Re: Where does m.offset point to?

[Joshua Lamorie <jpl@xxxxxxxxx>]

Well, I think I figured this out, so I'm responding to myself so that 
some sort of response will be in the archives.

In response to VIDIOC_REQBUFS I map a single chunk of memory with vmalloc.
The size is important so I establish the frame size and then use 
PAGE_ALIGN to get the memory allocation required for each buffer.  So 
the call to vmalloc is something like...

iFrameSizePage = PAGE_ALIGN(iFrameSize);
pcFrameBufferStart = (char *)vmalloc(iFrameCount * iFrameSizePage);

At the same time I create a list of struct v4l2_buffer.  For each 
v4l2_buffer I initialise the  .m.offset equal to the number of bytes 
away from the start of the allocated memory.  Something like..

for(i=0;i<iFrameCount;i++){
   pxBufferList[i].m.offset = i * iFrameSizePage;
}

For the mmap functions, I'm using a nopage function taken from Rubini's 
book (page 395, scullv) which takes a virtual page address and then 
searches the page table.

It works quite well, but I designed my embedded system before I started 
to do video and boy... image frames take a  lot of memory!

If anyone sees an error in what I've written, please let me know.

Joshua


Joshua Lamorie wrote:

> Gidday there,
>
> I'm building a Video4Linux driver for some proprietary hardware 
> (interface to a CMOS sensor).
>
> My initial driver was only doing read/write access, and now I want to 
> get the memory mapped stuff going.
>
> I have been looking at the example stuff in video-buf.[ch] but it 
> seems to do a bit more than I need.  I don't have PCI, I'm not doing 
> DMA and the driver stores the frames in system memory (as opposed to 
> in the device).
>
> I'm a little confused about what to do for v4l2_buffer.m.offset while 
> responding to VIDIOC_REQBUFS.
>
> I have my buffers each allocated as
> pxBufList[i] =  (struct v4l2_buffer 
> *)kmalloc(iPageAlignedBufSize,GFP_KERNEL)
>
> but I'm not sure what to do with
> pxBufList[i].m.offset
>
> As far as I understand, I need to have this set to something so that 
> when the application does VIDIOC_QUERYBUF m.offset has something 
> useful to pass to mmap.
>
> Any hints to a simpler implementation of this?
>
> Thanks
>
> Joshua


--

Xiphos Technologies
(514) 848-9640 x227
(514) 848-9644 fax

www.xiplink.com
_______________________________________________
The information transmitted is intended only for the
person or entity to which it is addressed and may contain
confidential and/or privileged material.  If you have
received this in error, please contact the sender and delete
this communication and any copy immediately. Thank you.

--

Unsubscribe mailto:video4linux-list-request@xxxxxxxxxx?subject=unsubscribe
https://www.redhat.com/mailman/listinfo/video4linux-list