Re: [PATCH 2/2] mmc: rtsx: add support for async request

[micky <micky_ching@realsil.com.cn>]

Hi Uffe,

do you accepted this patch?
if accepted, I will submit again with jones ack.

Best Regards.
micky.

On 06/19/2014 09:57 AM, micky wrote:
> On 06/18/2014 07:03 PM, Ulf Hansson wrote:
>> On 18 June 2014 12:08, micky <micky_ching@realsil.com.cn> wrote:
>>> On 06/18/2014 03:39 PM, Ulf Hansson wrote:
>>>> On 18 June 2014 03:17, micky <micky_ching@realsil.com.cn> wrote:
>>>>> On 06/17/2014 03:45 PM, Ulf Hansson wrote:
>>>>>> On 17 June 2014 03:04, micky <micky_ching@realsil.com.cn> wrote:
>>>>>>> On 06/16/2014 08:40 PM, Ulf Hansson wrote:
>>>>>>>> On 16 June 2014 11:09, micky <micky_ching@realsil.com.cn> wrote:
>>>>>>>>> On 06/16/2014 04:42 PM, Ulf Hansson wrote:
>>>>>>>>>>> @@ -36,7 +37,10 @@ struct realtek_pci_sdmmc {
>>>>>>>>>>>>             struct rtsx_pcr         *pcr;
>>>>>>>>>>>>             struct mmc_host         *mmc;
>>>>>>>>>>>>             struct mmc_request      *mrq;
>>>>>>>>>>>> +       struct workqueue_struct *workq;
>>>>>>>>>>>> +#define SDMMC_WORKQ_NAME "rtsx_pci_sdmmc_workq"
>>>>>>>>>>>>
>>>>>>>>>>>> +       struct work_struct      work;
>>>>>>>>>> I am trying to understand why you need a work/workqueue to 
>>>>>>>>>> implement
>>>>>>>>>> this feature. Is that really the case?
>>>>>>>>>>
>>>>>>>>>> Could you elaborate on the reasons?
>>>>>>>>> Hi Uffe,
>>>>>>>>>
>>>>>>>>> we need return as fast as possible in mmc_host_ops
>>>>>>>>> request(ops->request)
>>>>>>>>> callback,
>>>>>>>>> so the mmc core can continue handle next request.
>>>>>>>>> when next request everything is ready, it will wait previous 
>>>>>>>>> done(if
>>>>>>>>> not
>>>>>>>>> done),
>>>>>>>>> then call ops->request().
>>>>>>>>>
>>>>>>>>> we can't use atomic context, because we use mutex_lock() to 
>>>>>>>>> protect
>>>>>>>> ops->request should never executed in atomic context. Is that your
>>>>>>>> concern?
>>>>>>> Yes.
>>>>>> Okay. Unless I missed your point, I don't think you need the
>>>>>> work/workqueue.
>>>>> any other method?
>>>>>
>>>>>> Because, ops->request isn't ever executed in atomic context. That's
>>>>>> due to the mmc core, which handles the async mechanism, are waiting
>>>>>> for a completion variable in process context, before it invokes the
>>>>>> ops->request() callback.
>>>>>>
>>>>>> That completion variable will be kicked, from your host driver, when
>>>>>> you invoke mmc_request_done(), .
>>>>> Sorry, I don't understand here, how kicked?
>>>> mmc_request_done()
>>>>       ->mrq->done()
>>>>           ->mmc_wait_done()
>>>>               ->complete(&mrq->completion);
>>>>
>>>>> I think the flow is:
>>>>> - not wait for first req
>>>>> - init mrq->done
>>>>> - ops->request()                         --- A.rtsx: start queue
>>>>> work.
>>>>> - continue fetch next req
>>>>> - prepare next req ok,
>>>>> - wait previous done.                --- B.(mmc_request_done() may be
>>>>> called
>>>>> at any time from A to B)
>>>>> - init mrq->done
>>>>> - ops->request()                         --- C.rtsx: start queue
>>>>> next work.
>>>>> ...
>>>>> and seems no problem.
>>>> Right, I don't think there are any _problem_ by using the workqueue as
>>>> you have implemented, but I am questioning if it's correct. Simply
>>>> because I don't think there are any reasons to why you need a
>>>> workqueue, it doesn't solve any problem for you - it just adds
>>>> overhead.
>>> Hi Uffe,
>>>
>>> we have two driver under mfd, the rtsx-mmc and rtsx-ms,
>>> we use mutex lock(pcr_mutex) to protect resource,
>>> when we handle mmc request, we need hold the mutex until we finish the
>>> request,
>>> so it will not interruptted by rtsx-ms request.
>> Ahh, I see. Now, _that_ explains why you want the workqueue. :-) Thanks!
>>
>>> If we not use workq, once the request hold the mutex, we have to 
>>> wait until
>>> the request finish,
>>> then release mutex, so the mmc core is blocking at here.
>>> To implement nonblocking request, we have to use workq.
>> One minor suggestion below, please consider this as an optimization
>> which goes outside the context of this patch.
>>
>> There are cases when I think you should be able to skip the overhead
>> from scheduling the work from ->request(). Those cases can be
>> described as when the mutex are available which can be tested by using
>> mutex_trylock().
> Thanks for your suggestion.
>
> we need schedule the work every time mmc core call ops->request(),
> if we want to handle request, we need hold mutex and do the work.
> so mutex_trylock() will not help decrease overhead.
> if we not schedule the work, the ops->request will do nothing.
>
> Best Regards.
> micky
>> Kind regards
>> Uffe
>> .
>>
>