linux-kernel - Re: [PATCH v5 11/14] software node: move small properties inline when copying

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Message-ID: <20191016074857.GN32742@smile.fi.intel.com>
Date:   Wed, 16 Oct 2019 10:48:57 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        platform-driver-x86@...r.kernel.org
Subject: Re: [PATCH v5 11/14] software node: move small properties inline
 when copying

On Tue, Oct 15, 2019 at 11:25:53AM -0700, Dmitry Torokhov wrote:
> On Tue, Oct 15, 2019 at 03:20:28PM +0300, Andy Shevchenko wrote:
> > On Fri, Oct 11, 2019 at 04:07:18PM -0700, Dmitry Torokhov wrote:
> > > When copying/duplicating set of properties, move smaller properties that
> > > were stored separately directly inside property entry structures. We can
> > > move:
> > > 
> > > - up to 8 bytes from U8 arrays
> > > - up to 4 words
> > > - up to 2 double words
> > > - one U64 value
> > > - one or 2 strings.
> > 
> > Can you show where you extract such values?
> 
> the "value" union's largest member is u64, which is 8 bytes. Strings are
> pointers, so on 32-bit arches you can stuff 2 pointers into 8 bytes,
> while on 64-bits you have space for only one.
> 
> > 
> > > +	if (!dst->is_inline && dst->length <= sizeof(dst->value)) {
> > > +		/* We have an opportunity to move the data inline */
> > > +		const void *tmp = dst->pointer;
> > > +
> > 
> > > +		memcpy(&dst->value, tmp, dst->length);
> > 
> > ...because this is strange trick.
> 
> Not sure what is so strange about it. You just take data that is stored
> separately and move it into the structure, provided that it is not too
> big (i.e. it does not exceed sizeof(value union) size).

You store a value as union, but going to read as a member of union?
I'm pretty sure it breaks standard rules.

-- 
With Best Regards,
Andy Shevchenko