RE: [PATCH] usb: gadget: ffs: don't allow to open with O_NONBLOCK flag

[David Laight]

From: Robert Baldyga
> Hi Michal,
> 
> On 04/01/2015 05:17 PM, Michal Nazarewicz wrote:
> > On Wed, Apr 01 2015, Robert Baldyga <r.baldyga@xxxxxxxxxxx> wrote:
> >> FunctionFS can't support O_NONBLOCK because read/write operatons are
> >> directly translated into USB requests which are asynchoronous, so we
> >> can't know how long we will have to wait for request completion. For
> >> this reason in case of open with O_NONBLOCK flag we return
> >> -EWOULDBLOCK.
> >
> > cant is a bit strong of a word here though.  It can, but in a few
> > cases it doesnt.
> >
> > It kinda saddens me that this undoes all the lines of code that were put
> > into the file to support O_NONBLOCK (e.g. FFS_NO_SETUP path of
> > ffs_ep0_read).
> >
> > Im also worried this may break existing applications which, for better
> > or worse, open the file with O_NONBLOCK.
> >
> > Most importantly though, this does not stop users from using fcntl to
> > set O_NONBLOCK, so if you really want to stop O_NONBLOCK from being set,
> > that path should be checked as well (if possible).
> 
> I want rather to inform users that non-blocking i/o wouldn't work for
> epfiles. Indeed we can handle O_NONBLOCK for ep0 (for the same reason we
> can have poll), but for other epfiles there is no way to check if
> read/write operation can end up in short time. Everything is up to host.

Is that really necessary?
I'm sure there are a lot of device drivers that ignore O_NONBLOCK.

	David

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