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Date:   Fri, 18 Feb 2022 09:39:13 -0600
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Subject: Re: [PATCH v3 12/25] bus: mhi: ep: Add support for ring management
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To:     Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc:     mhi@lists.linux.dev, quic_hemantk@quicinc.com,
        quic_bbhatt@quicinc.com, quic_jhugo@quicinc.com,
        vinod.koul@linaro.org, bjorn.andersson@linaro.org,
        dmitry.baryshkov@linaro.org, quic_vbadigan@quicinc.com,
        quic_cang@quicinc.com, quic_skananth@quicinc.com,
        linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20220212182117.49438-1-manivannan.sadhasivam@linaro.org>
 <20220212182117.49438-13-manivannan.sadhasivam@linaro.org>
 <20504c14-6df5-83d1-53e9-85c682a5a3c7@linaro.org>
 <20220218080704.GD11964@workstation>
From:   Alex Elder <elder@linaro.org>
In-Reply-To: <20220218080704.GD11964@workstation>
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On 2/18/22 2:07 AM, Manivannan Sadhasivam wrote:
> On Tue, Feb 15, 2022 at 02:03:13PM -0600, Alex Elder wrote:
>> On 2/12/22 12:21 PM, Manivannan Sadhasivam wrote:
>>> Add support for managing the MHI ring. The MHI ring is a circular queue
>>> of data structures used to pass the information between host and the
>>> endpoint.
>>>
>>> MHI support 3 types of rings:
>>>
>>> 1. Transfer ring
>>> 2. Event ring
>>> 3. Command ring
>>>
>>> All rings reside inside the host memory and the MHI EP device maps it to
>>> the device memory using blocks like PCIe iATU. The mapping is handled in
>>> the MHI EP controller driver itself.
>>>
>>> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
>>
>> Great explanation.  One more thing to add, is that the command
>> and transfer rings are directed from the host to the MHI EP device,
>> while the event rings are directed from the EP device toward the
>> host.
>>
> 
> That's correct, will add.
> 
>> I notice that you've improved a few things I had notes about,
>> and I don't recall suggesting them.  I'm very happy about that.
>>
>> I have a few more comments here, some worth thinking about
>> at least.
>>
>> 					-Alex
>>
>>> ---
>>>    drivers/bus/mhi/ep/Makefile   |   2 +-
>>>    drivers/bus/mhi/ep/internal.h |  33 +++++
>>>    drivers/bus/mhi/ep/main.c     |  59 +++++++-
>>>    drivers/bus/mhi/ep/ring.c     | 267 ++++++++++++++++++++++++++++++++++
>>>    include/linux/mhi_ep.h        |  11 ++
>>>    5 files changed, 370 insertions(+), 2 deletions(-)
>>>    create mode 100644 drivers/bus/mhi/ep/ring.c
>>>
>>> diff --git a/drivers/bus/mhi/ep/Makefile b/drivers/bus/mhi/ep/Makefile
>>> index a1555ae287ad..7ba0e04801eb 100644
>>> --- a/drivers/bus/mhi/ep/Makefile
>>> +++ b/drivers/bus/mhi/ep/Makefile

. . .

>>> diff --git a/drivers/bus/mhi/ep/main.c b/drivers/bus/mhi/ep/main.c
>>> index 950b5bcabe18..2c8045766292 100644
>>> --- a/drivers/bus/mhi/ep/main.c
>>> +++ b/drivers/bus/mhi/ep/main.c
>>> @@ -18,6 +18,48 @@
>>>    static DEFINE_IDA(mhi_ep_cntrl_ida);
>>
>> The following function handles command or channel interrupt work.
>>
> 
> Both

What I meant was to suggest a comment that stated that it
is used for both of those.  Not really a bit deal though.

>>> +static void mhi_ep_ring_worker(struct work_struct *work)
>>> +{
>>> +	struct mhi_ep_cntrl *mhi_cntrl = container_of(work,
>>> +				struct mhi_ep_cntrl, ring_work);
>>> +	struct device *dev = &mhi_cntrl->mhi_dev->dev;
>>> +	struct mhi_ep_ring_item *itr, *tmp;
>>> +	struct mhi_ep_ring *ring;
>>> +	struct mhi_ep_chan *chan;
>>> +	unsigned long flags;
>>> +	LIST_HEAD(head);
>>> +	int ret;
>>> +
>>> +	/* Process the command ring first */
>>> +	ret = mhi_ep_process_ring(&mhi_cntrl->mhi_cmd->ring);
>>> +	if (ret) {
>>
>> At the moment I'm not sure where this work gets scheduled.
>> But what if there is no command to process?  It looks
>> like you go update the cached pointer no matter what
>> to see if there's anything new.  But it seems like you
>> ought to be able to do this when interrupted for a
>> command rather than all the time.
>>
> 
> No, ring cache is not getting updated all the time. If you look into
> process_ring(), first the write pointer is read from MMIO and there is a
> check to see if there are elements in the ring or not. Only if that
> check passes, the ring cache will get updated.
> 
> Since the same work item is used for both cmd and transfer rings, this
> check is necessary. The other option would be to use different work items
> for command and transfer rings. This is something I want to try once
> this initial version gets merged.

OK.  I accept your explanation (even though I confess I did not
go back and look at the code again...).

Thanks Mani.

					-Alex

> Thanks,
> Mani