[GIT PULL] Driver core fix for 4.20-rc5

[GIT PULL] Driver core fix for 4.20-rc5

[Greg KH]

The following changes since commit 651022382c7f8da46cb4872a545ee1da6d097d2a:

  Linux 4.20-rc1 (2018-11-04 15:37:52 -0800)

are available in the Git repository at:

  git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/driver-core.git tags/driver-core-4.20-rc5

for you to fetch changes up to a66d972465d15b1d89281258805eb8b47d66bd36:

  devres: Align data[] to ARCH_KMALLOC_MINALIGN (2018-11-11 11:40:04 -0800)

----------------------------------------------------------------
Driver core fix for 4.20-rc5

Here is a single driver core fix for 4.20-rc5

It resolves an issue with the data alignment in 'struct devres' for the
ARC platform.  The full details are in the commit changelog, but the
short summary is the change is a single line:
-       unsigned long long              data[]; /* guarantee ull alignment */
+       u8 __aligned(ARCH_KMALLOC_MINALIGN) data[];

This has been in linux-next for a while with no reported issues.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

----------------------------------------------------------------
Alexey Brodkin (1):
      devres: Align data[] to ARCH_KMALLOC_MINALIGN

 drivers/base/devres.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

Re: [GIT PULL] Driver core fix for 4.20-rc5

[Linus Torvalds]

On Fri, Nov 30, 2018 at 8:06 AM Greg KH <gregkh@linuxfoundation.org> wrote:
>
> It resolves an issue with the data alignment in 'struct devres' for the
> ARC platform.  The full details are in the commit changelog, but the
> short summary is the change is a single line:
>
> -       unsigned long long              data[]; /* guarantee ull alignment */
> +       u8 __aligned(ARCH_KMALLOC_MINALIGN) data[];

Hmm.

Are you aware that this is up to 128 bytes? Including on common
architectures like ARM64?

I've done the pull, but honestly, that seems a bit excessive, when a
fair amount of devres users seem to have fairly small data (ie looking
at "size", I see things like

    p = devres_alloc(dmam_device_release, sizeof(void *), GFP_KERNEL);

or

    dr = devres_alloc(devm_gpio_release, sizeof(unsigned), GFP_KERNEL);

that have allocations of a couple of bytes, and the new model means
that those allocations will be aligned to 128-byte boundaries, and
then (because ARCH_KMALLOC_MINALIGN, again) you'll end up actually
wasting 256 bytes for a tiny structure on ARM64.

Maybe it doesn't matter. But it does seem somewhat excessive to do
things like this.

Yeah, on x86, the alignment isn't even noticeable, being just 8 bytes.

                   Linus

Re: [GIT PULL] Driver core fix for 4.20-rc5

[pr-tracker-bot]

The pull request you sent on Fri, 30 Nov 2018 17:06:05 +0100:

> git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/driver-core.git tags/driver-core-4.20-rc5

has been merged into torvalds/linux.git:
https://git.kernel.org/torvalds/c/d7aca8a78c8bc5d3707691aab13cb4f7f6de696f

Thank you!

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