[PATCH 04/33] drm/amd/display: Use 4096 lut entries

[Harry Wentland]

On 2018-02-27 11:08 AM, Michel Dänzer wrote:
> On 2018-02-27 04:54 PM, Leo Li wrote:
>> On 2018-02-27 05:34 AM, Michel Dänzer wrote:
>>> On 2018-02-26 09:15 PM, Harry Wentland wrote:
>>>> From: "Leo (Sunpeng) Li" <sunpeng.li at amd.com>
>>>>
>>>> Non-legacy LUT size should reflect hw capability. Change size from 256
>>>> to 4096.
>>>>
>>>> However, X doesn't seem to play with legacy LUTs of such size.
>>>> Therefore, check for legacy lut when updating DC states, and update
>>>> accordingly.
>>>>
>>>> v2: Use a macro for the maximum drm LUT value.
>>>>
>>>> Signed-off-by: Leo (Sunpeng) Li <sunpeng.li at amd.com>
>>>> Reviewed-by: Harry Wentland <Harry.Wentland at amd.com>
>>>> ---
>>>>   drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c  |  2 +-
>>>>   drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h  |  4 +-
>>>>   .../drm/amd/display/amdgpu_dm/amdgpu_dm_color.c    | 77
>>>> ++++++++++++++++------
>>>>   3 files changed, 61 insertions(+), 22 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>>>> b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>>>> index bb9405daa087..f782518fc424 100644
>>>> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>>>> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>>>> @@ -3228,7 +3228,7 @@ static int amdgpu_dm_crtc_init(struct
>>>> amdgpu_display_manager *dm,
>>>>       dm->adev->mode_info.crtcs[crtc_index] = acrtc;
>>>>       drm_crtc_enable_color_mgmt(&acrtc->base, MAX_COLOR_LUT_ENTRIES,
>>>>                      true, MAX_COLOR_LUT_ENTRIES);
>>>> -    drm_mode_crtc_set_gamma_size(&acrtc->base, MAX_COLOR_LUT_ENTRIES);
>>>> +    drm_mode_crtc_set_gamma_size(&acrtc->base,
>>>> MAX_COLOR_LEGACY_LUT_ENTRIES);
>>>
>>> This means userspace can't set gamma ramps with more than 256 entries,
>>> since drm_mode_gamma_set_ioctl returns an error if the sizes don't
>>> match. We should just fix userspace which tries to use the wrong size.
>>
>> It seems X hard-codes 256 entries for the legacy lut, ignoring what the
>> kernel requests:
>>
>> xf86CrtcCreate in xserver/hw/xfree86/modes/xf86Crtc.c (~line 120):
>>
>> /* Preallocate gamma at a sensible size. */
>> crtc->gamma_size = 256;
>>
>>
>> I confirmed this as well via a printk on the user LUT size in
>> drm_mode_gamma_set_ioctl (after reverting to 4096 gamma size, of course).
>>
>> I'm not sure if modifying that line in X will be straightforward.
> 
> It could be overridden by the DDX driver. However, there is the issue of
> using an older DDX driver with a newer kernel.
> 
> 
>> It's worth noting that drm_mode_crtc_set_gamma_size() only sets the legacy
>> gamma size. Non-legacy gamma (de/regamma) sizes are reported via a mode
>> property, and does not conflict with legacy size. Therefore, reporting a
>> different size for legacy shouldn't be an issue, AFAIK; as long as we
>> check the DRM lut size and update DC accordingly, which we're doing.
> 
> Makes sense, but it means that gamma will not work for 30-bit colour in
> Xorg until the DDX driver uses the new gamma mechanism, which might
> require switching to the atomic KMS API as well. Is that okay?
> 

That's still current behavior, so this patch is not changing anything there. We could increase it but how would we deal with old DDX/X that assumes 256? Would it make sense instead to teach X to set the new gamma_lut property?

Harry

>