Re: [PATCHv2] dmaengine: hsu: use kzalloc_flex()

[Andy Shevchenko]

On Mon, Mar 30, 2026 at 11:41 PM Rosen Penev <rosenp@xxxxxxxxx> wrote:
> On Mon, Mar 30, 2026 at 1:46 AM Andy Shevchenko
> <andy.shevchenko@xxxxxxxxx> wrote:
> > On Sat, Mar 28, 2026 at 9:17 PM Rosen Penev <rosenp@xxxxxxxxx> wrote:

...

> > > -       hsu = devm_kzalloc(chip->dev, sizeof(*hsu), GFP_KERNEL);
> > > +       /* Calculate nr_channels from the IO space length */
> > > +       nr_channels = (chip->length - chip->offset) / HSU_DMA_CHAN_LENGTH;
> > > +       hsu = devm_kzalloc(chip->dev, struct_size(hsu, chan, nr_channels), GFP_KERNEL);
> > >         if (!hsu)
> > >                 return -ENOMEM;
> > >
> > > -       chip->hsu = hsu;
> > > -
> > > -       /* Calculate nr_channels from the IO space length */
> > > -       hsu->nr_channels = (chip->length - chip->offset) / HSU_DMA_CHAN_LENGTH;
> > > +       hsu->nr_channels = nr_channels;
> > >
> > > -       hsu->chan = devm_kcalloc(chip->dev, hsu->nr_channels,
> > > -                                sizeof(*hsu->chan), GFP_KERNEL);
> > > -       if (!hsu->chan)
> > > -               return -ENOMEM;
> > > +       chip->hsu = hsu;
> >
> > Don't know these _flex() APIs enough, but can we leave the chip->hsu =
> > hsu; in the same place as it's now?
> __counted_by requires the first assignment after allocation to be the
> counting variable. The _flex macros do this automatically for GCC15
> and above.

Why? The hsu member has nothing to do with VLA, where is this
requirement coming from? My understanding is that the check should
imply the minimum sizeof of the data structure and the compiler should
know that way before doing any allocations.

My understanding seems in align with what Gustavo blogged:
https://people.kernel.org/gustavoars/how-to-use-the-new-counted_by-attribute-in-c-and-linux

The same is written in the GCC patch description
https://gcc.gnu.org/pipermail/gcc-patches/2024-May/653123.html

--
With Best Regards,
Andy Shevchenko