[PATCH v2] drm/ast: astdp: fix loop timeout check

[PATCH v2] drm/ast: astdp: fix loop timeout check

[Dan Carpenter]

This code has an issue because it loops until "i" is set to UINT_MAX but
the test for failure assumes that "i" is set to zero.  The result is that
it will only print an error message if we succeed on the very last try.
Reformat the loop to count forwards instead of backwards.

Fixes: 2281475168d2 ("drm/ast: astdp: Perform link training during atomic_enable")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
---
v2: In version one, I introduced a bug where it would msleep(100) after failure
    and that is a pointless thing to do.  Also change the loop to a for loop.
---
 drivers/gpu/drm/ast/ast_dp.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/ast/ast_dp.c b/drivers/gpu/drm/ast/ast_dp.c
index 5d07678b502c..9bc21dd6a54d 100644
--- a/drivers/gpu/drm/ast/ast_dp.c
+++ b/drivers/gpu/drm/ast/ast_dp.c
@@ -146,18 +146,16 @@ void ast_dp_power_on_off(struct drm_device *dev, bool on)
 void ast_dp_link_training(struct ast_device *ast)
 {
 	struct drm_device *dev = &ast->base;
-	unsigned int i = 10;
+	int i;
 
-	while (i--) {
+	for (i = 0; i < 10; i++) {
 		u8 vgacrdc = ast_get_index_reg(ast, AST_IO_VGACRI, 0xdc);
 
 		if (vgacrdc & AST_IO_VGACRDC_LINK_SUCCESS)
-			break;
-		if (i)
-			msleep(100);
+			return;
+		msleep(100);
 	}
-	if (!i)
-		drm_err(dev, "Link training failed\n");
+	drm_err(dev, "Link training failed\n");
 }
 
 void ast_dp_set_on_off(struct drm_device *dev, bool on)
-- 
2.43.0

Re: [PATCH v2] drm/ast: astdp: fix loop timeout check

[Thomas Zimmermann]

Hi

Am 12.08.24 um 08:42 schrieb Dan Carpenter:
> This code has an issue because it loops until "i" is set to UINT_MAX but
> the test for failure assumes that "i" is set to zero.  The result is that
> it will only print an error message if we succeed on the very last try.
> Reformat the loop to count forwards instead of backwards.
>
> Fixes: 2281475168d2 ("drm/ast: astdp: Perform link training during atomic_enable")
> Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
> ---
> v2: In version one, I introduced a bug where it would msleep(100) after failure
>      and that is a pointless thing to do.  Also change the loop to a for loop.
> ---
>   drivers/gpu/drm/ast/ast_dp.c | 12 +++++-------
>   1 file changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/ast/ast_dp.c b/drivers/gpu/drm/ast/ast_dp.c
> index 5d07678b502c..9bc21dd6a54d 100644
> --- a/drivers/gpu/drm/ast/ast_dp.c
> +++ b/drivers/gpu/drm/ast/ast_dp.c
> @@ -146,18 +146,16 @@ void ast_dp_power_on_off(struct drm_device *dev, bool on)
>   void ast_dp_link_training(struct ast_device *ast)
>   {
>   	struct drm_device *dev = &ast->base;
> -	unsigned int i = 10;
> +	int i;
>   
> -	while (i--) {
> +	for (i = 0; i < 10; i++) {
>   		u8 vgacrdc = ast_get_index_reg(ast, AST_IO_VGACRI, 0xdc);
>   
>   		if (vgacrdc & AST_IO_VGACRDC_LINK_SUCCESS)
> -			break;
> -		if (i)
> -			msleep(100);
> +			return;
> +		msleep(100);

But we don't want to wait during the final iteration of this loop. If 
you want to use the for loop, it should be something like

for (i= 0; i < 10; ++i) {

     if (i)
       msleep(100)

     // now test vgacrdc
}

Best regards
Thomas

>   	}
> -	if (!i)
> -		drm_err(dev, "Link training failed\n");
> +	drm_err(dev, "Link training failed\n");
>   }
>   
>   void ast_dp_set_on_off(struct drm_device *dev, bool on)

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

Re: [PATCH v2] drm/ast: astdp: fix loop timeout check

[Dan Carpenter]

On Mon, Aug 12, 2024 at 09:42:53AM +0300, Dan Carpenter wrote:
> This code has an issue because it loops until "i" is set to UINT_MAX but
> the test for failure assumes that "i" is set to zero.  The result is that
> it will only print an error message if we succeed on the very last try.
> Reformat the loop to count forwards instead of backwards.
> 
> Fixes: 2281475168d2 ("drm/ast: astdp: Perform link training during atomic_enable")
> Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
> ---
> v2: In version one, I introduced a bug where it would msleep(100) after failure
>     and that is a pointless thing to do.  Also change the loop to a for loop.

