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Date:   Mon, 16 Nov 2020 12:56:16 -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.
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In-Reply-To: <20201116124332.GK3926@Mani-XPS-13-9360>
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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.
>> +		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.

> 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