Re: [PATCH v2 3/6] bus: mhi: core: Add support to stop or start channel data transfers

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

On Mon, Nov 16, 2020 at 12:56:16PM -0800, Bhaumik Bhatt wrote:
> Hi Mani,
> 
> On 2020-11-16 04:43, Manivannan Sadhasivam wrote:
> > On Wed, Nov 11, 2020 at 11:21:10AM -0800, Bhaumik Bhatt wrote:
> > > Some MHI client drivers may want to request a pause or halt of
> > > data transfer activity on their channels. Support for this does
> > > not exist and must be introduced, wherein the channel context is
> > > not reset or cleared but only the STOP channel command is issued.
> > > This would need to be paired with an API that allows resuming the
> > > data transfer activity on channels by use of the START channel
> > > command. This API assumes that the context information is already
> > > setup. Enable this using two new APIs, mhi_start_transfer() and
> > > mhi_stop_transfer().
> > > 
> > > Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>
> > > ---
> > >  drivers/bus/mhi/core/main.c | 41
> > > +++++++++++++++++++++++++++++++++++++++++
> > >  include/linux/mhi.h         | 19 +++++++++++++++++++
> > >  2 files changed, 60 insertions(+)
> > > 
> > > diff --git a/drivers/bus/mhi/core/main.c b/drivers/bus/mhi/core/main.c
> > > index 1226933..1a969f4 100644
> > > --- a/drivers/bus/mhi/core/main.c
> > > +++ b/drivers/bus/mhi/core/main.c
> > > @@ -1560,6 +1560,47 @@ void mhi_unprepare_from_transfer(struct
> > > mhi_device *mhi_dev)
> > >  }
> > >  EXPORT_SYMBOL_GPL(mhi_unprepare_from_transfer);
> > > 
> > > +static int mhi_update_transfer_state(struct mhi_device *mhi_dev,
> > > +				     enum mhi_ch_state_type to_state)
> > > +{
> > > +	struct mhi_controller *mhi_cntrl = mhi_dev->mhi_cntrl;
> > > +	struct mhi_chan *mhi_chan;
> > > +	int dir, ret;
> > > +
> > > +	for (dir = 0; dir < 2; dir++) {
> > > +		mhi_chan = dir ? mhi_dev->ul_chan : mhi_dev->dl_chan;
> > > +
> > > +		if (!mhi_chan)
> > > +			continue;
> > > +
> > > +		/*
> > > +		 * Bail out if one of the channels fail as client will reset
> > > +		 * both upon failure
> > > +		 */
> > > +		mutex_lock(&mhi_chan->mutex);
> > 
> > Hmm. The documentation about wait_for_completion*() used in
> > mhi_update_channel_state()says below,
> > 
> > "As all variants of wait_for_completion() can (obviously) block for a
> > long
> > time depending on the nature of the activity they are waiting for, so in
> > most cases you probably don't want to call this with held mutexes."
> > 
> Yes, that is understood. The mhi_chan->mutex is only used to lock any
> channel
> enable/start/stop/disable type operations, since these have to be in order,
> it
> is essential that we wait for one of the operations to finish before the
> next
> one.
> 
> Also we avoid a race, for example, at a time when a device crash forces a
> driver
> "remove" call, while an operation to start/stop a channel is already going
> on.

Can't you just drop the lock before calling wait_for_completion() and
acquire later? You should add a comment for that also!

> > > +		ret = mhi_update_channel_state(mhi_cntrl, mhi_chan, to_state);
> > > +		if (ret) {
> > > +			mutex_unlock(&mhi_chan->mutex);
> > > +			return ret;
> > > +		}
> > > +		mutex_unlock(&mhi_chan->mutex);
> > > +	}
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +int mhi_stop_transfer(struct mhi_device *mhi_dev)
> > > +{
> > > +	return mhi_update_transfer_state(mhi_dev, MHI_CH_STATE_TYPE_STOP);
> > > +}
> > > +EXPORT_SYMBOL_GPL(mhi_stop_transfer);
> > > +
> > > +int mhi_start_transfer(struct mhi_device *mhi_dev)
> > > +{
> > > +	return mhi_update_transfer_state(mhi_dev, MHI_CH_STATE_TYPE_START);
> > > +}
> > > +EXPORT_SYMBOL_GPL(mhi_start_transfer);
> > > +
> > >  int mhi_poll(struct mhi_device *mhi_dev, u32 budget)
> > >  {
> > >  	struct mhi_controller *mhi_cntrl = mhi_dev->mhi_cntrl;
> > > diff --git a/include/linux/mhi.h b/include/linux/mhi.h
> > > index 52b3c60..aee8494 100644
> > > --- a/include/linux/mhi.h
> > > +++ b/include/linux/mhi.h
> > > @@ -702,6 +702,25 @@ int mhi_prepare_for_transfer(struct mhi_device
> > > *mhi_dev);
> > >  void mhi_unprepare_from_transfer(struct mhi_device *mhi_dev);
> > > 
> > >  /**
> > > + * mhi_stop_transfer - Pauses ongoing channel activity by issuing
> > > the STOP
> > > + *                     channel command to both UL and DL channels.
> > > This command
> > > + *                     does not reset the channel context and the
> > > client drivers
> > > + *                     can issue mhi_start_transfer to resume
> > > activity.
> > > + * @mhi_dev: Device associated with the channels
> > > + */
> > > +int mhi_stop_transfer(struct mhi_device *mhi_dev);
> > > +
> > > +/**
> > > + * mhi_start_transfer - Resumes channel activity by issuing the
> > > START channel
> > > + *                      command to both UL and DL channels. This
> > > command assumes
> > > + *                      the channel context is already setup and
> > > the client
> > > + *                      drivers can issue mhi_stop_transfer to
> > > pause activity if
> > > + *                      required.
> > > + * @mhi_dev: Device associated with the channels
> > > + */
> > > +int mhi_start_transfer(struct mhi_device *mhi_dev);
> > > +
> > > +/**
> > 
> > Align the comment header properly.
> > 
> So I am trying to follow the documentation style for other functions in the
> same
> file. Is there any particular format you want me to refer to?
> 
> I use all spaces for the lines after the first one to align them just like
> the
> rest of them.
> 

The diff shows me of below style:

/**
+ *
+ *
...
+ /**

I just asked to fix this.

Thanks,
Mani

> > Thanks,
> > Mani
> > 
> > >   * mhi_poll - Poll for any available data in DL direction
> > >   * @mhi_dev: Device associated with the channels
> > >   * @budget: # of events to process
> > > --
> > > The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
> > > Forum,
> > > a Linux Foundation Collaborative Project
> > > 
> 
> Thanks,
> Bhaumik
> -- 
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project