Re: [PATCH] dmaengine: stm32-mdma: avoid 64-bit division

[Benjamin Gaignard]

2017-10-11 16:01 GMT+02:00 Arnd Bergmann <arnd@xxxxxxxx>:
> When building with a 64-bit dma_addr_t, we run into a link
> error:
>
> drivers/dma/stm32-mdma.o: In function `stm32_mdma_prep_dma_memcpy':
> stm32-mdma.c:(.text+0x16a3): undefined reference to `__umoddi3'
>
> Using a 64-bit division here is way too expensive, since the
> divisor is a known power-of-two value in reality. This moves
> the modulo operation into stm32_mdma_get_max_width(), where
> the compiler can optimize out that code, and we can use a 32-bit
> division to be on the safe side.
>
> Fixes: a4ffb13c8946 ("dmaengine: Add STM32 MDMA driver")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  drivers/dma/stm32-mdma.c | 27 ++++++++++++---------------
>  1 file changed, 12 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/dma/stm32-mdma.c b/drivers/dma/stm32-mdma.c
> index 0db59a7e80e0..55151c2c9fae 100644
> --- a/drivers/dma/stm32-mdma.c
> +++ b/drivers/dma/stm32-mdma.c
> @@ -387,7 +387,9 @@ static int stm32_mdma_get_width(struct stm32_mdma_chan *chan,
>         }
>  }
>
> -static enum dma_slave_buswidth stm32_mdma_get_max_width(u32 buf_len, u32 tlen)
> +static enum dma_slave_buswidth stm32_mdma_get_max_width(u32 buf_len,
> +                                                       u32 addr,
> +                                                       u32 tlen)
>  {
>         enum dma_slave_buswidth max_width = DMA_SLAVE_BUSWIDTH_8_BYTES;
>
> @@ -398,6 +400,9 @@ static enum dma_slave_buswidth stm32_mdma_get_max_width(u32 buf_len, u32 tlen)
>                         break;
>         }
>
> +       if (addr % max_width)
> +               max_width = DMA_SLAVE_BUSWIDTH_1_BYTE;
> +

I'm only half-convince by the implicite 32 bits cast done into
function prototype.
If we keep using dma_addr_t and use do_div() instead of %
does compiler can still optimize the code ?

>         return max_width;
>  }
>
> @@ -567,7 +572,7 @@ static int stm32_mdma_set_xfer_param(struct stm32_mdma_chan *chan,
>                 ctcr |= STM32_MDMA_CTCR_DBURST((ilog2(dst_best_burst)));
>
>                 /* Set memory data size */
> -               src_addr_width = stm32_mdma_get_max_width(buf_len, tlen);
> +               src_addr_width = stm32_mdma_get_max_width(buf_len, 0, tlen);
>                 chan->mem_width = src_addr_width;
>                 src_bus_width = stm32_mdma_get_width(chan, src_addr_width);
>                 if (src_bus_width < 0)
> @@ -611,7 +616,7 @@ static int stm32_mdma_set_xfer_param(struct stm32_mdma_chan *chan,
>                 ctcr |= STM32_MDMA_CTCR_SBURST((ilog2(src_best_burst)));
>
>                 /* Set memory data size */
> -               dst_addr_width = stm32_mdma_get_max_width(buf_len, tlen);
> +               dst_addr_width = stm32_mdma_get_max_width(buf_len, 0, tlen);
>                 chan->mem_width = dst_addr_width;
>                 dst_bus_width = stm32_mdma_get_width(chan, dst_addr_width);
>                 if (dst_bus_width < 0)
> @@ -956,9 +961,7 @@ stm32_mdma_prep_dma_memcpy(struct dma_chan *c, dma_addr_t dest, dma_addr_t src,
>                 ctcr |= STM32_MDMA_CTCR_TLEN((tlen - 1));
>
>                 /* Set source best burst size */
> -               max_width = stm32_mdma_get_max_width(len, tlen);
> -               if (src % max_width)
> -                       max_width = DMA_SLAVE_BUSWIDTH_1_BYTE;
> +               max_width = stm32_mdma_get_max_width(len, src, tlen);
>                 src_bus_width = stm32_mdma_get_width(chan, max_width);
>
>                 max_burst = tlen / max_width;
> @@ -971,9 +974,7 @@ stm32_mdma_prep_dma_memcpy(struct dma_chan *c, dma_addr_t dest, dma_addr_t src,
>                         STM32_MDMA_CTCR_SINCOS(src_bus_width);
>
>                 /* Set destination best burst size */
> -               max_width = stm32_mdma_get_max_width(len, tlen);
> -               if (dest % max_width)
> -                       max_width = DMA_SLAVE_BUSWIDTH_1_BYTE;
> +               max_width = stm32_mdma_get_max_width(len, dest, tlen);
>                 dst_bus_width = stm32_mdma_get_width(chan, max_width);
>
>                 max_burst = tlen / max_width;
> @@ -1014,9 +1015,7 @@ stm32_mdma_prep_dma_memcpy(struct dma_chan *c, dma_addr_t dest, dma_addr_t src,
>                                            STM32_MDMA_MAX_BLOCK_LEN);
>
>                         /* Set source best burst size */
> -                       max_width = stm32_mdma_get_max_width(len, tlen);
> -                       if (src % max_width)
> -                               max_width = DMA_SLAVE_BUSWIDTH_1_BYTE;
> +                       max_width = stm32_mdma_get_max_width(len, src, tlen);
>                         src_bus_width = stm32_mdma_get_width(chan, max_width);
>
>                         max_burst = tlen / max_width;
> @@ -1030,9 +1029,7 @@ stm32_mdma_prep_dma_memcpy(struct dma_chan *c, dma_addr_t dest, dma_addr_t src,
>                                 STM32_MDMA_CTCR_SINCOS(src_bus_width);
>
>                         /* Set destination best burst size */
> -                       max_width = stm32_mdma_get_max_width(len, tlen);
> -                       if (dest % max_width)
> -                               max_width = DMA_SLAVE_BUSWIDTH_1_BYTE;
> +                       max_width = stm32_mdma_get_max_width(len, dest, tlen);
>                         dst_bus_width = stm32_mdma_get_width(chan, max_width);
>
>                         max_burst = tlen / max_width;
> --
> 2.9.0
>
>
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-- 
Benjamin Gaignard

Graphic Study Group

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