diff options
| author | Per Forlin <per.forlin@linaro.org> | 2011-08-19 21:21:27 +0200 |
|---|---|---|
| committer | Felipe Balbi <balbi@ti.com> | 2011-09-09 13:06:04 +0300 |
| commit | 6532c7fdb2c3a2ec1b949ecd2ff5375069c1639a (patch) | |
| tree | 26afd8fdff7923f818f49aa405bb4208d1da914e /drivers/usb/gadget/Kconfig | |
| parent | 04eee25b1d754a837360504b7af426d1f86ffeb7 (diff) | |
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>
Diffstat (limited to 'drivers/usb/gadget/Kconfig')
| -rw-r--r-- | drivers/usb/gadget/Kconfig | 16 |
1 files changed, 16 insertions, 0 deletions
diff --git a/drivers/usb/gadget/Kconfig b/drivers/usb/gadget/Kconfig index fe5637983892..a60b472a0b39 100644 --- a/drivers/usb/gadget/Kconfig +++ b/drivers/usb/gadget/Kconfig @@ -96,6 +96,22 @@ 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 selecting USB_GADGET_DEBUG_FILES this value may be set by + a module parameter as well. + If unsure, say 2. + # # USB Peripheral Controller Support # |