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

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Date:	Wed, 18 Jun 2014 09:39:46 +0200
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	micky <micky_ching@...lsil.com.cn>
Cc:	Samuel Ortiz <sameo@...ux.intel.com>,
	Lee Jones <lee.jones@...aro.org>,
	Chris Ball <chris@...ntf.net>, devel@...uxdriverproject.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	Roger <rogerable@...ltek.com>, Wei WANG <wei_wang@...lsil.com.cn>
Subject: Re: [PATCH 2/2] mmc: rtsx: add support for async request

On 18 June 2014 03:17, micky <micky_ching@...lsil.com.cn> wrote:
> On 06/17/2014 03:45 PM, Ulf Hansson wrote:
>>
>> On 17 June 2014 03:04, micky <micky_ching@...lsil.com.cn> wrote:
>>>
>>> On 06/16/2014 08:40 PM, Ulf Hansson wrote:
>>>>
>>>> On 16 June 2014 11:09, micky <micky_ching@...lsil.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.

Kind regards
Ulf Hansson
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