linux-kernel - Re: [PATCH] usb: gadget: storage_common: make FSG_NUM

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Date:	Mon, 08 Aug 2011 11:40:09 +0200
From:	"Michal Nazarewicz" <mina86@...a86.com>
To:	"Felipe Balbi" <balbi@...com>,
	"Greg Kroah-Hartman" <gregkh@...e.de>, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org, linaro-dev@...ts.linaro.org,
	"Per Forlin" <per.forlin@...aro.org>
Subject: Re: [PATCH] usb: gadget: storage_common: make FSG_NUM_BUFFERS
 variable size

On Mon, 08 Aug 2011 02:22:14 +0200, Per Forlin <per.forlin@...aro.org>  
wrote:
> FSG_NUM_BUFFERS is set to 2 as default.
> Usually 2 buffers are enough to establish a good
> double 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 parallell with
> an ongoing transfer. Every time all buffers are consumed there is
> an additional cost for the next first request. Increasing the number
> of buffers decreases the risk of running out of buffers.

[... benchmark snip ...]

> There may not be one optimal number for all boards. That is the
> reason for adding the number to Kconfig,

It could actually be turned into a run-time configuration, but
that's a bit more intrusive, so adding Kconfig should be enough
for now.

> Signed-off-by: Per Forlin <per.forlin@...aro.org>

Acked-by: Michal Nazarewicz <mina86@...a86.com>


Some not very relevant comments below:

> diff --git a/drivers/usb/gadget/Kconfig b/drivers/usb/gadget/Kconfig
> index 029e288..bbd17f3 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 pipline buffers"

“pipeline”, missing “e”.

> +	range 2 64
> +	default 2
> +	help
> +	  Usually 2 buffers are enough to establish a good
> +	  double buffering pipeline. But when dealing with expensive

s/double//.  By definition, “double” buffering uses two buffers, isn't it?
So if we change number of buffers, it's no longer “double”.

> +	  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 parallell with

“parallel”, too many “l”s.

> +	  an ongoing transfer. Every time all buffers are consumed there is
> +	  an additional cost for the next first request. Increasing the number

“Next first request” sounds a bit illogical to me.

> +	  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..512d9cf 100644
> --- a/drivers/usb/gadget/storage_common.c
> +++ b/drivers/usb/gadget/storage_common.c
> @@ -262,8 +262,8 @@ 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  
> double-buffering */

s/double-buffering/good buffering pipeline/?

> +#define FSG_NUM_BUFFERS	CONFIG_USB_GADGET_STORAGE_NUM_BUFFERS
> /* Default size of buffer length. */
>  #define FSG_BUFLEN	((u32)16384)

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michal "mina86" Nazarewicz    (o o)
ooo +-----<email/xmpp: mnazarewicz@...gle.com>-----ooO--(_)--Ooo--
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