Re: [PATCH 0/8] u-boot: fs: add generic unaligned read handling

[Tom Rini <trini@konsulko.com>]

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On Tue, Jun 28, 2022 at 03:28:00PM +0800, Qu Wenruo wrote:
> [BACKGROUND]
> Unlike FUSE/Kernel which always pass aligned read range, U-boot fs code
> just pass the request range to underlying fses.
> 
> Under most case, this works fine, as U-boot only really needs to read
> the whole file (aka, 0 for both offset and len, len will be later
> determined using file size).
> 
> But if some advanced user/script wants to extract kernel/initramfs from
> combined image, we may need to do unaligned read in that case.
> 
> [ADVANTAGE]
> This patchset will handle unaligned read range in _fs_read():
> 
> - Get blocksize of the underlying fs
> 
> - Read the leading block contianing the unaligned range
>   The full block will be stored in a local buffer, then only copy
>   the bytes in the unaligned range into the destination buffer.
> 
>   If the first block covers the whole range, we just call it aday.
> 
> - Read the aligned range if there is any
> 
> - Read the tailing block containing the unaligned range
>   And copy the covered range into the destination.
> 
> [DISADVANTAGE]
> There are mainly two problems:
> 
> - Extra memory allocation for every _fs_read() call
>   For the leading and tailing block.
> 
> - Extra path resolving
>   All those supported fs will have to do extra path resolving up to 2
>   times (one for the leading block, one for the tailing block).
>   This may slow down the read.

This conceptually seems like a good thing.  Can you please post some
before/after times of reading large images from the supported
filesystems?

-- 
Tom

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