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

[Dan Carpenter]

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