linux-kernel - Re: [PATCH v3 15/25] bus: mhi: ep: Add support for processing MHI endpoint interrupts

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Date:   Tue, 22 Feb 2022 08:08:57 -0600
From:   Alex Elder <elder@...aro.org>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc:     mhi@...ts.linux.dev, quic_hemantk@...cinc.com,
        quic_bbhatt@...cinc.com, quic_jhugo@...cinc.com,
        vinod.koul@...aro.org, bjorn.andersson@...aro.org,
        dmitry.baryshkov@...aro.org, quic_vbadigan@...cinc.com,
        quic_cang@...cinc.com, quic_skananth@...cinc.com,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 15/25] bus: mhi: ep: Add support for processing MHI
 endpoint interrupts

On 2/22/22 2:18 AM, Manivannan Sadhasivam wrote:
> On Tue, Feb 15, 2022 at 04:39:30PM -0600, Alex Elder wrote:
>> On 2/12/22 12:21 PM, Manivannan Sadhasivam wrote:
>>> Add support for processing MHI endpoint interrupts such as control
>>> interrupt, command interrupt and channel interrupt from the host.
>>>
>>> The interrupts will be generated in the endpoint device whenever host
>>> writes to the corresponding doorbell registers. The doorbell logic
>>> is handled inside the hardware internally.
>>>
>>> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
>>
>> Unless I'm mistaken, you have some bugs here.
>>
>> Beyond that, I question whether you should be using workqueues
>> for handling all interrupts.  For now, it's fine, but there
>> might be room for improvement after this is accepted upstream
>> (using threaded interrupt handlers, for example).
>>
> 
> Only reason I didn't use bottom halves is that the memory for TRE buffers need
> to be allocated each time, so essentially the caller should not sleep.

Threaded interrupt handlers can sleep.  If scheduled, they run
immediately after hard interrupt handlers.  For receive buffers,
yes, replacing a receive buffer just consumed would require an
allocation, but for transmit I think it might be possible to
avoid the need to do a memory allocation.  (Things to think
about at some future date.)

> This is currently a limitation of iATU where there are only 8 windows for
> mapping the host memory and each memory region size is also limited.

Those are hard limitations, and probably what constrains you the most.

					-Alex
> 
>> 					-Alex
>>
>>> ---
>>>    drivers/bus/mhi/ep/main.c | 113 +++++++++++++++++++++++++++++++++++++-
>>>    include/linux/mhi_ep.h    |   2 +
>>>    2 files changed, 113 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/bus/mhi/ep/main.c b/drivers/bus/mhi/ep/main.c
>>> index ccb3c2795041..072b872e735b 100644
>>> --- a/drivers/bus/mhi/ep/main.c
>>> +++ b/drivers/bus/mhi/ep/main.c
>>> @@ -185,6 +185,56 @@ static void mhi_ep_ring_worker(struct work_struct *work)
>>>    	}
>>>    }
>>> +static void mhi_ep_queue_channel_db(struct mhi_ep_cntrl *mhi_cntrl,
>>> +				    unsigned long ch_int, u32 ch_idx)
>>> +{
>>> +	struct device *dev = &mhi_cntrl->mhi_dev->dev;
>>> +	struct mhi_ep_ring_item *item;
>>> +	struct mhi_ep_ring *ring;
>>> +	unsigned int i;
>>
>> Why not u32 i?  And why is the ch_int argument unsigned long?  The value
>> passed in is a u32.
>>
> 
> for_each_set_bit() expects the 2nd argument to be of type "unsigned long".
> 
> Thanks,
> Mani