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

[Pandiyan, Dhinakaran]

On Wed, 2016-08-03 at 14:44 +0000, Vivi, Rodrigo wrote:
> 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?
We could just return from snprintf() when it does not return
PIPE_CRC_LINE_LEN instead of reading all entries.

Anyway, shouldn't we fix hex to decimal conv.? Or is the value incorrect
when entry->frame in decimal exceeds 8 chars?
> > 
> > > 
> > >  }
> > >  
> > >  static const struct file_operations i915_pipe_crc_fops = {
> > > -- 
> > > 2.5.5
> > > 
> > > _______________________________________________
> > > Intel-gfx mailing list
> > > Intel-gfx at lists.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
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