Re: [PATCH v3 12/25] bus: mhi: ep: Add support for ring management

Alex Elder
2022-02-18T15:39:20Z

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