staging: comedi: check for more errors for zero-length write

If the "write" file operation handler, `comedi_write()` is passed 0 for
the amount to write, some error conditions are currently skipped and the
function just returns 0.  Change it to check those error conditions and
return an error value if appropriate.  The trickiest case is the check
for when the previously set up asynchronous command has terminated with
an error.  In that case, `-EPIPE` is returned (as it is for a write of
non-zero length) and the subdevice gets marked as non-busy.

A zero-length write that returns 0 has no other effects, in particular,
it does not cause the subdevice to be marked as non-busy.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
index 5a9c9d9782f37e54d483a5f2660ccde2802565e9..d57fadef47fcf8e8b363e3002690d606c8bdf16b 100644 (file)
--- a/drivers/staging/comedi/comedi_fops.c
+++ b/drivers/staging/comedi/comedi_fops.c
@@ -2331,8 +2331,6 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
        }
 
        async = s->async;
-       if (!nbytes)
-               goto out;
        if (s->busy != file || !(async->cmd.flags & CMDF_WRITE)) {
                retval = -EINVAL;
                goto out;
@@ -2349,9 +2347,12 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
                if (!comedi_is_runflags_running(runflags)) {
                        if (comedi_is_runflags_in_error(runflags))
                                retval = -EPIPE;
-                       become_nonbusy = true;
+                       if (retval || nbytes)
+                               become_nonbusy = true;
                        break;
                }
+               if (nbytes == 0)
+                       break;
 
                /* Allocate all free buffer space. */
                comedi_buf_write_alloc(s, async->prealloc_bufsz);