres in the current segment
>
> similarly req->buffer --- is also the data of current bio as it was written
> in LDD : text is as below
> "this field is simply the result of calling bio_data on the current
> bio.where bio_data(bio) Returns a kernel logical address pointing to the
> data to be transferre"
> therefore it seems it is only pointing to the data buffer of current bio
> only.....
>
> at one place number os sectors left in currnt segment at other place address
> to which transfer is made of current bio ....i am confused
>
Looks like you are a lot confused, see the implementation of
sbd_transfer() carefully, dont get confused, a segment is a part of
bio and a buffer may contain several segments.
Now lets see in detail,
A buffer points to a page,
a page consists of contagious memory location from where data has to
be transferred. These are called segments within a page.
So, to transfer a segment you need to specify the buffer that contains
the segment.
I hope this is clear.
If you see the implementation of sbd_transfer, offset of the segment
is calculated.
unsigned long offset = sector*KERNEL_SECTOR_SIZE;
and
no. of bytes to transfer
unsigned long nbytes = nsect*KERNEL_SECTOR_SIZE;
I hope now your confusion is clear.
I hope that helps.
Read about pages, segments and buffers from UTLK.
****************************************************************************************************************************************************************************************************************
> ********************************************************************
>
>
>
> On Fri, Mar 6, 2009 at 12:50 AM, Rohit Sharma <imreckless@xxxxxxxxx> wrote:
>>
>> Hi Nidhi,
>>
>> Seems like you are stuck up with this issue.
>>
>> Refer the following links if you haven't
>> Check out this link to understand block device driver
>> http://lwn.net/Articles/driver-porting/
>>
>> for bio structures check this link
>> http://lwn.net/Articles/26404/
>> >
>> > in gyst :
>> > when you actually transfer data in request processing function
>> > using req->current_nr_sectors
>> > from req->buffer
>> > in while elv_next_request loop
>>
>> Please try to provide with the code snippet, will help us understand the
>> problem better.
>>
>> > and if its transferred successfully book does end_request(req,1)
>> >
>> > then my ques is
>> > Q1. end_request doesnt end the request but ends the part of request
>> > what is that part --- a bio OR a segment or what ?
>> >
>> A request consists of a list of bios and bios consists of several
>> segments,
>> and the end request is called to finish the processing of current request.
>>
>> > by seeing current_nr_sectors it seems it transfer segment by segment ---
>> > but seeing req->buffer (equivalent to bio_data(current bio)) seems
>> > transferring bio by bio ??
>> >
>> i think you are referring to sbd_transfer() function,
>>
>> sbd_transfer(&Device, req->sector, req->current_nr_sectors,
>> req->buffer, rq_data_dir(req));
>>
>> what i understand is that it says transfer these many sectors
>> 'req->current_nr_sectors'
>> starting from sector no. 'req->sector' from buffer 'req->buffer' and
>> direction of data
>> 'rq_data_dir(req)' --> says READ or WRITE request.
>>
>>
>>
>> I hope that helps.
>>
>>
>>
>> Thanks.
>> >
>> > Nidhi
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> > --
>> > Thanks & Regards
>> > Nidhi
>
>
>
> --
> Thanks & Regards
> Nidhi
>
--
To unsubscribe from this list: send an email with
"unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx
Please read the FAQ at http://kernelnewbies.org/FAQ
