[PATCH] Revert "spi: bitbang: only toggle bitchanges"

[PATCH] Revert "spi: bitbang: only toggle bitchanges"

[Lars Persson]

This reverts commit 232a5adc5199 ("spi: bitbang: only toggle
bitchanges") because it breaks bitbanged SPI on our MIPS system. I
found two problems with the patch:
- oldbit must initially be computed from bit position 7, 15 or 31 of
  word depending on the value of bits.
- The optimization also does not eliminate consecutive ones because
  the compare of 1<<31 and 1 will be false (a bool only takes values 0
  or 1).

Signed-off-by: Lars Persson <larper-VrBV9hrLPhE@public.gmane.org>
---
 drivers/spi/spi-bitbang-txrx.h | 18 ++++--------------
 1 file changed, 4 insertions(+), 14 deletions(-)

diff --git a/drivers/spi/spi-bitbang-txrx.h b/drivers/spi/spi-bitbang-txrx.h
index 06b34e5..c616e41 100644
--- a/drivers/spi/spi-bitbang-txrx.h
+++ b/drivers/spi/spi-bitbang-txrx.h
@@ -49,17 +49,12 @@ bitbang_txrx_be_cpha0(struct spi_device *spi,
 {
 	/* if (cpol == 0) this is SPI_MODE_0; else this is SPI_MODE_2 */
 
-	bool oldbit = !(word & 1);
 	/* clock starts at inactive polarity */
 	for (word <<= (32 - bits); likely(bits); bits--) {
 
 		/* setup MSB (to slave) on trailing edge */
-		if ((flags & SPI_MASTER_NO_TX) == 0) {
-			if ((word & (1 << 31)) != oldbit) {
-				setmosi(spi, word & (1 << 31));
-				oldbit = word & (1 << 31);
-			}
-		}
+		if ((flags & SPI_MASTER_NO_TX) == 0)
+			setmosi(spi, word & (1 << 31));
 		spidelay(nsecs);	/* T(setup) */
 
 		setsck(spi, !cpol);
@@ -81,18 +76,13 @@ bitbang_txrx_be_cpha1(struct spi_device *spi,
 {
 	/* if (cpol == 0) this is SPI_MODE_1; else this is SPI_MODE_3 */
 
-	bool oldbit = !(word & (1 << 31));
 	/* clock starts at inactive polarity */
 	for (word <<= (32 - bits); likely(bits); bits--) {
 
 		/* setup MSB (to slave) on leading edge */
 		setsck(spi, !cpol);
-		if ((flags & SPI_MASTER_NO_TX) == 0) {
-			if ((word & (1 << 31)) != oldbit) {
-				setmosi(spi, word & (1 << 31));
-				oldbit = word & (1 << 31);
-			}
-		}
+		if ((flags & SPI_MASTER_NO_TX) == 0)
+			setmosi(spi, word & (1 << 31));
 		spidelay(nsecs); /* T(setup) */
 
 		setsck(spi, cpol);
-- 
2.1.4

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Re: [PATCH] Revert "spi: bitbang: only toggle bitchanges"

[Mark Brown]

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On Fri, Jul 24, 2015 at 07:37:16AM +0200, Lars Persson wrote:
> This reverts commit 232a5adc5199 ("spi: bitbang: only toggle
> bitchanges") because it breaks bitbanged SPI on our MIPS system. I
> found two problems with the patch:
> - oldbit must initially be computed from bit position 7, 15 or 31 of
>   word depending on the value of bits.

This might be a real issue but fixing it does not require a revert.

> - The optimization also does not eliminate consecutive ones because
>   the compare of 1<<31 and 1 will be false (a bool only takes values 0
>   or 1).

No, any non-zero value is true in C.  I assume you're talking about
this section of the diff:

> -		if ((flags & SPI_MASTER_NO_TX) == 0) {
> -			if ((word & (1 << 31)) != oldbit) {
> -				setmosi(spi, word & (1 << 31));
> -				oldbit = word & (1 << 31);

which looks fine - we see if the top bit is the same as the last top bit
we wrote, if it isn't we update the value output and record what we just
wrote.

