Re: O_DIRECT

["Peter T. Breuer" <ptb@it.uc3m.es>]

"A month of sundays ago kernel@street-vision.com wrote:"
> > What functions does a block driver have to implement in order to
> > support read/write when it has been opened with O_DIRECT from user
> > space.
> > 
> > I have made some experiments with plain read/write after opening with 
> > O_DIRECT:
> > 
> > 2.5.31:
> >    /dev/ram0      open fails
> >    file on ext2   r/w gives EINVAL
> >    /dev/hdaN      works
> > 
> > 2.4.19:
> >    /dev/ram0      r/w gives EINVAL
> >    file on ext2   r/w gives EINVAL
> >    /dev/hdaN      r/w gives EINVAL
> > 
> > WTF? It's not a library issue - strace shows the syscalls happening
> > with the right flag set on the open.
> 
> You should be able to get it to work on ext2. It works fine for me.
> Remeber that the memory you read/write from must be page aligned (ie
> mmap /dev/zero not malloc) and reads and writes must be multiples of the
> page size. I think block devices work on 2.4 too, but I forget (otherwise
> you can use raw devices).

Thanks for the input. Well, I used the same test program on all, and
the buffer was aligned at 512 bytes (because I intended it to work
with raw character devices too). You are saying that it was luck .. OK,
I'll retest in a little while.

So I simply have to do "nothing" in the driver?

Peter