Re: [PATCH v3 22/25] bus: mhi: ep: Add support for processing transfer ring

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

On Tue, Feb 15, 2022 at 04:40:18PM -0600, Alex Elder wrote:
> On 2/12/22 12:21 PM, Manivannan Sadhasivam wrote:
> > Add support for processing the transfer ring from host. For the transfer
> > ring associated with DL channel, the xfer callback will simply invoked.
> > For the case of UL channel, the ring elements will be read in a buffer
> > till the write pointer and later passed to the client driver using the
> > xfer callback.
> > 
> > The client drivers should provide the callbacks for both UL and DL
> > channels during registration.
> 
> I think you already checked and guaranteed that.
> 
> I have a question and suggestion below.  But it could
> be considered an optimization that could be implemented
> in the future, so:
> 
> Reviewed-by: Alex Elder <elder@linaro.org>
> 
> > 
> > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > ---
> >   drivers/bus/mhi/ep/main.c | 49 +++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 49 insertions(+)
> > 
> > diff --git a/drivers/bus/mhi/ep/main.c b/drivers/bus/mhi/ep/main.c
> > index b937c6cda9ba..baf383a4857b 100644
> > --- a/drivers/bus/mhi/ep/main.c
> > +++ b/drivers/bus/mhi/ep/main.c
> > @@ -439,6 +439,55 @@ static int mhi_ep_read_channel(struct mhi_ep_cntrl *mhi_cntrl,
> >   	return 0;
> >   }
> > +int mhi_ep_process_tre_ring(struct mhi_ep_ring *ring, struct mhi_ep_ring_element *el)
> > +{
> > +	struct mhi_ep_cntrl *mhi_cntrl = ring->mhi_cntrl;
> > +	struct mhi_result result = {};
> > +	u32 len = MHI_EP_DEFAULT_MTU;
> > +	struct mhi_ep_chan *mhi_chan;
> > +	int ret;
> > +
> > +	mhi_chan = &mhi_cntrl->mhi_chan[ring->ch_id];
> > +
> > +	/*
> > +	 * Bail out if transfer callback is not registered for the channel.
> > +	 * This is most likely due to the client driver not loaded at this point.
> > +	 */
> > +	if (!mhi_chan->xfer_cb) {
> > +		dev_err(&mhi_chan->mhi_dev->dev, "Client driver not available\n");
> > +		return -ENODEV;
> > +	}
> > +
> > +	if (ring->ch_id % 2) {
> > +		/* DL channel */
> > +		result.dir = mhi_chan->dir;
> > +		mhi_chan->xfer_cb(mhi_chan->mhi_dev, &result);
> > +	} else {
> > +		/* UL channel */
> > +		do {
> > +			result.buf_addr = kzalloc(len, GFP_KERNEL);
> 
> So you allocate an 8KB buffer into which you copy
> received data, then pass that to the ->xfer_cb()
> function.  Then you free that buffer.  Repeatedly.
> 
> Two questions about this:
> - This suggests that after copying the data in, the
>   ->xfer_cb() function will copy it again, is that
>   correct?
> - If that is correct, why not just reuse the same 8KB
>   buffer, allocated once outside the loop?
> 

The allocation was moved into the loop so that the TRE length buffer could be
allocated but I somehow decided to allocate the Max length buffer. So this could
be moved outside of the loop.

Thanks,
Mani

> It might also be nice to consider whether you could
> allocate the buffer here and have the ->xfer_cb()
> function be responsible for freeing it (and ideally,
> pass it along rather than copying it again).
> 
> > +			if (!result.buf_addr)
> > +				return -ENOMEM;
> > +
> > +			ret = mhi_ep_read_channel(mhi_cntrl, ring, &result, len);
> > +			if (ret < 0) {
> > +				dev_err(&mhi_chan->mhi_dev->dev, "Failed to read channel\n");
> > +				kfree(result.buf_addr);
> > +				return ret;
> > +			}
> > +
> > +			result.dir = mhi_chan->dir;
> > +			mhi_chan->xfer_cb(mhi_chan->mhi_dev, &result);
> > +			kfree(result.buf_addr);
> > +			result.bytes_xferd = 0;
> > +
> > +			/* Read until the ring becomes empty */
> > +		} while (!mhi_ep_queue_is_empty(mhi_chan->mhi_dev, DMA_TO_DEVICE));
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> >   static int mhi_ep_cache_host_cfg(struct mhi_ep_cntrl *mhi_cntrl)
> >   {
> >   	struct device *dev = &mhi_cntrl->mhi_dev->dev;
>