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Re: [PATCH] Revert "spi: bitbang: only toggle bitchanges"

[Lars Persson]

On Fri, 2015-07-24 at 12:44 +0200, Mark Brown wrote:
> On Fri, Jul 24, 2015 at 07:37:16AM +0200, Lars Persson wrote:
> > This reverts commit 232a5adc5199 ("spi: bitbang: only toggle
> > bitchanges") because it breaks bitbanged SPI on our MIPS system. I
> > found two problems with the patch:
> > - oldbit must initially be computed from bit position 7, 15 or 31 of
> >   word depending on the value of bits.
> 
> This might be a real issue but fixing it does not require a revert.
> 
> > - The optimization also does not eliminate consecutive ones because
> >   the compare of 1<<31 and 1 will be false (a bool only takes values 0
> >   or 1).
> 
> No, any non-zero value is true in C.  I assume you're talking about
> this section of the diff:
> 
> > -		if ((flags & SPI_MASTER_NO_TX) == 0) {
> > -			if ((word & (1 << 31)) != oldbit) {
> > -				setmosi(spi, word & (1 << 31));
> > -				oldbit = word & (1 << 31);
> 
> which looks fine - we see if the top bit is the same as the last top bit
> we wrote, if it isn't we update the value output and record what we just
> wrote.

No this is a misunderstanding about the semantics of the bool type.
When assigning to the bool any non-zero value becomes 1.
When comparing an integer against the bool, the bool is promoted to an
integer.

If we previously transmitted a one, oldbit equals 1.
If current bit is a one, then (word & (1 << 31)) equals 1<<31.

((1<<31) != 1) is TRUE so we call setmosi again, hence no optimization
for consecutive ones.

Please do not mix bool and bitwise operations.

- Lars

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Re: [PATCH] Revert "spi: bitbang: only toggle bitchanges"

[Jonas Gorski]

On Fri, Jul 24, 2015 at 12:44 PM, Mark Brown <broonie-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> wrote:
> On Fri, Jul 24, 2015 at 07:37:16AM +0200, Lars Persson wrote:
>> This reverts commit 232a5adc5199 ("spi: bitbang: only toggle
>> bitchanges") because it breaks bitbanged SPI on our MIPS system. I
>> found two problems with the patch:
>> - oldbit must initially be computed from bit position 7, 15 or 31 of
>>   word depending on the value of bits.
>
> This might be a real issue but fixing it does not require a revert.
>
>> - The optimization also does not eliminate consecutive ones because
>>   the compare of 1<<31 and 1 will be false (a bool only takes values 0
>>   or 1).
>
> No, any non-zero value is true in C.  I assume you're talking about
> this section of the diff:

While any value will be treated as true, the inverse of a value (!)
can only ever be 0 or 1, so

>> -             bool oldbit = !(word & 1);

makes oldbit to be either 0 or 1, and here

>> -                     if ((word & (1 << 31)) != oldbit) {

we compare the bool with an unsigned integer, thus the bool gets
promoted to unsigned integer as well, and we compare
0 or 1 with 0 or (1 << 31), so if oldbit is 1 this condition will
always evaluate to true.

Feel free to verify with a compiler of your choice ;-)

The fix for this is to use !!(word & (1 << 31)).


Jonas
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Re: [PATCH] Revert "spi: bitbang: only toggle bitchanges"

[Lars-Peter Clausen]

On 07/24/2015 01:48 PM, Lars Persson wrote:
> On Fri, 2015-07-24 at 12:44 +0200, Mark Brown wrote:
>> On Fri, Jul 24, 2015 at 07:37:16AM +0200, Lars Persson wrote:
>>> This reverts commit 232a5adc5199 ("spi: bitbang: only toggle
>>> bitchanges") because it breaks bitbanged SPI on our MIPS system. I
>>> found two problems with the patch:
>>> - oldbit must initially be computed from bit position 7, 15 or 31 of
>>>    word depending on the value of bits.
>>
>> This might be a real issue but fixing it does not require a revert.
>>
>>> - The optimization also does not eliminate consecutive ones because
>>>    the compare of 1<<31 and 1 will be false (a bool only takes values 0
>>>    or 1).
>>
>> No, any non-zero value is true in C.  I assume you're talking about
>> this section of the diff:
>>
>>> -		if ((flags & SPI_MASTER_NO_TX) == 0) {
>>> -			if ((word & (1 << 31)) != oldbit) {
>>> -				setmosi(spi, word & (1 << 31));
>>> -				oldbit = word & (1 << 31);
>>
>> which looks fine - we see if the top bit is the same as the last top bit
>> we wrote, if it isn't we update the value output and record what we just
>> wrote.
>
> No this is a misunderstanding about the semantics of the bool type.
> When assigning to the bool any non-zero value becomes 1.
> When comparing an integer against the bool, the bool is promoted to an
> integer.

