Re: [PATCH] drm/bridge: analogix_dp: Don't return -EBUSY when msg->size is 0 in aux transaction

[Zain Wang]

Hi Tomasz,

å 2017/2/20 10:40, Tomasz Figa åé:
> Hi Zain,
>
> On Mon, Feb 13, 2017 at 6:27 PM, zain wang <wzz@xxxxxxxxxxxxxx> wrote:
> > The analogix_dp_transfer() will return -EBUSY if num_transferred is zero.
> > But sometimes we will send a bare address packet to start the transaction,
> > like drm_dp_i2c_xfer() show:
> >          ......
> >          /* Send a bare address packet to start the transaction.
> >           * Zero sized messages specify an address only (bare
> >           * address) transaction.
> >           */
> >          msg.buffer = NULL;
> >          msg.size = 0;
> >          err = drm_dp_i2c_do_msg(aux, &msg);
> >          ......
> >
> > In this case, the msg->size is zero, so the num_transferred will be zero too.
> > We can't return -EBUSY here, let's we return num_transferred if num_transferred
> > equals msg->size.
> Please see my question inline.
>
> > Signed-off-by: zain wang <wzz@xxxxxxxxxxxxxx>
> > ---
> >   drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c b/drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c
> > index 303083a..5384aca 100644
> > --- a/drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c
> > +++ b/drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c
> > @@ -1162,5 +1162,5 @@ ssize_t analogix_dp_transfer(struct analogix_dp_device *dp,
> >                   (msg->request & ~DP_AUX_I2C_MOT) == DP_AUX_NATIVE_READ)
> >                  msg->reply = DP_AUX_NATIVE_REPLY_ACK;
> >
> > -       return num_transferred > 0 ? num_transferred : -EBUSY;
> > +       return (num_transferred == msg->size) ? num_transferred : -EBUSY;
> I might be missing something but, looking at the code, I don't see any
> possibility of num_transferred ever being different than msg->size. To
> be honest, it doesn't seem to even make any sense keeping the local
> variable there, because msg->size can be simply always returned, as
> errors are handled by jumping to aux_error label.
Yeah, I agree with you.
The better way to fix this issue is to revert the changes 
https://patchwork.kernel.org/patch/9411711/
(returning num_transferred directly may be better here)

Maybe we can revert the changes above with some new comment.
@Sean, How do you think about Tomasz's comment?

Thanks
Zain
> Best regards,
> Tomasz