Re: [RFC] regmap_range_cfg usage with v4l2-cci

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 10/31/23 19:07, Kieran Bingham wrote:
> Quoting Hans de Goede (2023-10-31 17:26:58)
>> Hi,
>>
>> On 10/31/23 18:05, Alain Volmat wrote:
>>> Hi Hans,
>>>
>>> On Tue, Oct 31, 2023 at 10:53:16AM +0100, Hans de Goede wrote:
>>>> <resend with Alain added to the To: for some reason reply-to-all did not add Alain>
>>>
>>> No pb, I also received it via the mailing-list ;-)
>>>
>>>>
>>>> Hi Alain,
>>>>
>>>> On 10/30/23 18:36, Alain Volmat wrote:
>>>>> Hi,
>>>>>
>>>>> Goal of this email is to get first comments prior to posting a patch.
>>>>>
>>>>> Could we consider enhancements within the v4l2-cci in order to also
>>>>> allow regmap_range_cfg usage for paged register access ?
>>>>
>>>> Yes definitely.
>>>>
>>>> Extending v4l2-cci for other use cases was already briefly discussed
>>>> between Kieran (Cc-ed) and me:
>>>>
>>>> The CCI part of the MIPI CSI spec says that multi-byte registers are
>>>> always in big endian format, but some of the Sony IMX sensors actually
>>>> use little-endian format for multi-byte registers.
>>>>
>>>> The main reason why we need v4l2-cci and cannot use regmap directly is
>>>> because of the variable register width in CCI, where as regmap only
>>>> supports a single width. v4l2 cci uses 8 bits width in the underlying
>>>> regmap-config and then takes care of multy-byte registers by e.g.
>>>> reading multiple bytes and calling e.g. get_unaligned_be16() on
>>>> the read bytes.
>>>>
>>>> For the IMX scenario the plan is to add the notion of v4l2-cci
>>>> flags by adding this to include/media/v4l2-cci.h :
>>>>
>>>> struct v4l2_cci {
>>>>      struct regmap *map;
>>>>      long flags;
>>>> }
>>>>
>>>> And then change the prototype for devm_cci_regmap_init_i2c() to:
>>>>
>>>> struct v4l2_cci *devm_cci_regmap_init_i2c(struct i2c_client *client,
>>>>                                           int reg_addr_bits, long flags);
>>>>
>>>> And have devm_cci_regmap_init_i2c():
>>>> 1. devm_kmalloc() a struct v4l2_cci
>>>> 2. store the regmap there
>>>> 3. copy over flags from the function argument
>>>>
>>>> Combined with modifying all the other functions to take
>>>> "struct v4l2_cci *cci" as first argument instead of
>>>> "struct regmap *map".
>>>>
>>>> This change will require all existing sensor drivers using
>>>> v4l2-cci to be converted for the "struct regmap *map" ->
>>>> "struct v4l2_cci *cci" change, this all needs to be done
>>>> in one single commit adding the new struct + flags argument
>>>> to avoid breaking the compilation.
>>>>
>>>> Then once we have this a second patch can add:
>>>>
>>>> /* devm_cci_regmap_init_i2c() flags argument defines */
>>>> #define V4L2_CCI_DATA_LE     BIT(0)
>>>>
>>>> to include/media/v4l2-cci.h and make v4l2-cci.h honor
>>>> this flag solving the IMX scenario.
>>>
>>> I understand that in case of IMX sensors, ALL the multi-registers
>>> value are encoded in little-endian right ?
>>
>> Yes I believe so, Laurent, Kieran ?
> 
> I'm not 100% sure here, I think there are some IMX sensors with Little
> Endian - and some with Big Endian ... because all the same would be too
> easy.
> 
> I haven't seen a single device with mixed big and little endian yet
> though.
> 
> 
>>> In case of the GalaxyCore
>>> GC2145, most of the registers (page 0 / 1 and 2) are correctly
>>> encoded in big-endian, however page 3 (MIPI configuration) are
>>> 2 or 3 registers in little-endian.  So far maybe this is minor
>>> case, but the approach of having the endianness part of the v4l2_cci
>>> struct wouldn't allow to address such case ?
>>>
>>> Originally I thought we could have CCI_REG macros for little endian
>>> as well, such as CCI_REG16_LE etc etc since we anyway still have spare
>>> space I guess on top of the width part.  Drawback is that in drivers
>>> for IMX we would end-up with longer macros CCI_REG16_LE(...) instead
>>> of CCI_REG16(...).
>>
>> Hmm, that (CCI_REG16_LE etc) is an interesting proposal, that
>> would avoid the need to add a struct with flags and if I understand
>> things correctly then you would also not need any extra data
>> on top of the regmap, right ?
>>
>> I did not take the mixed endian case for data registers into
>> account yet. Since that apparently is a thing I think that
>> your CCI_REG16_LE etc proposal is better then adding a struct
>> with flags.
>>
>> Laurent, Kieran what do you think ?
> 
> Especially given there are some devices with multiple encodings, I like
> this.
> 
> I think it can simplify things if we don't need a separate 
> struct v4l2_cci allocated too.

Ok, lets go with (CCI_REG16_LE etc) then for the IMX sensors
and for the mixed endianess in the GalaxyCore sensors

Regards,

Hans