[bug report] mt76: implement functions to get the response skb for MCU calls

[Dan Carpenter]

I would like to revisit this question.  Last time I complained about
this Johannes responded but he misread what mt76_mcu_send_and_get_msg()
does.  I have looked at it as well and I also cannot explain what is
going on in that function.

I have looked at the callers and my first instinct is that maybe this
is dead stub code?  But then when I look at mt76x02u_mcu_send_msg() I
think "No, this is not stub code.  This should be returning the newly
allocated skb to the caller."

But then I think, surely at some point someone tested this code???  It
must be stub code.

Could we get some clarity on this?

regards,
dan carpenter

On Fri, Oct 08, 2021 at 05:27:35PM +0300, Dan Carpenter wrote:
> On Fri, Oct 08, 2021 at 04:03:10PM +0200, Johannes Berg wrote:
> > On Fri, 2021-10-08 at 16:00 +0300, Dan Carpenter wrote:
> > > 
> > >     1146         ret = mt76_mcu_send_and_get_msg(&dev->mt76, MCU_EXT_CMD_EFUSE_ACCESS, &req,
> > >     1147                                         sizeof(req), true, &skb);
> > > 
> > > If mt76_mcu_send_and_get_msg() calls the dev->mcu_ops->mcu_send_msg()
> > > then "skb" is not initialized.
> > > 
> > >     1148         if (ret)
> > >     1149                 return ret;
> > >     1150 
> > > --> 1151         res = (struct mt7921_mcu_eeprom_info *)skb->data;
> > 
> > Looks like possibly 'skb' is always initialized if
> > mt76_mcu_send_and_get_msg() returns 0 (success)?
> > 
> 
> This build is with cross function analysis enabled so Smatch looks for
> that.
> 
> The problem is that the caller has to know if dev->mcu_ops->mcu_send_msg
> is NULL or not because if it's non-NULL "skb" is not set.  Perhaps that
> means it should be separated into two functions and we pick which one
> to call depending on whether the pointer is set.
> 
> drivers/net/wireless/mediatek/mt76/mcu.c
>     54  int mt76_mcu_send_and_get_msg(struct mt76_dev *dev, int cmd, const void *data,
>     55                                int len, bool wait_resp, struct sk_buff **ret_skb)
>                                                                                 ^^^^^^^
> This is the parameter.
> 
>     56  {
>     57          struct sk_buff *skb;
>     58  
>     59          if (dev->mcu_ops->mcu_send_msg)
>     60                  return dev->mcu_ops->mcu_send_msg(dev, cmd, data, len, wait_resp);
> 
> The function pointer doesn't set *ret_skb at all.
> 
>     61  
>     62          skb = mt76_mcu_msg_alloc(dev, data, len);
>     63          if (!skb)
>     64                  return -ENOMEM;
>     65  
>     66          return mt76_mcu_skb_send_and_get_msg(dev, skb, cmd, wait_resp, ret_skb);
> 
> But this does.
> 
>     67  }
> 
> regards,
> dan carpenter