Re: [RFC] [PATCH 1/6] SoC Camera: add driver for OMAP1 camera interface

[Marin Mitov <mitov@issp.bas.bg>]

On Friday, August 13, 2010 10:13:08 pm Janusz Krzysztofik wrote:
> Friday 13 August 2010 11:11:52 Marin Mitov napisał(a):
> > On Friday, August 13, 2010 11:52:41 am Guennadi Liakhovetski wrote:
> > > On Fri, 13 Aug 2010, Janusz Krzysztofik wrote:
> > > > Thursday 12 August 2010 23:38:17 Guennadi Liakhovetski napisał(a):
> > > > > 1. We've discussed this dynamic switching a bit on IRC today. The
> > > > > first reaction was - you probably should concentrate on getting the
> > > > > contiguous version to work reliably. I.e., to reserve the memory in
> > > > > the board init code similar, how other contig users currently do it.
> > > >
> > > > I already tried before to find out how I could allocate memory at init
> > > > without reinventing a new videobuf-dma-contig implementation. Since in
> > > > the Documentation/video4linux/videobuf I've read that videobuf does not
> > > > currently play well with drivers that play tricks by allocating DMA
> > > > space at system boot time, I've implemented the alternate sg path.
> > > >
> > > > If it's not quite true what the documentation says and you can give me
> > > > a hint how this could be done, I might try again.
> > >
> > > For an example look at
> > > arch/arm/mach-mx3/mach-pcm037.c::pcm037_camera_alloc_dma().
> 
> Yes, this is the solution that suffers from the already discussed limitation 
> of not being able to remap a memory with different attributes, which affects 
> OMAP1 as well.
> 
> > For preallocating dma-coherent memory for device personal use during device
> > probe() time (when the memory is less fragmented compared to open() time)
> > see also dt3155_alloc_coherent/dt3155_free_coherent in
> > drivers/staging/dt3155v4l/dt3155vfl.c (for x86 arch, I do not know if it
> > works for arm arch)
> 
> With this workaround applied, I get much better results, thank you Marin. 
> However, it seems not bullet proof, since mmap still happens to fail for a 
> reason not quite clear to me:
> 

This is just a preallocation of coherent memory kept for further private driver use, 
should not be connected to mmap problem.

> 
> Maybe I should preallocate a few more pages than will be actually used by the 
> driver?
> 
> Anyways, I'm not sure if this piece of code could be accepted for inclusion 
> into the mainline tree, perhaps only under drivers/staging.

The idea for the piece of code I have proposed to you is taken from the functions
dma_declare_coherent_memory()/dma_release_declared_memory() in mainline 
drivers/base/dma-coherent.c

> 
> Thanks,
> Janusz
>