I mean this version also sleeps on the last failed iteration but at least there
isn't always true if statement to try prevent optimize away the last sleep.
I'm okay with a sleep on the slow path but not with pointless if statements. ;)

regards,
dan carpenter

Re: [PATCH v2] drm/ast: astdp: fix loop timeout check

[Dan Carpenter]

On Mon, Aug 12, 2024 at 08:48:16AM +0200, Thomas Zimmermann wrote:
> Hi
> 
> Am 12.08.24 um 08:42 schrieb Dan Carpenter:
> > This code has an issue because it loops until "i" is set to UINT_MAX but
> > the test for failure assumes that "i" is set to zero.  The result is that
> > it will only print an error message if we succeed on the very last try.
> > Reformat the loop to count forwards instead of backwards.
> > 
> > Fixes: 2281475168d2 ("drm/ast: astdp: Perform link training during atomic_enable")
> > Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
> > ---
> > v2: In version one, I introduced a bug where it would msleep(100) after failure
> >      and that is a pointless thing to do.  Also change the loop to a for loop.
> > ---
> >   drivers/gpu/drm/ast/ast_dp.c | 12 +++++-------
> >   1 file changed, 5 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/ast/ast_dp.c b/drivers/gpu/drm/ast/ast_dp.c
> > index 5d07678b502c..9bc21dd6a54d 100644
> > --- a/drivers/gpu/drm/ast/ast_dp.c
> > +++ b/drivers/gpu/drm/ast/ast_dp.c
> > @@ -146,18 +146,16 @@ void ast_dp_power_on_off(struct drm_device *dev, bool on)
> >   void ast_dp_link_training(struct ast_device *ast)
> >   {
> >   	struct drm_device *dev = &ast->base;
> > -	unsigned int i = 10;
> > +	int i;
> > -	while (i--) {
> > +	for (i = 0; i < 10; i++) {
> >   		u8 vgacrdc = ast_get_index_reg(ast, AST_IO_VGACRI, 0xdc);
> >   		if (vgacrdc & AST_IO_VGACRDC_LINK_SUCCESS)
> > -			break;
> > -		if (i)
> > -			msleep(100);
> > +			return;
> > +		msleep(100);
> 
> But we don't want to wait during the final iteration of this loop. If you
> want to use the for loop, it should be something like
> 
> for (i= 0; i < 10; ++i) {
> 
>     if (i)
>       msleep(100)
> 
>     // now test vgacrdc
> }
> 
> Best regards
> Thomas

I feel like if we really hit this failure path then we won't care about the
tenth msleep().  I can resend if you want, but I'd prefer to just leave it.

regards,
dan carpenter

Re: [PATCH v2] drm/ast: astdp: fix loop timeout check

[Thomas Zimmermann]

Hi

Am 12.08.24 um 08:54 schrieb Dan Carpenter:
> On Mon, Aug 12, 2024 at 08:48:16AM +0200, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 12.08.24 um 08:42 schrieb Dan Carpenter:
>>> This code has an issue because it loops until "i" is set to UINT_MAX but
>>> the test for failure assumes that "i" is set to zero.  The result is that
>>> it will only print an error message if we succeed on the very last try.
>>> Reformat the loop to count forwards instead of backwards.
>>>
>>> Fixes: 2281475168d2 ("drm/ast: astdp: Perform link training during atomic_enable")
>>> Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
>>> ---
>>> v2: In version one, I introduced a bug where it would msleep(100) after failure
>>>       and that is a pointless thing to do.  Also change the loop to a for loop.
>>> ---
>>>    drivers/gpu/drm/ast/ast_dp.c | 12 +++++-------
>>>    1 file changed, 5 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/ast/ast_dp.c b/drivers/gpu/drm/ast/ast_dp.c
>>> index 5d07678b502c..9bc21dd6a54d 100644
>>> --- a/drivers/gpu/drm/ast/ast_dp.c
>>> +++ b/drivers/gpu/drm/ast/ast_dp.c
>>> @@ -146,18 +146,16 @@ void ast_dp_power_on_off(struct drm_device *dev, bool on)
>>>    void ast_dp_link_training(struct ast_device *ast)
>>>    {
>>>    	struct drm_device *dev = &ast->base;
>>> -	unsigned int i = 10;
>>> +	int i;
>>> -	while (i--) {
>>> +	for (i = 0; i < 10; i++) {
>>>    		u8 vgacrdc = ast_get_index_reg(ast, AST_IO_VGACRI, 0xdc);
>>>    		if (vgacrdc & AST_IO_VGACRDC_LINK_SUCCESS)
>>> -			break;
>>> -		if (i)
>>> -			msleep(100);
>>> +			return;
>>> +		msleep(100);
>> But we don't want to wait during the final iteration of this loop. If you
>> want to use the for loop, it should be something like
>>
>> for (i= 0; i < 10; ++i) {
>>
>>      if (i)
>>        msleep(100)
>>
>>      // now test vgacrdc
>> }
>>
>> Best regards
>> Thomas
> I feel like if we really hit this failure path then we won't care about the
> tenth msleep().  I can resend if you want, but I'd prefer to just leave it.

Please resend. Even if the link training ultimately fails, the rest of 
DRM keeps running. 100 msec is not so short to shrug it off IMHO.

Best regards
Thomas

>
> regards,
> dan carpenter
>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

Re: [PATCH v2] drm/ast: astdp: fix loop timeout check

[Dan Carpenter]

On Mon, Aug 12, 2024 at 09:30:00AM +0200, Thomas Zimmermann wrote:
> > I feel like if we really hit this failure path then we won't care about the
> > tenth msleep().  I can resend if you want, but I'd prefer to just leave it.
> 
> Please resend. Even if the link training ultimately fails, the rest of DRM
> keeps running. 100 msec is not so short to shrug it off IMHO.
> 

Sure.  No problem.

regards,
dan carpenter