usb: gadget: storage: make FSG_NUM_BUFFERS variable size
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>
diff --git a/drivers/usb/gadget/Kconfig b/drivers/usb/gadget/Kconfig
index fe56379..a60b472 100644
--- a/drivers/usb/gadget/Kconfig
+++ b/drivers/usb/gadget/Kconfig
@@ -96,6 +96,22 @@
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 selecting USB_GADGET_DEBUG_FILES this value may be set by
+ a module parameter as well.
+ If unsure, say 2.
+
#
# USB Peripheral Controller Support
#