linux-kernel - RE: [PATCH v2 1/5] cramfs: direct memory access support

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Date:   Wed, 16 Aug 2017 18:28:15 +0000
From:   Chris Brandt <Chris.Brandt@...esas.com>
To:     Nicolas Pitre <nicolas.pitre@...aro.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
CC:     "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-embedded@...r.kernel.org" <linux-embedded@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 1/5] cramfs: direct memory access support

On Wednesday, August 16, 2017, Nicolas Pitre wrote:
> Small embedded systems typically execute the kernel code in place (XIP)
> directly from flash to save on precious RAM usage. This adds the ability
> to consume filesystem data directly from flash to the cramfs filesystem
> as well. Cramfs is particularly well suited to this feature as it is
> very simple and its RAM usage is already very low, and with this feature
> it is possible to use it with no block device support and even lower RAM
> usage.
> 
> This patch was inspired by a similar patch from Shane Nay dated 17 years
> ago that used to be very popular in embedded circles but never made it
> into mainline. This is a cleaned-up implementation that uses far fewer
> memory address at run time when both methods are configured in. In the
> context of small IoT deployments, this functionality has become relevant
> and useful again.
> 
> To distinguish between both access types, the cramfs_physmem filesystem
> type must be specified when using a memory accessible cramfs image, and
> the physaddr argument must provide the actual filesystem image's physical
> memory location.
> 
> Signed-off-by: Nicolas Pitre <nico@...aro.org>
> ---
>  fs/cramfs/Kconfig |  30 ++++++-
>  fs/cramfs/inode.c | 264 +++++++++++++++++++++++++++++++++++++++++++------
> -----
>  2 files changed, 242 insertions(+), 52 deletions(-)
> 
> diff --git a/fs/cramfs/Kconfig b/fs/cramfs/Kconfig
> index 11b29d491b..5eed4ad2d5 100644
> --- a/fs/cramfs/Kconfig
> +++ b/fs/cramfs/Kconfig
> @@ -1,6 +1,5 @@
>  config CRAMFS
>  	tristate "Compressed ROM file system support (cramfs) (OBSOLETE)"
> -	depends on BLOCK
>  	select ZLIB_INFLATE
>  	help
>  	  Saying Y here includes support for CramFs (Compressed ROM File
> @@ -20,3 +19,32 @@ config CRAMFS
>  	  in terms of performance and features.
> 
>  	  If unsure, say N.
> +
> +config CRAMFS_BLOCKDEV
> +	bool "Support CramFs image over a regular block device" if EXPERT
> +	depends on CRAMFS && BLOCK
> +	default y
> +	help
> +	  This option allows the CramFs driver to load data from a regular
> +	  block device such a disk partition or a ramdisk.
> +


trailing whitespace


> +config CRAMFS_PHYSMEM
> +	bool "Support CramFs image directly mapped in physical memory"
> +	depends on CRAMFS
> +	default y if !CRAMFS_BLOCKDEV
> +	help
> +	  This option allows the CramFs driver to load data directly from
> +	  a linear adressed memory range (usually non volatile memory
> +	  like flash) instead of going through the block device layer.
> +	  This saves some memory since no intermediate buffering is
> +	  necessary.
> +
> +	  The filesystem type for this feature is "cramfs_physmem".
> +	  The location of the CramFs image in memory is board
> +	  dependent. Therefore, if you say Y, you must know the proper
> +	  physical address where to store the CramFs image and specify
> +	  it using the physaddr=0x******** mount option (for example:
> +	  "mount -t cramfs_physmem -o physaddr=0x100000 none /mnt").
> +
> +	  If unsure, say N.
> +


new blank line at EOF



-Chris