[v3,21/25] bus: mhi: ep: Add support for reading from the host

Data transfer between host and the ep device happens over the transfer
ring associated with each bi-directional channel pair. Host defines the
transfer ring by allocating memory for it. The read and write pointer
addresses of the transfer ring are stored in the channel context.

Once host places the elements in the transfer ring, it increments the
write pointer and rings the channel doorbell. Device will receive the
doorbell interrupt and will process the transfer ring elements.

This commit adds support for reading the transfer ring elements from
the transfer ring till write pointer, incrementing the read pointer and
finally sending the completion event to the host through corresponding
event ring.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
---
 drivers/bus/mhi/ep/main.c | 103 ++++++++++++++++++++++++++++++++++++++
 include/linux/mhi_ep.h    |   9 ++++
 2 files changed, 112 insertions(+)

On 2/12/22 12:21 PM, Manivannan Sadhasivam wrote:
> Data transfer between host and the ep device happens over the transfer
> ring associated with each bi-directional channel pair. Host defines the
> transfer ring by allocating memory for it. The read and write pointer
> addresses of the transfer ring are stored in the channel context.
> 
> Once host places the elements in the transfer ring, it increments the
> write pointer and rings the channel doorbell. Device will receive the
> doorbell interrupt and will process the transfer ring elements.
> 
> This commit adds support for reading the transfer ring elements from
> the transfer ring till write pointer, incrementing the read pointer and
> finally sending the completion event to the host through corresponding
> event ring.
> 
> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>

Indentation nits mentioned.

Reviewed-by: Alex Elder <elder@linaro.org>

> ---
>   drivers/bus/mhi/ep/main.c | 103 ++++++++++++++++++++++++++++++++++++++
>   include/linux/mhi_ep.h    |   9 ++++
>   2 files changed, 112 insertions(+)
> 
> diff --git a/drivers/bus/mhi/ep/main.c b/drivers/bus/mhi/ep/main.c
> index 4c2ee517832c..b937c6cda9ba 100644
> --- a/drivers/bus/mhi/ep/main.c
> +++ b/drivers/bus/mhi/ep/main.c
> @@ -336,6 +336,109 @@ int mhi_ep_process_cmd_ring(struct mhi_ep_ring *ring, struct mhi_ep_ring_element
>   	return ret;
>   }
>   
> +bool mhi_ep_queue_is_empty(struct mhi_ep_device *mhi_dev, enum dma_data_direction dir)
> +{
> +	struct mhi_ep_chan *mhi_chan = (dir == DMA_FROM_DEVICE) ? mhi_dev->dl_chan :
> +								mhi_dev->ul_chan;
> +	struct mhi_ep_cntrl *mhi_cntrl = mhi_dev->mhi_cntrl;
> +	struct mhi_ep_ring *ring = &mhi_cntrl->mhi_chan[mhi_chan->chan].ring;
> +
> +	return !!(ring->rd_offset == ring->wr_offset);
> +}
> +EXPORT_SYMBOL_GPL(mhi_ep_queue_is_empty);
> +
> +static int mhi_ep_read_channel(struct mhi_ep_cntrl *mhi_cntrl,
> +				struct mhi_ep_ring *ring,
> +				struct mhi_result *result,
> +				u32 len)
> +{
> +	struct mhi_ep_chan *mhi_chan = &mhi_cntrl->mhi_chan[ring->ch_id];
> +	size_t bytes_to_read, read_offset, write_offset;
> +	struct device *dev = &mhi_cntrl->mhi_dev->dev;
> +	struct mhi_ep_ring_element *el;
> +	bool td_done = false;
> +	void *write_to_loc;
> +	u64 read_from_loc;
> +	u32 buf_remaining;
> +	int ret;
> +
> +	buf_remaining = len;
> +
> +	do {
> +		/* Don't process the transfer ring if the channel is not in RUNNING state */
> +		if (mhi_chan->state != MHI_CH_STATE_RUNNING)
> +			return -ENODEV;
> +
> +		el = &ring->ring_cache[ring->rd_offset];
> +
> +		/* Check if there is data pending to be read from previous read operation */
> +		if (mhi_chan->tre_bytes_left) {
> +			dev_dbg(dev, "TRE bytes remaining: %d\n", mhi_chan->tre_bytes_left);
> +			bytes_to_read = min(buf_remaining, mhi_chan->tre_bytes_left);
> +		} else {
> +			mhi_chan->tre_loc = MHI_EP_TRE_GET_PTR(el);
> +			mhi_chan->tre_size = MHI_EP_TRE_GET_LEN(el);
> +			mhi_chan->tre_bytes_left = mhi_chan->tre_size;
> +
> +			bytes_to_read = min(buf_remaining, mhi_chan->tre_size);
> +		}
> +
> +		read_offset = mhi_chan->tre_size - mhi_chan->tre_bytes_left;
> +		write_offset = len - buf_remaining;
> +		read_from_loc = mhi_chan->tre_loc + read_offset;
> +		write_to_loc = result->buf_addr + write_offset;
> +
> +		dev_dbg(dev, "Reading %zd bytes from channel (%d)\n", bytes_to_read, ring->ch_id);
> +		ret = mhi_cntrl->read_from_host(mhi_cntrl, read_from_loc, write_to_loc,
> +						bytes_to_read);
> +		if (ret < 0)
> +			return ret;
> +
> +		buf_remaining -= bytes_to_read;
> +		mhi_chan->tre_bytes_left -= bytes_to_read;
> +
> +		/*
> +		 * Once the TRE (Transfer Ring Element) of a TD (Transfer Descriptor) has been
> +		 * read completely:
> +		 *
> +		 * 1. Send completion event to the host based on the flags set in TRE.
> +		 * 2. Increment the local read offset of the transfer ring.