 From the patch context alone it is not clear that oldbit is of type bool. I 
think this is where the confusion came from. By just looking at the path 
fragment itself you'd think that oldbit is a unsigned int given its usage 
pattern. But it becomes obvious that the code wont work if you know oldbit 
is a bool.

- Lars

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Re: [PATCH] Revert "spi: bitbang: only toggle bitchanges"

[Michael Grzeschik]

On Fri, Jul 24, 2015 at 07:37:16AM +0200, Lars Persson wrote:
> This reverts commit 232a5adc5199 ("spi: bitbang: only toggle
> bitchanges") because it breaks bitbanged SPI on our MIPS system. I
> found two problems with the patch:
> - oldbit must initially be computed from bit position 7, 15 or 31 of
>   word depending on the value of bits.
> - The optimization also does not eliminate consecutive ones because
>   the compare of 1<<31 and 1 will be false (a bool only takes values 0
>   or 1).
> 
> Signed-off-by: Lars Persson <larper-VrBV9hrLPhE@public.gmane.org>
> ---
>  drivers/spi/spi-bitbang-txrx.h | 18 ++++--------------
>  1 file changed, 4 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/spi/spi-bitbang-txrx.h b/drivers/spi/spi-bitbang-txrx.h
> index 06b34e5..c616e41 100644
> --- a/drivers/spi/spi-bitbang-txrx.h
> +++ b/drivers/spi/spi-bitbang-txrx.h
> @@ -49,17 +49,12 @@ bitbang_txrx_be_cpha0(struct spi_device *spi,
>  {
>  	/* if (cpol == 0) this is SPI_MODE_0; else this is SPI_MODE_2 */
>  
> -	bool oldbit = !(word & 1);
>  	/* clock starts at inactive polarity */
>  	for (word <<= (32 - bits); likely(bits); bits--) {
>  
>  		/* setup MSB (to slave) on trailing edge */
> -		if ((flags & SPI_MASTER_NO_TX) == 0) {

> -			if ((word & (1 << 31)) != oldbit) {
This needs to become:
			if (!!(word & (1 << 31)) != oldbit) {

so oldbit really get compared by bool and not by int.

> -				setmosi(spi, word & (1 << 31));
> -				oldbit = word & (1 << 31);
> -			}
> -		}
> +		if ((flags & SPI_MASTER_NO_TX) == 0)
> +			setmosi(spi, word & (1 << 31));
>  		spidelay(nsecs);	/* T(setup) */
>  
>  		setsck(spi, !cpol);
> @@ -81,18 +76,13 @@ bitbang_txrx_be_cpha1(struct spi_device *spi,
>  {
>  	/* if (cpol == 0) this is SPI_MODE_1; else this is SPI_MODE_3 */
>  
> -	bool oldbit = !(word & (1 << 31));
>  	/* clock starts at inactive polarity */
>  	for (word <<= (32 - bits); likely(bits); bits--) {
>  
>  		/* setup MSB (to slave) on leading edge */
>  		setsck(spi, !cpol);
> -		if ((flags & SPI_MASTER_NO_TX) == 0) {
> -			if ((word & (1 << 31)) != oldbit) {

and this needs to become:
			if (!!(word & (1 << 31)) != oldbit) {

> -				setmosi(spi, word & (1 << 31));
> -				oldbit = word & (1 << 31);
> -			}
> -		}
> +		if ((flags & SPI_MASTER_NO_TX) == 0)
> +			setmosi(spi, word & (1 << 31));
>  		spidelay(nsecs); /* T(setup) */
>  
>  		setsck(spi, cpol);


Sorry for the issue,
Michael

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Re: [PATCH] Revert "spi: bitbang: only toggle bitchanges"

[Mark Brown]

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On Fri, Jul 24, 2015 at 01:55:30PM +0200, Lars-Peter Clausen wrote:
> On 07/24/2015 01:48 PM, Lars Persson wrote:

> >No this is a misunderstanding about the semantics of the bool type.
> >When assigning to the bool any non-zero value becomes 1.
> >When comparing an integer against the bool, the bool is promoted to an
> >integer.

> From the patch context alone it is not clear that oldbit is of type bool. I
> think this is where the confusion came from. By just looking at the path
> fragment itself you'd think that oldbit is a unsigned int given its usage
> pattern. But it becomes obvious that the code wont work if you know oldbit
> is a bool.

...and obviously the clear intention is that it should be an unsigned
integer so the appropriate thing to do is to fix that.

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