data transfer from kernel to user space

[Daniel Baluta]

On Tue, Jul 12, 2011 at 9:56 AM, Amit Nagal <helloin.amit at gmail.com> wrote:
> On Tue, Jul 12, 2011 at 11:51 AM, Tirtha Ghosh <gtirtha at gmail.com> wrote:
>> Use a shared memory between user space and kernel space.
>> The same physical memory can be translated as user-space virtual address as
>> well as kernel space.
>> See more about mmap, io_remap_pfn_range etc. and you will get an idea.
>> The main advantage with this u can avoid copying data between two spaces but
>> disadvantage associated is u need to request  a fixed memory chunk from
>> kernel for this purpose.
>>
>>
> Thanx for reply . Regarding mmap usage , As per  LDD3 , page 423 :
> "Not every device lends itself to the mmap abstraction; it makes no
> sense, for instance, for serial ports and other stream-oriented
> devices." .
>
> why mmap is not useful for stream-oriented devices ?
> After all even for stream-oriented device (as in my case ) , we need
> to copy data from kernel buffer to user-space buffer ,
> and if copy can be avoided via any mechanism , it will be much faster  .

Usually, character oriented devices, are only aware of 'next byte' to
send/receive.
Think what would happen, if you map a memory area of 4K, but only 1
byte has arrived?
You would need to find a way to tell user, that mmapped area is not
yet ready to be read,
or only the first byte is valid.

Also, think if lseek makes sense on a serial device.

thanks,
Daniel.