[Intel-gfx] [PATCH 2/2] drm/i915: Fix the return value of pipe crc read function.

[Vivi, Rodrigo]

On Wed, 2016-08-03 at 10:31 +0300, Ville Syrjälä wrote:
> On Tue, Aug 02, 2016 at 09:42:07PM -0700, Rodrigo Vivi wrote:
> > 
> > A read(fd, buf, len) function should return the number
> > of bytes read. In our case we need to return the
> > number of bytes we copy to user, instead of returning
> > the number of bytes we read internally.
> > 
> > It was really strange when I saw i-g-t test case using
> > len '54' but getting '56' as return. First thought was
> > how do we read more than we asked? But also I checked
> > and there was really only 54. Until I realized it
> > was all our fault. EFAULT!
> > 
> > Signed-off-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
> > ---
> >  drivers/gpu/drm/i915/i915_debugfs.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_debugfs.c
> > b/drivers/gpu/drm/i915/i915_debugfs.c
> > index 7052c47..b7b8d79 100644
> > --- a/drivers/gpu/drm/i915/i915_debugfs.c
> > +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> > @@ -3659,7 +3659,7 @@ i915_pipe_crc_read(struct file *filep, char
> > __user *user_buf, size_t count,
> >  
> >  	spin_unlock_irq(&pipe_crc->lock);
> >  
> > -	return bytes_read;
> > +	return PIPE_CRC_LINE_LEN;
> Nope. We can read multiple entries in one go.
> 
> bytes_read += snprintf()
> 
> so it's mostly good. The only case where it fails if we don't have
> enough space for the snprintf(), as snprintf() will then return the
> number of bytes it would have written. That can actually happen on
> account of the hex vs. decimal mess with the frame counter.

Oh, I see.

So what about:
 if (bytes_read > count)
	return -E-something?

> 
> > 
> >  }
> >  
> >  static const struct file_operations i915_pipe_crc_fops = {
> > -- 
> > 2.5.5
> > 
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