Re: [PATCH v2 11/16] bus: mhi: core: Add support for data transfer

[Manivannan Sadhasivam]

Hi Arnd,

On Mon, Feb 17, 2020 at 10:17:51PM +0530, Manivannan Sadhasivam wrote:
> Hi Arnd,
> 
> On Mon, Feb 17, 2020 at 05:13:37PM +0100, Arnd Bergmann wrote:
> > On Fri, Jan 31, 2020 at 2:51 PM Manivannan Sadhasivam
> > <manivannan.sadhasivam@xxxxxxxxxx> wrote:
> > 
> > > @@ -648,6 +715,31 @@ static int parse_ch_cfg(struct mhi_controller *mhi_cntrl,
> > >                 mhi_chan->db_cfg.pollcfg = ch_cfg->pollcfg;
> > >                 mhi_chan->xfer_type = ch_cfg->data_type;
> > >
> > > +               switch (mhi_chan->xfer_type) {
> > > +               case MHI_BUF_RAW:
> > > +                       mhi_chan->gen_tre = mhi_gen_tre;
> > > +                       mhi_chan->queue_xfer = mhi_queue_buf;
> > > +                       break;
> > > +               case MHI_BUF_SKB:
> > > +                       mhi_chan->queue_xfer = mhi_queue_skb;
> > > +                       break;
> > > +               case MHI_BUF_SCLIST:
> > > +                       mhi_chan->gen_tre = mhi_gen_tre;
> > > +                       mhi_chan->queue_xfer = mhi_queue_sclist;
> > > +                       break;
> > > +               case MHI_BUF_NOP:
> > > +                       mhi_chan->queue_xfer = mhi_queue_nop;
> > > +                       break;
> > > +               case MHI_BUF_DMA:
> > > +               case MHI_BUF_RSC_DMA:
> > > +                       mhi_chan->queue_xfer = mhi_queue_dma;
> > > +                       break;
> > > +               default:
> > > +                       dev_err(mhi_cntrl->dev,
> > > +                               "Channel datatype not supported\n");
> > > +                       goto error_chan_cfg;
> > > +               }
> > > +
> > 
> > While looking through the driver to see how the DMA gets handled, I came
> > across the multitude of mhi_queue_* functions, which seems like a
> > layering violation to me, given that they are all implemented by the
> > core code as well, and the client driver needs to be aware of
> > which one to call. Are you able to lift these out of the common interface
> > and make the client driver call these directly, or maybe provide a direct
> > interface based on mhi_buf_info to replace these?
> > 
> 
> It sounds reasonable to me. Let me discuss this internally with Qcom guys to
> see if they have any objections.
> 

I looked into this in detail and found that the queue_xfer callbacks are tied
to the MHI channels. For instance, the callback gets attached to ul_chan (uplink
channel) and dl_chan (downlink channel) during controller registration. And
when the device driver calls the callback, the MHI stack calls respective queue
function for the relevant channel. For instance,

```
int mhi_queue_transfer(struct mhi_device *mhi_dev,
                       enum dma_data_direction dir, void *buf, size_t len,
                       enum mhi_flags mflags)
{
        if (dir == DMA_TO_DEVICE)
                return mhi_dev->ul_chan->queue_xfer(mhi_dev, mhi_dev->ul_chan,
                                                    buf, len, mflags);
        else
                return mhi_dev->dl_chan->queue_xfer(mhi_dev, mhi_dev->dl_chan,
                                                    buf, len, mflags);
}
```

If we use the direct queue API's this will become hard to handle. So, I'll keep
it as it is.

Thanks,
Mani

> Thanks,
> Mani
> 
> >       Arnd