The feature of runtime pm in mmc_get_card& mmc_put_card

今天在porting sandisk 開發的flow，發現有兩個function是目前使用的kernel沒有定義的。
mmc_get_card 以及 mmc_put_card。
google了一下其實就是把原本mmc_claim_host 以及mmc_release_host加上runtime pm相關的function包起來而成。

[PATCH V3 3/4] mmc: block: Enable runtime pm for mmc blkdevice

從這版patch中可以看到mmc_get_card 以及 mmc_put_card的定義。也取代了原本的mmc_claim_host 以及mmc_release_host。

void mmc_get_card(struct mmc_card *card)
{
 pm_runtime_get_sync(&card->dev);
 mmc_claim_host(card->host);
}
EXPORT_SYMBOL(mmc_get_card);

/*
 * This is a helper function, which releases the host and drops the runtime
 * pm reference for the card device.
 */
void mmc_put_card(struct mmc_card *card)
{
 mmc_release_host(card->host);
 pm_runtime_mark_last_busy(&card->dev);
 pm_runtime_put_autosuspend(&card->dev);
}
EXPORT_SYMBOL(mmc_put_card);

從Documentation/power/runtime_pm.txt可以找到這幾個相關function的簡單敘述。

int pm_runtime_get_sync(struct device *dev);

    - increment the device's usage counter, run       

    pm_runtime_resume(dev) and return its result

int pm_runtime_resume(struct device *dev);

    - execute the subsystem-level resume callback for the device; 

   returns 0 on success, 1 if the device's runtime PM status was already 'active' 

   or error code on failure, where -EAGAIN means it may be safe to attempt to

   resume the device again in future, but 'power.runtime_error' should be checked 

   additionally, and -EACCES means that 'power.disable_depth' is different from 0

void pm_runtime_mark_last_busy(struct device *dev);
    - set the power.last_busy field to the current time

int pm_runtime_put_autosuspend(struct device *dev);
    - decrement the device's usage counter; if the result is 0 then run
    pm_request_autosuspend(dev) and return its result

int pm_request_autosuspend(struct device *dev);
    - schedule the execution of the subsystem-level suspend callback for the
    device when the autosuspend delay has expired; if the delay has already
    expired then the work item is queued up immediately

在文件中有下面這麼一段，這邊想提到的是既然mmc_get_card會引用到pm_runtime_resume，根據文件應該在其他地方會先enable runtime pm.

If the default initial runtime PM status of the device (i.e. 'suspended')
reflects the actual state of the device, its bus type's or its driver's
->probe() callback will likely need to wake it up using one of the PM core's
helper functions described in Section 4.  In that case, pm_runtime_resume()
should be used.  Of course, for this purpose the device's runtime PM has to be
enabled earlier by calling pm_runtime_enable().

再來文件中提到下面這段，也就理解了為何在mmc_put_card呼叫這兩個function。

Inactivity is determined based on the power.last_busy field.  Drivers should
call pm_runtime_mark_last_busy() to update this field after carrying out I/O,
typically just before calling pm_runtime_put_autosuspend(). 

Error: Program “/NDK-build” not found in PATH

Recently, I want to develop android applications with opencv. 

After downloaded opencv sdk and import to my eclipse, Some errors occurred.

One of them is Error: Program “/NDK-build” not found in PATH.

My OS is Ubuntu12.04, it is a linux environment, so i modified the Build command: ${NDKROOT}/ndk-build.cmd to ${NDKROOT}/ndk-build. The reason is that *.cmd is used for Windows version.

Second, in my Environment variables,  I don't have NDKROOT, so I add this variable and set the relative path to my ndk folder. 

As a result, Error: Program “/NDK-build” not found in PATH fixed.

分析eMMC packed command in Linux kernel 之 mmc_blk_prep_packed_list

針對device是否有packed能力做一些篩選條件，若沒有的話會直接goto mmc_blk_clear_packed離開此函式。

分析一、

max_blk_count = min(card->host->max_blk_count,

                        card->host->max_req_size >> 9);

        if (unlikely(max_blk_count > 0xffff))

                max_blk_count = 0xffff;

也就是不管如何host端最多最多一次送的blk_count就是65535個blocks.

分析二、

max_phys_segs = queue_max_segments(q);

        req_sectors += blk_rq_sectors(cur);

        phys_segments += cur->nr_phys_segments;

        if (rq_data_dir(cur) == WRITE) {

                req_sectors++;

                phys_segments++;

        }

藉由queue_max_segments得到request_queue的max_segmants值。

藉由blk_rq_sectors得到目前request要傳送sector(512bytes)數目。

/* Number of scatter-gather DMA addr+len pairs after

* physical address coalescing is performed.

*/

unsigned short nr_phys_segments;

藉由nr_phys_segments的註解說明，我們知道紀錄的是scatter-gather DMA addr+len pairs在做完physical address coalescing的數量。

至於physical address coalescing是怎麼樣的行為目前先不探討。

接著如果當前的request是一個寫入要求，req_sectors以及phys_segments各加一。WHY? (直覺想到的是要傳packed command的header，但是若為packed read也是需要header，目前暫留此懸疑。)

再來進入此函數的重頭戲，

while (reqs < max_packed_rw - 1) {

                spin_lock_irq(q->queue_lock);

                next = blk_fetch_request(q);

                spin_unlock_irq(q->queue_lock);

                if (!next)

                        break;

先利用spin_lock機制鎖住此request的request_queue。確保在做blk_fecth_request時的同步問題。

/**

 * blk_fetch_request - fetch a request from a request queue

 * @q: request queue to fetch a request from

 *

 * Description:

 *     Return the request at the top of @q.  The request is started on

 *     return and LLD can start processing it immediately.

