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Date:   Mon, 23 Nov 2020 16:45:37 -0800
From:   Bhaumik Bhatt <bbhatt@codeaurora.org>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc:     linux-arm-msm@vger.kernel.org, hemantk@codeaurora.org,
        jhugo@codeaurora.org, loic.poulain@linaro.org,
        kvalo@codeaurora.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 3/6] bus: mhi: core: Add support to stop or start
 channel data transfers
Organization: Qualcomm Innovation Center, Inc.
Reply-To: bbhatt@codeaurora.org
Mail-Reply-To: bbhatt@codeaurora.org
In-Reply-To: <20201121170519.GA2343@thinkpad>
References: <1605122473-12179-1-git-send-email-bbhatt@codeaurora.org>
 <1605122473-12179-4-git-send-email-bbhatt@codeaurora.org>
 <20201116124332.GK3926@Mani-XPS-13-9360>
 <3bf88d90e4006ba17e2e86c76a926581@codeaurora.org>
 <20201121170519.GA2343@thinkpad>
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On 2020-11-21 09:05 AM, Manivannan Sadhasivam wrote:
> 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!
> 
So, based on the MHI device sending a command completion, we do not 
expect to
receive it out of order and for tighter control, we also do not want to 
allow
multiple commands to be issued unless one has been responded to or times 
out due
to reasons outside of MHI's control.

We share a common completion variable for a channel and multiple 
commands issued
for channel state changes can result in the completions being 
mishandled.

For example, if we send two commands for the same channel - stop and 
reset and
end up dropping/re-acquiring the lock while waiting for stop channel 
completion
as a response, we could see a reinit_completion() before the complete() 
and the
waiter keeps waiting as a result because the complete() is not seen till 
the
next command response comes in. We may also have to move to 
complete_all() for
this purpose, as these could come from different threads.

Another example is a failure to move states could be seen when dropping 
and
re-acquiring the lock if stop and start channel commands are issued back 
to back
whereas we will not see the issue with current behavior as a start will 
wait for
a stop to fully complete.
>> > > +		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.
> 
I don't see an extra space. Is that it? All functions I see are of the 
style:
/**
  * foo - function desc
  *
  * @param: Desc
  *
  */
> 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

Thanks,
Bhaumik
---
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora 
Forum,
a Linux Foundation Collaborative Project