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

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

[Arnd Bergmann]

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@arndb.de>
---
 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;
+
 	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

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

[Benjamin Gaignard]

2017-10-11 16:01 GMT+02:00 Arnd Bergmann <arnd@arndb.de>:
> 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@arndb.de>
> ---
>  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
>
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel



-- 
Benjamin Gaignard

Graphic Study Group

Linaro.org │ Open source software for ARM SoCs

Follow Linaro: Facebook | Twitter | Blog

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

[Arnd Bergmann]

On Wed, Oct 11, 2017 at 4:27 PM, Benjamin Gaignard
<benjamin.gaignard@linaro.org> wrote:
> 2017-10-11 16:01 GMT+02:00 Arnd Bergmann <arnd@arndb.de>:
>
>> @@ -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 ?
>

I wouldn't want to add a do_div() here, since it's guaranteed
not to be needed. Would you prefer an explicit cast here
and leave the argument as dma_addr_t?

We could also use a bit mask here like

  if (addr & (max_width-1))

or we could combined it with the check above:

                if ((((buf_len | addr) & (max_width - 1)) == 0) &&
                   (tlen >= max_width))

       Arnd

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

[Benjamin Gaignard]

2017-10-11 16:39 GMT+02:00 Arnd Bergmann <arnd@arndb.de>:
> On Wed, Oct 11, 2017 at 4:27 PM, Benjamin Gaignard
> <benjamin.gaignard@linaro.org> wrote:
>> 2017-10-11 16:01 GMT+02:00 Arnd Bergmann <arnd@arndb.de>:
>>
>>> @@ -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 ?
>>
>
> I wouldn't want to add a do_div() here, since it's guaranteed
> not to be needed. Would you prefer an explicit cast here
> and leave the argument as dma_addr_t?
>
> We could also use a bit mask here like
>
>   if (addr & (max_width-1))

That sound better for me since it doesn't limit the code to 32 bits architecture

>
> or we could combined it with the check above:
>
>                 if ((((buf_len | addr) & (max_width - 1)) == 0) &&
>                    (tlen >= max_width))

No it is more simple to read with two checks

Benjamin
>
>        Arnd

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

[Arnd Bergmann]

On Wed, Oct 11, 2017 at 4:46 PM, Benjamin Gaignard
<benjamin.gaignard@linaro.org> wrote:
> 2017-10-11 16:39 GMT+02:00 Arnd Bergmann <arnd@arndb.de>:
>> On Wed, Oct 11, 2017 at 4:27 PM, Benjamin Gaignard
>> <benjamin.gaignard@linaro.org> wrote:
>>> 2017-10-11 16:01 GMT+02:00 Arnd Bergmann <arnd@arndb.de>:
>>>
>>>> @@ -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 ?
>>>
>>
>> I wouldn't want to add a do_div() here, since it's guaranteed
>> not to be needed. Would you prefer an explicit cast here
>> and leave the argument as dma_addr_t?
>>
>> We could also use a bit mask here like
>>
>>   if (addr & (max_width-1))
>
> That sound better for me since it doesn't limit the code to 32 bits architecture

FWIW, I used the u32 type here because that's the limit of the
dma driver, the dma_addr_t gets converted to that anyway
later.

>>
>> or we could combined it with the check above:
>>
>>                 if ((((buf_len | addr) & (max_width - 1)) == 0) &&
>>                    (tlen >= max_width))
>
> No it is more simple to read with two checks

I should have mentioned that this variant would also change
behavior: the current code falls back to byte access when
the address alignment is less than the length alignment.
The change I suggested here would change that to use
the maximum possible address width that fits the alignment
of either size or address.

I don't know what behavior we actually want though, or
if that change would be correct.

      Arnd

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

[Pierre Yves MORDRET]

On 10/11/2017 05:13 PM, Arnd Bergmann wrote:
> On Wed, Oct 11, 2017 at 4:46 PM, Benjamin Gaignard
> <benjamin.gaignard@linaro.org> wrote:
>> 2017-10-11 16:39 GMT+02:00 Arnd Bergmann <arnd@arndb.de>:
>>> On Wed, Oct 11, 2017 at 4:27 PM, Benjamin Gaignard
>>> <benjamin.gaignard@linaro.org> wrote:
>>>> 2017-10-11 16:01 GMT+02:00 Arnd Bergmann <arnd@arndb.de>:
>>>>
>>>>> @@ -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 ?
>>>>
>>>
>>> I wouldn't want to add a do_div() here, since it's guaranteed
>>> not to be needed. Would you prefer an explicit cast here
>>> and leave the argument as dma_addr_t?
>>>
>>> We could also use a bit mask here like
>>>
>>>   if (addr & (max_width-1))
>>
>> That sound better for me since it doesn't limit the code to 32 bits architecture
> 
> FWIW, I used the u32 type here because that's the limit of the
> dma driver, the dma_addr_t gets converted to that anyway
> later.
> 
>>>
>>> or we could combined it with the check above:
>>>
>>>                 if ((((buf_len | addr) & (max_width - 1)) == 0) &&
>>>                    (tlen >= max_width))
>>
>> No it is more simple to read with two checks
> 
> I should have mentioned that this variant would also change
> behavior: the current code falls back to byte access when
> the address alignment is less than the length alignment.
> The change I suggested here would change that to use
> the maximum possible address width that fits the alignment
> of either size or address.

Both alignment are required on address and length.
The main advantage result is maximized in term of width. As for now I don't see
any drawback except a short explanation.
Nonetheless I need to think a little bit more about this change.

> 
> I don't know what behavior we actually want though, or
> if that change would be correct.
> 
>       Arnd
> 

Regards
Py