 *

 * Return:

 *     Pointer to the request at the top of @q if available.  Null

 *     otherwise.

 *

 * Context:

 *     queue_lock must be held.

 */

struct request *blk_fetch_request(struct request_queue *q)

{

        struct request *rq;

        rq = blk_peek_request(q);

        if (rq)

                blk_start_request(rq);

        return rq;

}

EXPORT_SYMBOL(blk_fetch_request);

分析三、

註解說明其實已經很清楚說明此函數(blk_fetch_request)的任務，這邊我還不了解的是LLD是甚麼? Wiki: LLD(Lower level design, a stage in project management of SW development or other computer-related projects, following the High-level design) 在此LLD是指將此request交給dirver去處理。

分析三-甲、

/**

 * blk_peek_request - peek at the top of a request queue

 * @q: request queue to peek at

 *

 * Description:

 *     Return the request at the top of @q.  The returned request

 *     should be started using blk_start_request() before LLD starts

 *     processing it.

 *

 * Return:

 *     Pointer to the request at the top of @q if available.  Null

 *     otherwise.

 *

 * Context:

 *     queue_lock must be held.

 */

struct request *blk_peek_request(struct request_queue *q)

再度藉由清楚明瞭的註解知道此函數的目的。(良好的註解習慣是優秀程式設計師必備) 另外會再寫一篇詳細分析blk_peek_request跟io-scheduler相關的探討。

分析三-乙、

/**

 * blk_start_request - start request processing on the driver

 * @req: request to dequeue

 *

 * Description:

 *     Dequeue @req and start timeout timer on it.  This hands off the

 *     request to the driver.

 *

 *     Block internal functions which don't want to start timer should

 *     call blk_dequeue_request().

 *

 * Context:

 *     queue_lock must be held.

 */

void blk_start_request(struct request *req)

{

        blk_dequeue_request(req);

        /*

         * We are now handing the request to the hardware, initialize

         * resid_len to full count and add the timeout handler.

         */

        req->resid_len = blk_rq_bytes(req);

        if (unlikely(blk_bidi_rq(req)))

                req->next_rq->resid_len = blk_rq_bytes(req->next_rq);

        blk_add_timer(req);

}

EXPORT_SYMBOL(blk_start_request);

在註解中提到若是不想要啟動timeout timer，應該直接呼叫blk_dequeue_request。呼叫blk_dequeue_request後，取得目前request要傳送/接收的data長度，並且藉由blk_bidi_rq知道此request後是否接著還有request，接著取得下個request要傳送/接收的data長度，我想這邊是為了提升request收送的效率。

/**

 * blk_add_timer - Start timeout timer for a single request

 * @req: request that is about to start running.

 *

 * Notes:

 *    Each request has its own timer, and as it is added to the queue, we

 *    set up the timer. When the request completes, we cancel the timer.

 */

void blk_add_timer(struct request *req)

這邊就不再多做說明blk_add_timer了。

接著有幾個條件會讓程序走向blk_reqeue_request:

1.      下個request是REQ_DISCARD 或是 REQ_FLUSH

2.      下個request的IO方向不同

3.      下個request是reliable write但是device不支援

4.      加上下個request要求的data block count後，大於host最大的block count

5.      加上下個request的nr_phys_segments後，大於host max_phys_segs

再來，我們看看blk_reqeue_request在做甚麼事。

/**

 * blk_requeue_request - put a request back on queue

 * @q:             request queue where request should be inserted

 * @rq:           request to be inserted

 *

 * Description:

 *    Drivers often keep queueing requests until the hardware cannot accept

 *    more, when that condition happens we need to put the request back

 *    on the queue. Must be called with queue lock held.

 */

void blk_requeue_request(struct request_queue *q, struct request *rq)

因此當hardware還能負荷request或是沒有觸動到上面提到的幾個條件，就會持續呼叫list_add_tail將entry加到packed_list中。

/**

 * list_add_tail - add a new entry

 * @new: new entry to be added

 * @head: list head to add it before

 *

 * Insert a new entry before the specified head.

 * This is useful for implementing queues.

 */

static inline void list_add_tail(struct list_head *new, struct list_head *head)

{

        __list_add(new, head->prev, head);

}

離開重頭戲while迴圈後，得知reqs需要執行之數量，呼叫list_add並設定相關packed_num, packed_retries參數後結束整個函式。

/**

 * list_add - add a new entry

 * @new: new entry to be added

 * @head: list head to add it after

 *

 * Insert a new entry after the specified head.

 * This is good for implementing stacks.

 */

static inline void list_add(struct list_head *new, struct list_head *head)

{

        __list_add(new, head, head->next);

}

來看一下，__list_add吧。

static inline void __list_add(struct list_head *new,

                              struct list_head *prev, struct list_head *next)

{

        next->prev = new;   //1

        new->next = next;   //2

        new->prev = prev;   //3

        prev->next = new;   //4

}