Re: [PATCH v3 2/3] mailbox: mediatek: add support for adsp mailbox controller

[Tzung-Bi Shih <tzungbi@google.com>]

On Thu, Nov 25, 2021 at 09:51:27AM +0800, allen-kh.cheng wrote:
> On Wed, 2021-11-24 at 18:25 +0800, Tzung-Bi Shih wrote:
> > > On Wed, Nov 24, 2021 at 04:45:13PM +0800, allen-kh.cheng wrote:
> > > > > +static int mtk_adsp_mbox_send_data(struct mbox_chan *chan,
> > > void
> > > > > *data)
> > > > > +{
> > > > > +  struct adsp_mbox_ch_info *ch_info = chan->con_priv;
> > > > > +  void __iomem *reg = ch_info->va_reg;
> > > > > +
> > > > > +  spin_lock(&ch_info->lock);
> > > > > +  writel(ch_info->ipc_op_val, reg + MTK_ADSP_MBOX_IN_CMD);
> > > > > +  spin_unlock(&ch_info->lock);
> > 
> > >
> > > Why does it need the lock?
> > >
> > > Is the write to MTK_ADSP_MBOX_IN_CMD a synchronous operation?
> > > - If yes, I failed to understand why does it need the lock.  Every
> > > calls to mtk_adsp_mbox_send_data() should wait for the data
> > transfer
> > > completion.
> > > - If no, I also failed to understand
> > why.  mtk_adsp_mbox_send_data()
> > > has no way to be aware of the transfer completion.  Would expect a
> > > loop for checking the completion for the case.
> > >
> >  
> 
> In ADSP MBOX IPC flow,
>  
> Host would call mbox send data when the shared data transfer completed.
>  
> (mtk_adsp_mbox_send_data will notice client using MTK_ADSP_MBOX_IN_CMD)
>  
> It’s more like a signal.
>  
> In general case,
>  
> There may be some hosts use the same mbox channel.
>  
> I think it’s better using lock to protect access to
> MTK_ADSP_MBOX_IN_CMD registers

I still failed to understand.  What if 2 hosts notify the same client by writing MTK_ADSP_MBOX_IN_CMD at the same time?