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usb: gadget: storage: make FSG_NUM_BUFFERS variable size

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FSG_NUM_BUFFERS is set to 2 as default.
Usually 2 buffers are enough to establish a good buffering pipeline.
The number may be increased in order to compensate a for bursty VFS
behaviour.

Here follows a description of system that may require more than
2 buffers.
 * CPU ondemand governor active
 * latency cost for wake up and/or frequency change
 * DMA for IO

Use case description.
 * Data transfer from MMC via VFS to USB.
 * DMA shuffles data from MMC and to USB.
 * The CPU wakes up every now and then to pass data in and out from VFS,
   which cause the bursty VFS behaviour.

Test set up
 * Running dd on the host reading from the mass storage device
 * cmdline: dd if=/dev/sdb of=/dev/null bs=4k count=$((256*100))
 * Caches are dropped on the host and on the device before each run

Measurements on a Snowball board with ondemand_governor active.

FSG_NUM_BUFFERS 2
104857600 bytes (105 MB) copied, 5.62173 s, 18.7 MB/s
104857600 bytes (105 MB) copied, 5.61811 s, 18.7 MB/s
104857600 bytes (105 MB) copied, 5.57817 s, 18.8 MB/s

FSG_NUM_BUFFERS 4
104857600 bytes (105 MB) copied, 5.26839 s, 19.9 MB/s
104857600 bytes (105 MB) copied, 5.2691 s, 19.9 MB/s
104857600 bytes (105 MB) copied, 5.2711 s, 19.9 MB/s

There may not be one optimal number for all boards. This is why
the number is added to Kconfig. If selecting USB_GADGET_DEBUG_FILES
this value may be set by a module parameter as well.

Signed-off-by: Per Forlin <per.forlin@linaro.org>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>
This commit is contained in:
Per Forlin
2011-08-19 21:21:27 +02:00
committed by Felipe Balbi
parent 04eee25b1d
commit 6532c7fdb2
4 changed files with 85 additions and 20 deletions
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33
drivers/usb/gadget/storage_common.c
View File

@@ -52,6 +52,12 @@
* characters rather then a pointer to void.
*/
/*
* When USB_GADGET_DEBUG_FILES is defined the module param num_buffers
* sets the number of pipeline buffers (length of the fsg_buffhd array).
* The valid range of num_buffers is: num >= 2 && num <= 4.
*/
#include <linux/usb/storage.h>
#include <scsi/scsi.h>
@@ -264,8 +270,31 @@ static struct fsg_lun *fsg_lun_from_dev(struct device *dev)
#define EP0_BUFSIZE 256
#define DELAYED_STATUS (EP0_BUFSIZE + 999) /* An impossibly large value */
/* Number of buffers we will use. 2 is enough for double-buffering */
#define FSG_NUM_BUFFERS 2
#ifdef CONFIG_USB_GADGET_DEBUG_FILES
static unsigned int fsg_num_buffers = CONFIG_USB_GADGET_STORAGE_NUM_BUFFERS;
module_param_named(num_buffers, fsg_num_buffers, uint, S_IRUGO);
MODULE_PARM_DESC(num_buffers, "Number of pipeline buffers");
#else
/*
* Number of buffers we will use.
* 2 is usually enough for good buffering pipeline
*/
#define fsg_num_buffers CONFIG_USB_GADGET_STORAGE_NUM_BUFFERS
#endif /* CONFIG_USB_DEBUG */
/* check if fsg_num_buffers is within a valid range */
static inline int fsg_num_buffers_validate(void)
{
if (fsg_num_buffers >= 2 && fsg_num_buffers <= 4)
return 0;
pr_err("fsg_num_buffers %u is out of range (%d to %d)\n",
fsg_num_buffers, 2 ,4);
return -EINVAL;
}
/* Default size of buffer length. */
#define FSG_BUFLEN ((u32)16384)