[LEDE-DEV] [PATCH] bcm53xx: use iflag=skip_bytes for dd command during sysupgrade

[Rafał Miłecki]

On 2017-12-24 00:26, Etienne Champetier wrote:
> 2017-12-23 14:14 GMT-08:00 Rafał Miłecki <zajec5 at gmail.com>:
>> From: Rafał Miłecki <rafal at milecki.pl>
>> 
>> Since BusyBox 1.25.0 dd command supports iflag=skip_bytes which allows
>> skipping requested amount of bytes without reducing blocksize. Thanks 
>> to
>> this we can leave default blocksize and let dd work more efficiently.
>> 
>> On Netgear R6250 "dd skip=58 iflag=skip_bytes" can be 5 times faster
>> than "dd bs=58 skip=1" when extracting TRX out of CHK.
>> 
>> Signed-off-by: Rafał Miłecki <rafal at milecki.pl>
>> ---
>>  target/linux/bcm53xx/base-files/lib/upgrade/platform.sh | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/target/linux/bcm53xx/base-files/lib/upgrade/platform.sh 
>> b/target/linux/bcm53xx/base-files/lib/upgrade/platform.sh
>> index 1a33e3a447..06451f17fd 100644
>> --- a/target/linux/bcm53xx/base-files/lib/upgrade/platform.sh
>> +++ b/target/linux/bcm53xx/base-files/lib/upgrade/platform.sh
>> @@ -274,11 +274,11 @@ platform_pre_upgrade() {
>>  platform_trx_from_chk_cmd() {
>>         local header_len=$((0x$(get_magic_long_at "$1" 4)))
>> 
>> -       echo -n dd bs=$header_len skip=1
>> +       echo -n dd skip=$header_len iflag=skip_bytes
>>  }
>> 
>>  platform_trx_from_cybertan_cmd() {
>> -       echo -n dd bs=32 skip=1
>> +       echo -n dd skip=32 iflag=skip_bytes
> 
> haven't checked, but would "tail -c+32" work (or maybe "tail -c+33") 
> works ?
> I know I add a similar old dd version to handle in the past (but
> nothing to do with OpenWRT) and "tail" was enough in the end

I didn't test it, but I believe we could use tail somehow for that.

The question is: why should we?

We use "dd" in general for a lot of sysupgrade-related operations. It
has more fearures like controlling a block size which we may want to use
in a future for more optimizations. It can be used to "cut" a part of
file from the middle of it.

I don't see any real advantage of switching to tail.