diff mbox series

drm/amd/display: use a more accurate check in dm_helpers_dp_read_dpcd()

Message ID 20230309213027.256243-1-hamza.mahfooz@amd.com
State New
Headers
Series drm/amd/display: use a more accurate check in dm_helpers_dp_read_dpcd() |

Commit Message

Hamza Mahfooz March 9, 2023, 9:30 p.m. UTC 
  We should be checking if drm_dp_dpcd_read() returns the size that we are
asking it to read instead of just checking if it is greater than zero.
Also, we should WARN_ON() here since this condition is only ever met, if
there is an issue worth investigating. So, compare the return value of
drm_dp_dpcd_read() to size and WARN_ON() if they aren't equal.

Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
---
 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Rodrigo Siqueira Jordao March 10, 2023, 5:42 p.m. UTC | #1 
On 3/9/23 14:30, Hamza Mahfooz wrote:
> We should be checking if drm_dp_dpcd_read() returns the size that we are
> asking it to read instead of just checking if it is greater than zero.
> Also, we should WARN_ON() here since this condition is only ever met, if
> there is an issue worth investigating. So, compare the return value of
> drm_dp_dpcd_read() to size and WARN_ON() if they aren't equal.
> 
> Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
> ---
>   drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> index 8d598b322e5b..ed2ed7b1d869 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> @@ -511,8 +511,8 @@ bool dm_helpers_dp_read_dpcd(
>   		return false;
>   	}
>   
> -	return drm_dp_dpcd_read(&aconnector->dm_dp_aux.aux, address,
> -			data, size) > 0;
> +	return !WARN_ON(drm_dp_dpcd_read(&aconnector->dm_dp_aux.aux, address,
> +					 data, size) != size);
>   }
>   
>   bool dm_helpers_dp_write_dpcd(


Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>

and pushed to amd-staging-drm-next.

Thanks
Siqueira
Ville Syrjälä March 10, 2023, 5:48 p.m. UTC | #2 
On Thu, Mar 09, 2023 at 04:30:27PM -0500, Hamza Mahfooz wrote:
> We should be checking if drm_dp_dpcd_read() returns the size that we are
> asking it to read instead of just checking if it is greater than zero.
> Also, we should WARN_ON() here since this condition is only ever met, if
> there is an issue worth investigating. So, compare the return value of
> drm_dp_dpcd_read() to size and WARN_ON() if they aren't equal.
> 
> Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
> ---
>  drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> index 8d598b322e5b..ed2ed7b1d869 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> @@ -511,8 +511,8 @@ bool dm_helpers_dp_read_dpcd(
>  		return false;
>  	}
>  
> -	return drm_dp_dpcd_read(&aconnector->dm_dp_aux.aux, address,
> -			data, size) > 0;
> +	return !WARN_ON(drm_dp_dpcd_read(&aconnector->dm_dp_aux.aux, address,
> +					 data, size) != size);

Just FYI there are devices out there that violate the DP spec and reads
from specific DPCD registers simply fail instead of returning the
expected 0.

>  }
>  
>  bool dm_helpers_dp_write_dpcd(
> -- 
> 2.39.2
Ville Syrjälä March 10, 2023, 5:51 p.m. UTC | #3 
On Fri, Mar 10, 2023 at 07:48:04PM +0200, Ville Syrjälä wrote:
> On Thu, Mar 09, 2023 at 04:30:27PM -0500, Hamza Mahfooz wrote:
> > We should be checking if drm_dp_dpcd_read() returns the size that we are
> > asking it to read instead of just checking if it is greater than zero.
> > Also, we should WARN_ON() here since this condition is only ever met, if
> > there is an issue worth investigating. So, compare the return value of
> > drm_dp_dpcd_read() to size and WARN_ON() if they aren't equal.
> > 
> > Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
> > ---
> >  drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> > index 8d598b322e5b..ed2ed7b1d869 100644
> > --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> > +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> > @@ -511,8 +511,8 @@ bool dm_helpers_dp_read_dpcd(
> >  		return false;
> >  	}
> >  
> > -	return drm_dp_dpcd_read(&aconnector->dm_dp_aux.aux, address,
> > -			data, size) > 0;
> > +	return !WARN_ON(drm_dp_dpcd_read(&aconnector->dm_dp_aux.aux, address,
> > +					 data, size) != size);
> 
> Just FYI there are devices out there that violate the DP spec and reads
> from specific DPCD registers simply fail instead of returning the
> expected 0.

And of course anyone can yank the cable anytime, so in
fact pretty much any DPCD read can fail.
Harry Wentland March 20, 2023, 8:25 p.m. UTC | #4 
On 3/10/23 12:51, Ville Syrjälä wrote:
> On Fri, Mar 10, 2023 at 07:48:04PM +0200, Ville Syrjälä wrote:
>> On Thu, Mar 09, 2023 at 04:30:27PM -0500, Hamza Mahfooz wrote:
>>> We should be checking if drm_dp_dpcd_read() returns the size that we are
>>> asking it to read instead of just checking if it is greater than zero.
>>> Also, we should WARN_ON() here since this condition is only ever met, if
>>> there is an issue worth investigating. So, compare the return value of
>>> drm_dp_dpcd_read() to size and WARN_ON() if they aren't equal.
>>>
>>> Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
>>> ---
>>>  drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
>>> index 8d598b322e5b..ed2ed7b1d869 100644
>>> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
>>> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
>>> @@ -511,8 +511,8 @@ bool dm_helpers_dp_read_dpcd(
>>>  		return false;
>>>  	}
>>>  
>>> -	return drm_dp_dpcd_read(&aconnector->dm_dp_aux.aux, address,
>>> -			data, size) > 0;
>>> +	return !WARN_ON(drm_dp_dpcd_read(&aconnector->dm_dp_aux.aux, address,
>>> +					 data, size) != size);
>>
>> Just FYI there are devices out there that violate the DP spec and reads
>> from specific DPCD registers simply fail instead of returning the
>> expected 0.
> 
> And of course anyone can yank the cable anytime, so in
> fact pretty much any DPCD read can fail.
> 

Thanks for making this very important point. It seems like drm_dp_dpcd_access
checks for that, though, and returns -EPROTO if !(ret == size). So I don't
expect this patch to change any behavior.

Harry
diff mbox series

Patch

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
index 8d598b322e5b..ed2ed7b1d869 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
@@ -511,8 +511,8 @@  bool dm_helpers_dp_read_dpcd(
 		return false;
 	}
 
-	return drm_dp_dpcd_read(&aconnector->dm_dp_aux.aux, address,
-			data, size) > 0;
+	return !WARN_ON(drm_dp_dpcd_read(&aconnector->dm_dp_aux.aux, address,
+					 data, size) != size);
 }
 
 bool dm_helpers_dp_write_dpcd(