Re: [PATCH 00/10] Support for dumb picture keychain

[Michael Karcher]

Am Donnerstag, den 26.03.2009, 10:08 +0100 schrieb Alexandre Julliard:
> > - The FILE_NO_INTERMEDIATE_BUFFER flag seemed to require the O_DIRECT
> >
> > flag during the Unix open. This was proven by having standard Linux code
> > access the device. #1/10 and #9/10 fixed this.
>
> I don't see why you'd need O_DIRECT.

O_DIRECT avoids buffering in the Linux kernel. If that dumb device makes
"block-mapped I/O", i.e. uses read/write requests to perform *actions*
on that device, buffering renders the communication unusable.
> > - Linux requires the I/O buffer for special files (/dev/..) to be
> >
> > aligned on 512 byte boundary to get the correct data. #4/10 corrects
> > that problem for both reading and writing.
>
> That's really ugly.

That's only the case if the device has been opened with O_DIRECT. For
buffered read/write, no alignment is needed.

Regards,
  Michael Karcher