linux-kernel - Re: [PATCH 3/6] bitops: bitmap helper to set variable length values

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Message-ID: <Ys8uKUKErpoWL873@yury-laptop>
Date:   Wed, 13 Jul 2022 13:42:17 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Sebastian Fricke <sebastian.fricke@...labora.com>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>, knaerzche@...il.com,
        Collabora Kernel ML <kernel@...labora.com>,
        bob.beckett@...labora.com,
        Ezequiel Garcia <ezequiel@...guardiasur.com.ar>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        linux-staging@...ts.linux.dev, nicolas.dufresne@...labora.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH 3/6] bitops: bitmap helper to set variable length values

On Wed, Jul 13, 2022 at 10:14:24PM +0200, Andy Shevchenko wrote:
> On Wed, Jul 13, 2022 at 9:44 PM Yury Norov <yury.norov@...il.com> wrote:
> > On Wed, Jul 13, 2022 at 09:10:33PM +0200, Andy Shevchenko wrote:
> > > On Wed, Jul 13, 2022 at 8:56 PM Yury Norov <yury.norov@...il.com> wrote:
> > > > On Wed, Jul 13, 2022 at 06:31:59PM +0200, Sebastian Fricke wrote:
> > >
> > > ...
> > >
> > > > I'd suggest you to try implementing
> > > >         bitmap_copy_from(dst, src, dst_off, len)
> > > > or even
> > > >         bitmap_copy_from(dst, dst_off, src, src_off, len)
> > > > if you expect that you'll need more flexibility in the future.
> > >
> > > Do you think it would be useful?
> > >
> > > We have bitmap_replace() & bitmap_remap(). Wouldn't that be enough?
> >
> > bitmap_replace and bitmap_remap have no an 'offset' parameter.
> 
> True.
> 
> But then it's a bit too generic to have this src_off, no?

That's why I said:

> > > > if you expect that you'll need more flexibility in the future.

My preferred option is bitmap_copy_from(dst, src, dst_off, len).

> I would rather expect for asymmetrical bitmaps that the other side
> will be either one of the fixed width types (it makes sense to have
> for 32- or 64-bit arguments.

Look at patch #6 - it copies 1,4,5,9,10,32,37... - pretty much a random
number number of bits.
 
> When you have a source bitmap of x bits and  you would like to copy it
> into a y-bit one, I would think that either you have a small amount of
> bits in x anyway, or x is a full-sized bitmap (same order as y).

It sounds like a speculation to me. Why shouldn't we let people to
copy with an offset any number of bits? 

> Also
> keep in mind that granularity is long, so less than long it makes no
> sense.
> 
>   bitmap_copy_from_T(unsigned long *map, start, len, T src),
> 
> where T is type, start is the offset in map, len is the amount of bits
> from src starting from 0. That's what is required in most of the cases
> I believe.

But not in Sebastian's case, according to patch #6.

Thanks,
Yury