From: Per Forlin per.forlin@linaro.org

FSG_NUM_BUFFERS is set to 2 as default. Usually 2 buffers are enough to establish a good buffering pipeline. But when dealing with expensive request preparation (i.e. dma_map) there may be benefits of increasing the number of buffers. There is an extra cost for every first request, the others are prepared in parallel with an ongoing transfer. Every time all buffers are consumed there is an additional cost for the first request. Increasing the number of buffers decreases the risk of running out of buffers.

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 104857600 bytes (105 MB) copied, 5.57769 s, 18.8 MB/s 104857600 bytes (105 MB) copied, 5.59654 s, 18.7 MB/s 104857600 bytes (105 MB) copied, 5.58948 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 104857600 bytes (105 MB) copied, 5.27174 s, 19.9 MB/s 104857600 bytes (105 MB) copied, 5.27261 s, 19.9 MB/s 104857600 bytes (105 MB) copied, 5.27135 s, 19.9 MB/s 104857600 bytes (105 MB) copied, 5.27249 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 --- drivers/usb/gadget/Kconfig | 15 +++++++++++++++ drivers/usb/gadget/storage_common.c | 7 +++++-- 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/gadget/Kconfig b/drivers/usb/gadget/Kconfig index 029e288..1888a16 100644 --- a/drivers/usb/gadget/Kconfig +++ b/drivers/usb/gadget/Kconfig @@ -96,6 +96,21 @@ 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 64 + default 2 + help + Usually 2 buffers are enough to establish a good + buffering pipeline. But when dealing with expensive + request preparation (i.e. dma_map) there may be benefits of + increasing the number of buffers. There is an extra cost for + every first request, the others are prepared in parallel with + an ongoing transfer. Every time all buffers are consumed there is + an additional cost for the first request. Increasing the number + of buffers decreases the risk of running out of buffers. + 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)