Scull Driver - Read

[sahil aggarwal]

Hi Pranay

Well explained.

Thank you.

On 20 March 2015 at 11:46, Pranay Srivastava <pranjas at gmail.com> wrote:
> Hi Sahil
>
> On Sun, Mar 15, 2015 at 10:17 AM, sahil aggarwal <sahil.agg15 at gmail.com> wrote:
>> hi all,
>>
>> Going through scull driver code, i see read function reads only till
>> end of 1 quantum, so do kernel call read multiple times if count from
>> q_pos exceeds quantum size limit.?
>>
>>
>> Ref: ldd3.
>>
>> if(count > quantum - q_pos)
>> count = quantum - q_pos;
>> if(copy_to_user(buf, dptr->data[s_pos] + q_pos,count)){
>> retval = -EFAULT;
>> goto out;
>> }
>
> If you are not using the default read/write routines, then its up to
> you to code that.
>
> Kernel doesn't do multiple read calls. It'll do only what you asked it
> for no more but it can do less. For example, a file is say 100 KiB and
> you are reading say 4KiB in a loop when do you stop?
>
> Simply put it's the user space application which is doing the looping
> and repeatedly doing read calls because it assumes that file isn't
> finished.
>
> So again when do you stop reading from the file, without knowing size
> of the file.?
>
>> *f_pos += count;
>> retval = count;
>>
>> Thanks
>> Regards
>>
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>
>
>
> --
>         ---P.K.S