linux-kernel - Re: [PATCH v2] dma-coherent: add support for multi coherent rmems per dev

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Message-ID: <CAJDAHvYYUcn2HL1QTSG-KA0QcuAOotk5hY4cpuasOu_H=X-V0A@mail.gmail.com>
Date: Thu, 8 Feb 2024 15:53:37 +0800
From: Howard Yen <howardyen@...gle.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: hch@....de, m.szyprowski@...sung.com, robin.murphy@....com, 
	gregkh@...uxfoundation.org, rafael@...nel.org, broonie@...nel.org, 
	james@...iv.tech, james.clark@....com, masahiroy@...nel.org, 
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev
Subject: Re: [PATCH v2] dma-coherent: add support for multi coherent rmems per dev

On Tue, Feb 6, 2024 at 11:43 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Mon, Feb 05, 2024 at 02:08:00PM +0200, Andy Shevchenko wrote:
> > On Mon, Feb 05, 2024 at 07:23:00AM +0000, Howard Yen wrote:
>
> ...
>
> > > @@ -18,15 +18,9 @@ struct dma_coherent_mem {
> > >     unsigned long   *bitmap;
> > >     spinlock_t      spinlock;
> > >     bool            use_dev_dma_pfn_offset;
> > > +   struct list_head        node;
> >
> > Have you run `pahole`? Here I see wasted bytes for nothing.
>
> On top of that one may make container_of() to be no-op, by placing this member
> to be the first one. But, double check this with bloat-o-meter (that it indeed
> does better code generation) and on the other hand check if the current first
> member is not performance critical and having additional pointer arithmetics is
> okay.
>
> > >  };
>
>
> --
> With Best Regards,
> Andy Shevchenko
>
>

I'm trying to re-org the members as below

from ===>

struct dma_coherent_mem {
void *                     virt_base;            /*     0     8 */
dma_addr_t                 device_base;          /*     8     8 */
unsigned long              pfn_base;             /*    16     8 */
int                        size;                 /*    24     4 */

/* XXX 4 bytes hole, try to pack */

unsigned long *            bitmap;               /*    32     8 */
spinlock_t                 spinlock;             /*    40     4 */
bool                       use_dev_dma_pfn_offset; /*    44     1 */

/* XXX 3 bytes hole, try to pack */

struct list_head           node;                 /*    48    16 */

/* size: 64, cachelines: 1, members: 8 */
/* sum members: 57, holes: 2, sum holes: 7 */
};


to ===>

struct dma_coherent_mem {
struct list_head           node;                 /*     0    16 */
void *                     virt_base;            /*    16     8 */
dma_addr_t                 device_base;          /*    24     8 */
unsigned long              pfn_base;             /*    32     8 */
int                        size;                 /*    40     4 */
spinlock_t                 spinlock;             /*    44     4 */
unsigned long *            bitmap;               /*    48     8 */
bool                       use_dev_dma_pfn_offset; /*    56     1 */

/* size: 64, cachelines: 1, members: 8 */
/* padding: 7 */
};

Looks like there is about 7 bytes padding at the end of the structure.
Should I add __attribute__((__packed__)) to not add the padding?

-- 
Regards,

Howard