Re: [PATCH v2 3/3] mmc: Add driver for LiteX's LiteSDCard interface

["Gabriel L. Somlo" <gsomlo@gmail.com>]

On Thu, Dec 09, 2021 at 09:31:49AM +0100, Geert Uytterhoeven wrote:
> Hi Gabriel,
> 
> On Wed, Dec 8, 2021 at 9:14 PM Gabriel L. Somlo <gsomlo@gmail.com> wrote:
> > I did *some* of this for v3, but since figured out how to use `pahole` :)
> 
> Right, pahole.
> 
> > On Mon, Dec 06, 2021 at 11:07:56AM +0100, Geert Uytterhoeven wrote:
> > > > +struct litex_mmc_host {
> > > > +       struct mmc_host *mmc;
> > > > +       struct platform_device *dev;
> > > > +
> > > > +       void __iomem *sdphy;
> > > > +       void __iomem *sdcore;
> > > > +       void __iomem *sdreader;
> > > > +       void __iomem *sdwriter;
> > > > +       void __iomem *sdirq;
> > > > +
> > > > +       u32 resp[4];
> > > > +       u16 rca;
> > > > +
> > > > +       void *buffer;
> > > > +       size_t buf_size;
> > > > +       dma_addr_t dma;
> > > > +
> > > > +       unsigned int freq;
> > > > +       unsigned int clock;
> > > > +       bool is_bus_width_set;
> > > > +       bool app_cmd;
> > > > +
> > > > +       int irq;
> > > > +       struct completion cmd_done;
> > >
> > > You may want to reorder the members to avoid implicit gaps
> > > (i.e. structs first, followed by integral types in decreasing size).
> >
> > So, for v4, I'll have it looking like this, which `pahole` says is
> > optimally packed:
> >
> > struct litex_mmc_host {
> >         struct mmc_host *          mmc;                  /*     0     8 */
> >         struct platform_device *   dev;                  /*     8     8 */
> >         void *                     sdphy;                /*    16     8 */
> >         void *                     sdcore;               /*    24     8 */
> >         void *                     sdreader;             /*    32     8 */
> >         void *                     sdwriter;             /*    40     8 */
> >         void *                     sdirq;                /*    48     8 */
> >         void *                     buffer;               /*    56     8 */
> >         /* --- cacheline 1 boundary (64 bytes) --- */
> >         size_t                     buf_size;             /*    64     8 */
> 
> size_t is 32-bit on RV32, so you may want to move it below cmd_done.
> 
> >         dma_addr_t                 dma;                  /*    72     8 */
> >         struct completion          cmd_done;             /*    80    32 */
> >         int                        irq;                  /*   112     4 */
> >         unsigned int               ref_clk;              /*   116     4 */
> >         unsigned int               sd_clk;               /*   120     4 */
> >         u32                        resp[4];              /*   124    16 */
> >         /* --- cacheline 2 boundary (128 bytes) was 12 bytes ago --- */
> >         u16                        rca;                  /*   140     2 */
> >         bool                       is_bus_width_set;     /*   142     1 */
> >         bool                       app_cmd;              /*   143     1 */
> >
> >         /* size: 144, cachelines: 3, members: 18 */
> >         /* last cacheline: 16 bytes */
> > };

After a bit of a fight, I managed to wrestle `pahole` to display useful
information for 32-bit (rv32imac) builds:

struct litex_mmc_host {
	struct mmc_host *          mmc;                  /*     0     4 */
	struct platform_device *   dev;                  /*     4     4 */
	void *                     sdphy;                /*     8     4 */
	void *                     sdcore;               /*    12     4 */
	void *                     sdreader;             /*    16     4 */
	void *                     sdwriter;             /*    20     4 */
	void *                     sdirq;                /*    24     4 */
	void *                     buffer;               /*    28     4 */
	size_t                     buf_size;             /*    32     4 */
	dma_addr_t                 dma;                  /*    36     4 */
	struct completion          cmd_done;             /*    40    16 */
	int                        irq;                  /*    56     4 */
	unsigned int               ref_clk;              /*    60     4 */
	/* --- cacheline 1 boundary (64 bytes) --- */
	unsigned int               sd_clk;               /*    64     4 */
	u32                        resp[4];              /*    68    16 */
	u16                        rca;                  /*    84     2 */
	bool                       is_bus_width_set;     /*    86     1 */
	bool                       app_cmd;              /*    87     1 */

	/* size: 88, cachelines: 2, members: 18 */
	/* last cacheline: 24 bytes */
};

Looks like even with `size_t buf_size` where it is right now, there
still are no holes. I like it where it is, as it's related to the
field immediately preceding it (`buffer`). I'd rather not move it,
particularly since we're not actually eliminating any additional
holes.

What do you think (i.e., is there a configuration where there's still
a chance we may run into trouble)?

Thanks,
--Gabriel