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

Gabriel L. Somlo
2021-12-09T20:58:12Z

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