On 02/09/18 14:04, Laurent Pinchart wrote:
> Hi Hans,
> On Friday, 9 February 2018 15:00:53 EET Hans Verkuil wrote:
>> On 02/09/18 13:44, Sakari Ailus wrote:
>>> On Fri, Feb 09, 2018 at 01:18:18PM +0100, Hans Verkuil wrote:
>>>> On 02/09/18 13:01, Sakari Ailus wrote:
>>>>> On Thu, Feb 08, 2018 at 09:36:46AM +0100, Hans Verkuil wrote:
>>>>>> The VIDIOC_DBG_G/S_REGISTER ioctls imply that VIDIOC_DBG_G_CHIP_INFO is
>>>>>> also present, since without that you cannot use v4l2-dbg.
>>>>>> Just like the implementation in v4l2-ioctl.c this can be implemented in
>>>>>> the
>>>>>> core and no drivers need to be modified.
>>>>>> It also makes it possible for v4l2-compliance to properly test the
>>>>>> VIDIOC_DBG_G/S_REGISTER ioctls.
>>>>>> Signed-off-by: Hans Verkuil <hans.verk...@cisco.com>
>>>>>> ---
>>>>>>  drivers/media/v4l2-core/v4l2-subdev.c | 13 +++++++++++++
>>>>>>  1 file changed, 13 insertions(+)
>>>>>> diff --git a/drivers/media/v4l2-core/v4l2-subdev.c
>>>>>> b/drivers/media/v4l2-core/v4l2-subdev.c index
>>>>>> 6cabfa32d2ed..2a5b5a3fa7a3 100644
>>>>>> --- a/drivers/media/v4l2-core/v4l2-subdev.c
>>>>>> +++ b/drivers/media/v4l2-core/v4l2-subdev.c
>>>>>> @@ -255,6 +255,19 @@ static long subdev_do_ioctl(struct file *file,
>>>>>> unsigned int cmd, void *arg)>>>> 
>>>>>>                          return -EPERM;
>>>>>>                  return v4l2_subdev_call(sd, core, s_register, p);
>>>>>>          }
>>>>>> +        case VIDIOC_DBG_G_CHIP_INFO:
>>>>>> +        {
>>>>>> +                struct v4l2_dbg_chip_info *p = arg;
>>>>>> +
>>>>>> +                if (p->match.type != V4L2_CHIP_MATCH_SUBDEV || 
>>>>>> p->match.addr)
>>>>>> +                        return -EINVAL;
>>>>>> +                if (sd->ops->core && sd->ops->core->s_register)
>>>>>> +                        p->flags |= V4L2_CHIP_FL_WRITABLE;
>>>>>> +                if (sd->ops->core && sd->ops->core->g_register)
>>>>>> +                        p->flags |= V4L2_CHIP_FL_READABLE;
>>>>>> +                strlcpy(p->name, sd->name, sizeof(p->name));
>>>>>> +                return 0;
>>>>>> +        }
>>>>> This is effectively doing the same as debugfs except that it's specific
>>>>> to V4L2. I don't think we should endorse its use, and especially not
>>>>> without a real use case.
>>>> We (Cisco) use it all the time. Furthermore, this works for any bus, not
>>>> just i2c. Also spi, internal register busses, etc.
>>>> It's been in use for many years. More importantly, there is no excuse to
>>>> have only half the API implemented.
>>>> It's all fine to talk about debugfs, but are you going to make that? This
>>>> API works, it's supported by v4l2-dbg, it's in use. Now, let's at least
>>>> make it pass v4l2-compliance.
>>>> I agree, if we would redesign it, we would use debugfs. But I think it
>>>> didn't even exist when this was made. So this API is here to stay and
>>>> all it takes is this ioctl of code to add the missing piece for subdevs.
>>>> Nobody is going to make a replacement for this using debugfs. Why spend
>>>> effort on it if we already have an API for this?
>>> It's not the first case when a more generic API replaces a subsystem
>>> specific one. We have another conversion to make, switching from
>>> implementing s_power() callback in drivers to runtime PM for instance.
>>> I simply want to point out that this patch is endorsing something which is
>>> obsolete and not needed: no-one has complained about the lack of this for
>>> sub-devices, haven't they?
>>> I'd just remove the check from v4l-compliance or make it optional. New
>>> drivers should use debugfs instead if something like that is needed.
>> You are correct in one respect: we use this API, but with video devices.
>> So subdevices support the g/s_register ops, and they are called via
>> /dev/videoX.
>> We can remove the ioctl support from v4l2-subdev.c (not the g/s_register
>> ops!). Without VIDIOC_DBG_G_CHIP_INFO I don't think v4l2-dbg is usable.
>> Although it is always possible to call the ioctl directly, of course.
>> So if Mauro would agree to this, the DBG ioctl support in v4l2-subdev can be
>> removed.
> That would be my preferred option.
>> But either remove them, or add this ioctl. Don't leave it in a zombie state.
>> Personally I see no harm whatsoever in just adding VIDIOC_DBG_G_CHIP_INFO.
>> If someone ever makes a patch to switch over to debugfs then these ioctls
>> can be removed.
>> BTW, how would new drivers use debugfs for this? Does regmap provide such
>> access?
> Before attempting to provide an answer, as I've never used those ioctls 
> myself, could you please give us a bit more information about the use cases 
> you have at Cisco for this ?

Exactly what it was made for: debugging issues by reading/writing registers
on the fly. It's very similar to using i2cget/set for i2c devices, except it
can also be used for registers in e.g. an IP block. It's also a bit safer
since you can filter (if needed) which addresses can be written.

One thing I have seen in a code review from Sakari for a sensor driver that
implemented the g/s_dbg_register ops: I think it is not right to reject a
patch based on that. It is used (albeit not through a v4l-subdev device
node) and unless someone can provide a working alternative (and 'make something
in debugfs' is not a valid alternative) it should not be a reason for
rejecting it.

It works, it's been there for ages, and we never said that it is suddenly
no longer allowed to be used. And we certainly have not provided an alternative
to this API.



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