[PATCH v3 3/5] arm64: mte: implement CONFIG_ARM64_MTE_COMP

[Alexander Potapenko]

On Tue, Jul 18, 2023 at 7:18 PM Andy Shevchenko
<andriy.shevchenko at linux.intel.com> wrote:

> > However it doesn't seem to be very picky.
> >
> >   $ scripts/kernel-doc -v  -none arch/arm64/include/asm/mtecomp.h
> >
> > warns about e.g. parameter name mismatch, but does not care about the
> > missing return section.
>
> -Wreturn is missing

Nice, adding it (or -Wall) uncovered more problems.

> ...
>
> > > Also
> > > why you put the descriptions in to the header file? It's a bit unusual for the
> > > exported ones.
> >
> > https://docs.kernel.org/doc-guide/kernel-doc.html was not specific on
> > this, and I thought anyone wanting to understand how an interface
> > works would prefer reading the header rather than the implementation.
> > I can move the comments to arch/arm64/mm/mtecomp.c if you think it's a
> > better place for them.
>
> With the kernel doc in the C file you may also comment the internal ones and
> generate documentation only for exported ones. This is not possible with h.

I see.
After some hesitation and looking at the existing practices in the
kernel, I moved the kernel doc comments to mtecomp.c



> > but all those values are >= 0
> > and <= 256, so there should be no problems.
> > (shorts could have been ints as well, we are just saving some stack
> > space in ea0_compress()/ea0_decompress()).
>
> Then why they are signed? Please, justify that.
> Signdness prone to subtle and hard to hunt errors due to integer promotions.

Switched to unsigned shorts.

> ...
>
> > > > +#include <linux/bits.h>
> > > > +#include <linux/bitmap.h>
> > >
> > > bitmap guarantees that bits.h will be included.
> >
> > I am following the IWYU principle here, and I believe it's a good thing to do.
> > I've seen cases where these transitive dependencies rotted after some
> > refactoring, but the fact was only noticed in certain configurations.
> > Also, there's an ongoing work by Ingo Molnar to speed up kernel builds
> > by optimizing headers
> > (https://lwn.net/ml/linux-kernel/YdIfz+LMewetSaEB@gmail.com/), and it
> > relies on explicit dependencies which are easier to untangle.
>
> Yes, but we have some guarantees. E.g., we don't include compiler*.h
> when types.h is included, because of the guarantees.
Ok, I removed bits.h

> Otherwise your code misses _a lot_ of headers.

... and tried to expand the list of headers where applicable.

>
> ...
>
> > > Can you make it unsigned and start from 0?
> >
> > Changed to start with 0, but I am a bit hesitant about making it
> > unsigned: it is now no more special than a loop variable.
>
> Signdness is a beast in C, needs always an additional justification.

Changed all ints to unsigned, because, well, why not :)


> > Shall I undef the constants above as well (e.g. BITS_PER_TAG)?
> > The intuitive answer is "no",
>
> Correct.
>
> > but then there should be some difference between those and RANGES_INLINE?
>
> Yes, one is widely used constant and one is a _localized_ helper.

Ack

>
> ...
>
> > > > +static void bitmap_write(unsigned long *bitmap, unsigned long value,
> > > > +                      unsigned long *pos, unsigned long bits)
> > >
> > > Please, don't use reserved namespace. Yours is ea0, use it:
> > > ea0_bitmap_write()! Same to other similarly named functions.
> >
> > Done.
> > However there are two parallel namespaces now: "ea0" for the
> > compression algorithm, and "memcomp" for the module initialization and
> > data structures.
> > Dunno if it makes sense to merge them (and rename the .c file accordingly).
>
> Your choice. Mu point, just do prefix it with something meaningful.
Fine, I'll go with "ea0" for the algorithm and "memcomp" for the
module-specific stuff.
Let's also see what ARM maintainers say.

> ...
>
> > > > +     u8 r_tags[256];
> > > > +     int r_len = ARRAY_SIZE(r_tags);
> > >
> > No, it is the length of the arrays (both r_tags and r_sizes).
> > Below you make a good point it will spare us a kernel.h dependency,
> > but for the sake of keeping the code error-prone we probably shouldn't
> > assume r_tags is a byte array.
>
> It's a common practice even outside of Linux kernel to use sizeof() against
> char arrays. I don't see the point to have the ARRAY_SIZE() complication here.

Fixed.

> ...
>
> > > > +             snprintf(name, ARRAY_SIZE(name), "mte-tags-%d", size);
>
> You see, if you grep for similar calls I'm pretty sure the order of 2 of power
> of 10 will be the difference between sizeof()/ARRAY_SIZE() if the latter even
> occurs at least once.

You are right, it is 3700+ vs. 66 :)
Changed ARRAY_SIZE() to sizeof() here as well.