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_govenor 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.

Signed-off-by: Per Forlin per.forlin@linaro.org Acked-by: Michal Nazarewicz mina86@mina86.com --- Change log. v2: Update after proofreading comments from Michal Nazarewicz v3: Clarify the description of this patch based on input from Alan Stern

drivers/usb/gadget/Kconfig | 14 ++++++++++++++ drivers/usb/gadget/storage_common.c | 7 +++++-- 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/gadget/Kconfig b/drivers/usb/gadget/Kconfig index 029e288..24d3ef5 100644 --- a/drivers/usb/gadget/Kconfig +++ b/drivers/usb/gadget/Kconfig @@ -96,6 +96,20 @@ config USB_GADGET_VBUS_DRAW This value will be used except for system-specific gadget drivers that have more specific information.

+config USB_GADGET_STORAGE_NUM_BUFFERS + int "Number of storage pipeline buffers" + range 2 4 + default 2 + help + Usually 2 buffers are enough to establish a good buffering + pipeline. The number may be increased in order to compensate + for a bursty VFS behaviour. For instance there may be cpu wake up + latencies that makes the VFS to appear bursty in a system with + an cpu on-demand governor. Especially if DMA is doing IO to + offload the CPU. In this case the CPU will go into power + save often and spin up occasionally to move data within VFS. + If unsure, say 2. + config USB_GADGET_SELECTED boolean

diff --git a/drivers/usb/gadget/storage_common.c b/drivers/usb/gadget/storage_common.c index 1fa4f70..683c972 100644 --- a/drivers/usb/gadget/storage_common.c +++ b/drivers/usb/gadget/storage_common.c @@ -262,8 +262,11 @@ 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 +/* + * Number of buffers we will use. + * 2 is usually enough for good buffering pipeline + */ +#define FSG_NUM_BUFFERS CONFIG_USB_GADGET_STORAGE_NUM_BUFFERS

/* Default size of buffer length. */ #define FSG_BUFLEN ((u32)16384)