Your comments in this section explain some things that
I did not completely understand for a *very* long time.
The same flags are used in IPA, but are not as well
documented as they are for MHI.

> +		 */
> +		if (!mhi_chan->tre_bytes_left) {
> +			/*
> +			 * The host will split the data packet into multiple TREs if it can't fit
> +			 * the packet in a single TRE. In that case, CHAIN flag will be set by the
> +			 * host for all TREs except the last one.
> +			 */
> +			if (MHI_EP_TRE_GET_CHAIN(el)) {
> +				/*
> +				 * IEOB (Interrupt on End of Block) flag will be set by the host if
> +				 * it expects the completion event for all TREs of a TD.
> +				 */
> +				if (MHI_EP_TRE_GET_IEOB(el))
> +					mhi_ep_send_completion_event(mhi_cntrl,
> +					ring, MHI_EP_TRE_GET_LEN(el), MHI_EV_CC_EOB);

Check your indentation above.

> +			} else {
> +				/*
> +				 * IEOT (Interrupt on End of Transfer) flag will be set by the host
> +				 * for the last TRE of the TD and expects the completion event for
> +				 * the same.
> +				 */
> +				if (MHI_EP_TRE_GET_IEOT(el))
> +					mhi_ep_send_completion_event(mhi_cntrl,
> +					ring, MHI_EP_TRE_GET_LEN(el), MHI_EV_CC_EOT);

Indentation here too.

> +				td_done = true;
> +			}
> +
> +			mhi_ep_ring_inc_index(ring);
> +		}
> +
> +		result->bytes_xferd += bytes_to_read;
> +	} while (buf_remaining && !td_done);
> +
> +	return 0;
> +}
> +
>   static int mhi_ep_cache_host_cfg(struct mhi_ep_cntrl *mhi_cntrl)
>   {
>   	struct device *dev = &mhi_cntrl->mhi_dev->dev;
> diff --git a/include/linux/mhi_ep.h b/include/linux/mhi_ep.h
> index 276d29fef465..aaf4b6942037 100644
> --- a/include/linux/mhi_ep.h
> +++ b/include/linux/mhi_ep.h
> @@ -268,4 +268,13 @@ int mhi_ep_power_up(struct mhi_ep_cntrl *mhi_cntrl);
>    */
>   void mhi_ep_power_down(struct mhi_ep_cntrl *mhi_cntrl);
>   
> +/**
> + * mhi_ep_queue_is_empty - Determine whether the transfer queue is empty
> + * @mhi_dev: Device associated with the channels
> + * @dir: DMA direction for the channel
> + *
> + * Return: true if the queue is empty, false otherwise.
> + */
> +bool mhi_ep_queue_is_empty(struct mhi_ep_device *mhi_dev, enum dma_data_direction dir);
> +
>   #endif