Re: [PATCH 2/2] hexdump: Allow skipping identical lines

[Miquel Raynal <miquel.raynal@bootlin.com>]

Hi Andy,

andriy.shevchenko@linux.intel.com wrote on Mon, 26 Aug 2024 20:35:36
+0300:

> On Mon, Aug 26, 2024 at 06:24:16PM +0200, Miquel Raynal wrote:
> > When dumping long buffers (especially for debug purposes) it may be very
> > convenient to sometimes avoid spitting all the lines of the buffer if
> > the lines are identical. Typically on embedded devices, the console
> > would be wired to a UART running at 115200 bauds, which makes the dumps
> > very (very) slow. In this case, having a flag to avoid printing
> > duplicated lines is handy.
> > 
> > Example of a made up repetitive output:
> > 0f 53 63 47 56 55 78 7a aa b7 8c ff ff ff ff ff
> > ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> > ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> > ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> > ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> > ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> > ff ff ff ff ff ff ff ff ff ff ff ff 01 2a 39 eb
> > 
> > Same but with the flag enabled:
> > 0f 53 63 47 56 55 78 7a aa b7 8c ff ff ff ff ff
> > ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> > *
> > ff ff ff ff ff ff ff ff ff ff ff ff 01 2a 39 eb  
> 
> The problem here is that without offset we can't see how many lines were
> skipped.

Yes, this is intended, I prefer to mimic userspace tools behavior.

> Two ways to solve (that come to my mind immediately, maybe more and better):
> 1) make sure that new flag implies or expects (otherwise BUILD_BUG_ON() or so)
>   the OFFSET to be set;

It depends what you are looking for. When I print a 2kiB page and want
to compare the output with some other dump, I will immediately see if
there are more or less skipped lines in the diff. When I want to just
grab the UBI header and skip all the ff's following while asking a full
buffer to be dumped (for kernel development reasons), the amount of
skipped lines is not of interest to me either. Of course this is my own
use case, but I guess there are others.

However this is true it is sometimes also useful to know where we are in
the dump, but the hexdump helpers already include all the interesting
bits for that through the 'prefix_type' parameter :

enum {
	DUMP_PREFIX_NONE,
	DUMP_PREFIX_ADDRESS,
	DUMP_PREFIX_OFFSET
};

See https://elixir.bootlin.com/linux/v4.20.17/source/include/linux/printk.h

I anyway understand the request and will change the example with
something more common, probably, by using one of the two other
prefixes.

> 2) [OR] add number of lines skipped in that * line.

As mentioned above, this is not the intended output.

> Personally I prefer the 1) as I think that you tried to follow the existing
> format of user space tools and there is a chance that there are other tools or
> scripts that parse the dump to restore the binary contents.

Exactly. Also, just simply using the diff command over two dumps
without being polluted by any additions on one side or the other is very
convenient.

Thanks,
Miquèl