SDK16 USB MSC - Understanding working buffers

[Nikosant03]

Hi everyone,

I am working on USB MSC example. I have two questions.

1. What is the MSC_WORKBUFFER_SIZE defines in the example? Is the last parameter of APP_USBD_MSC_GLOBAL_DEF function. The documentation says that is the " Work buffer size (need to fit into all block devices from block device list)", What does it mean?

    a.  What's the purpose? I understand that this is somehow related to the number of block devices?

    b. How does it calculated for each specific project? For instance, why the example defines MSC_WORKBUFFER_SIZE (1024)?

#define MSC_WORKBUFFER_SIZE (1024)

/*lint -save -e26 -e64 -e123 -e505 -e651*/
/**
 * @brief Mass storage class instance
 */
APP_USBD_MSC_GLOBAL_DEF(m_app_msc,
    0,
    msc_user_ev_handler,
    ENDPOINT_LIST(),
    BLOCKDEV_LIST(),
    MSC_WORKBUFFER_SIZE);

2. The second question is related to the f_mkfs() function. 

    a. What is the purpose of the 4th parameter

    b. How does it calculated for each specific project? For instance, why the example sets the working buffer to 512bytes?

static uint8_t buf[512];
ff_result = f_mkfs("", FM_FAT, 1024, buf, sizeof(buf)); 

Thanks in advance

Nick

[Kenneth]

I think I will need to look into question 1, I assume it related to the work buffer size for the mass storage drive, larger should typically be faster.

Regarding 2. From (http://elm-chan.org/fsw/ff/doc/mkfs.html):

work

Pointer to the working buffer used for the format process. When a null pointer is given with FF_USE_LFN == 3, the function obtains a memory block in this function for the working buffer.

len

Size of the working buffer in unit of byte. It needs to be FF_MAX_SS at least. Plenty of working buffer reduces number of write transactions to the drive and the format process will finish quickly.

[Nikosant03]

Thank you for your support Keneth,

Kenneth said:

Regarding 2. From (http://elm-chan.org/fsw/ff/doc/mkfs.html):

work

Pointer to the working buffer used for the format process. When a null pointer is given with FF_USE_LFN == 3, the function obtains a memory block in this function for the working buffer.

len

Size of the working buffer in unit of byte. It needs to be FF_MAX_SS at least. Plenty of working buffer reduces number of write transactions to the drive and the format process will finish quickly.

I've seen this reference, however I cannot understand from what the size of the working buffer is depended from. That is, it should be always 1024? I suppose no..

[Kenneth]

Increasing the buffer size can reduce the number of write transactions to the external flash memory, so bigger will likely be faster. Not sure if that was entirely clear, but it's a third party library so don't have more details than documented: http://elm-chan.org/fsw/ff/doc/appnote.html 

Edit: I could find there is a forum: http://elm-chan.org/fsw/ff/bd/ 

[Nikosant03]

Hi Keneth,

Kenneth said:

Increasing the buffer size can reduce the number of write transactions to the external flash memory, so bigger will likely be faster

I think that's clear to me, thanks!!

Do you have any update regarding the MSC_WORKBUFFER_SIZE defined in the example?

[Kenneth]

MSC_WORKBUFFER_SIZE is the buffer that gets actual payload from the USB. This buffer is subdivided into 2 smaller buffers and they should have the size of a single block. So for a FATFS formatted with 512B sectors the size is 1024, so twice as large as the sector size.

Hope that helps,
Kenneth