Re: [PATCH 09/22] drm/i915: Create color management files

["Sharma, Shashank" <shashank.sharma@intel.com>]

Thanks for the review Emil.
Please find my comments inline

Regards
Shashank
On 10/13/2015 6:29 PM, Emil Velikov wrote:
> On 10 October 2015 at 05:55, Sharma, Shashank <shashank.sharma@intel.com> wrote:
>> On 10/10/2015 4:17 AM, Emil Velikov wrote:
>>>
>>> Hi Shashank,
>>>
>>> On 9 October 2015 at 20:28, Shashank Sharma <shashank.sharma@intel.com>
>>> wrote:
>>> [snip]
>>>>
>>>> +
>>>> +/* Color management bit utilities */
>>>> +#define GET_BIT_MASK(n) ((1 << n) - 1)
>>>> +
>>>> +/* Read bits of a word from bit no. 'start'(lsb) till 'n' bits */
>>>> +#define GET_BITS(x, start, nbits) ((x >> start) & GET_BIT_MASK(nbits))
>>>> +
>>>> +/* Round off by adding 1 to the immediate lower bit */
>>>> +#define GET_BITS_ROUNDOFF(x, start, nbits) \
>>>> +       ((GET_BITS(x, start, (nbits + 1)) + 1) >> 1)
>>>> +
>>>> +/* Clear bits of a word from bit no. 'start' till nbits */
>>>> +#define CLEAR_BITS(x, start, nbits) ( \
>>>> +               x &= ~((GET_BIT_MASK(nbits) << start)))
>>>> +
>>>> +/* Write bit_pattern of no_bits bits in a target word */
>>>> +#define SET_BITS(target, bit_pattern, start_bit, no_bits) \
>>>> +               do { \
>>>> +                       CLEAR_BITS(target, start_bit, no_bits); \
>>>> +                       target |= (bit_pattern << start_bit);  \
>>>> +               } while (0)
>>>
>>> It feels suspicious that the kernel does not have macros for either
>>> one of these.
>>>
>>> I would invite you to take a look at include/linux/bitops.h and other
>>> kernel headers.
>>>
>> Thanks for pointing this out, but if you closely observe, these macros are
>> well tested, and created for color management operations, which have
>> specific requirements, like:
>> - pick 8 bits from 16th bit onwards, make them LSB, and give result:
>> GET_BITS
>> - take these 8 bits and move to bit 17th of the word, clearing the existing
>> ones: SET_BITS
>> For core register programming, this was required, so we created it. I would
>> still have a look
>> at the existing ones which you pointed out to avoid any duplication, if they
>> fall directly in the implementation, else I would like to continue with
>> these.
> Unless I'm missing something, these are generic bit manipulation
> macros, are they not ? As such I'd imagine we have some of these
> already available, but I cannot say which ones off-hand.
>
If you closely observe, what set_bit does is picks up the bit pattern 
from nth to n+reqd bit, moves it to LSB and returns it. similarly set 
bits clears the bits, then copy the bit pattern in the respective bits 
and manipulates the shifts. I could not find any such examples which I 
can directly use from suggested macros.

> Regards,
> Emil
>
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