RFC: DSI host capabilities (was: [PATCH RFC 03/10] drm/panel: Add LGD panel driver for Sony Xperia XZ3)

[AngeloGioacchino Del Regno]

Il 30/05/23 17:44, Neil Armstrong ha scritto:
> On 30/05/2023 14:36, Dmitry Baryshkov wrote:
>> On 30/05/2023 15:15, AngeloGioacchino Del Regno wrote:
>>> Il 30/05/23 13:44, Dmitry Baryshkov ha scritto:
>>>> On Tue, 30 May 2023 at 10:24, Neil Armstrong <neil.armstrong at linaro.org> wrote:
>>>>>
>>>>> Hi Marijn, Dmitry, Caleb, Jessica,
>>>>>
>>>>> On 29/05/2023 23:11, Marijn Suijten wrote:
>>>>>> On 2023-05-22 04:16:20, Dmitry Baryshkov wrote:
>>>>>> <snip>
>>>>>>>> +   if (ctx->dsi->dsc) {
>>>>>>>
>>>>>>> dsi->dsc is always set, thus this condition can be dropped.
>>>>>>
>>>>>> I want to leave room for possibly running the panel without DSC (at a
>>>>>> lower resolution/refresh rate, or at higher power consumption if there
>>>>>> is enough BW) by not assigning the pointer, if we get access to panel
>>>>>> documentation: probably one of the magic commands sent in this driver
>>>>>> controls it but we don't know which.
>>>>>
>>>>> I'd like to investigate if DSC should perhaps only be enabled if we
>>>>> run non certain platforms/socs ?
>>>>>
>>>>> I mean, we don't know if the controller supports DSC and those particular
>>>>> DSC parameters so we should probably start adding something like :
>>>>>
>>>>> static drm_dsc_config dsc_params_qcom = {}
>>>>>
>>>>> static const struct of_device_id panel_of_dsc_params[] = {
>>>>>          { .compatible = "qcom,sm8150", , .data = &dsc_params_qcom },
>>>>>          { .compatible = "qcom,sm8250", , .data = &dsc_params_qcom },
>>>>>          { .compatible = "qcom,sm8350", , .data = &dsc_params_qcom },
>>>>>          { .compatible = "qcom,sm8450", , .data = &dsc_params_qcom },
>>>>> };
>>>>
>>>> I think this would damage the reusability of the drivers. The panel
>>>> driver does not actually care if the SoC is SM8350, sunxi-something or
>>>> RCar.
>>>> Instead it cares about host capabilities.
>>>>
>>>> I think instead we should extend mipi_dsi_host:
>>>>
>>>> #define MIPI_DSI_HOST_MODE_VIDEO BIT(0)
> 
> I assume all DSI controller supports Video mode, so it should be a negative here
> if for a reason it's not the case.

Either all positive or all negative... and yes I agree that all DSI controllers
support video mode nowadays, but:
  - Will that be true for future controllers? (likely yes, but you never know)
  - Is there any controller driver not implementing video mode?
    - Will there be one in the future?

> 
> There should also be a flag to tell if sending LP commands sending while
> in HS Video mode is supported.
> 

+1. This is the case for both qcom and mtk.

>>>> #define MIPI_DSI_HOST_MODE_CMD  BIT(1)
>>>> #define MIPI_DSI_HOST_VIDEO_SUPPORTS_COMMANDS BIT(2)
>>>> // FIXME: do we need to provide additional caps here ?
>>>>
>>>> #define MIPI_DSI_DSC_1_1 BIT(0)
>>>> #define MIPI_DSI_DSC_1_2 BIT(1)
>>>> #define MIPI_DSI_DSC_NATIVE_422 BIT(2)
>>>> #define MIPI_DSI_DSC_NATIVE_420 BIT(3)
>>>> #define MIPI_DSI_DSC_FRAC_BPP BIT(4)
>>>> // etc.
>>>>
>>>> struct mipi_dsi_host {
>>>>   // new fields only
>>>>    unsigned long mode_flags;
>>>>    unsigned long dsc_flags;
>>>> };
>>>>
>>>> Then the panel driver can adapt itself to the host capabilities and
>>>> (possibly) select one of the internally supported DSC profiles.
>>>>
>>>
>>> I completely agree about extending mipi_dsi_host, other SoCs could reuse that and
>>> support for DSC panels would become a lot cleaner.
>>
>> Sounds good. I will wait for one or two more days (to get the possible feedback 
>> on fields/flags/etc) and post an RFC patch to dri-devel.
> 
> Good, I was waiting until a DSC panel appears on the list (and I failed to be the 
> first), it's now the case.
> 
> For VTRD6130, the panel is capable of the 4 modes:
> - video mode
> - command mode
> - video mode & DSC
> - command mode & DSC
> 
> So it would need such info to enable one of the mode in some order to determine.
> 

Dynamically determining is not trivial, as that depends on multiple variables:
  - Availability of the modes (obviously)
  - Available lanes
    - Available bandwidth per lane
      - Available total bandwidth
  - Power consumption considerations (DSC IP may be using more or less power
    depending on the actual SoC//controller)
    - Thermal management: DSC may make no thermal sense as in, more heat output
      vs thermal envelope (laptop vs embedded vs handset)
  - Others

Hence, the implementation should also provide a way of choosing a preferred mode
on a per-controller basis (DSC or no compression).

Just a few considerations that came to mind with a good sleep.

Cheers!

> Thanks,
> Neil
>>
>>>
>>> For example, on MediaTek DRM there's some support for DSC, more or less the same
>>> for SPRD DRM and some DSI bridge drivers... having a clean infrastructure would
>>> definitely help.
>>>
>>> I'm sad I cannot offer testing in that case because despite being sure that there
>>> are MTK smartphones around with DSI panels using DSC, I have none... and all of the
>>> Chromebooks are not using DSC anyway (but using DisplayPort compression, which is
>>> obviously an entirely different beast).
>>>
>>>>>
>>>>> ...
>>>>> static int sony_akatsuki_lgd_probe(struct mipi_dsi_device *dsi)
>>>>> ...
>>>>>          const struct of_device_id *match;
>>>>>
>>>>> ...
>>>>>          match = of_match_node(panel_of_dsc_params, of_root);
>>>>>          if (match && match->data) {
>>>>>                  dsi->dsc = devm_kzalloc(&dsi->dev, sizeof(*dsc), GFP_KERNEL);
>>>>>                  memcpy(dsi->dsc, match->data, sizeof(*dsc));
>>>>>          } else {
>>>>>                  dev_warn(&dsi->dev, "DSI controller is not marked as 
>>>>> supporting DSC\n");
>>>>>          }
>>>>> ...
>>>>> }
>>>>>
>>>>> and probably bail out if it's a DSC only panel.
>>>>>
>>>
>>> Usually DDICs support both DSC and non-DSC modes, depending on the initial
>>> programming (read: init commands)... but the usual issue is that many DDICs
>>> are not publicly documented for reasons, so yes, bailing out if DSC is not
>>> supported would be the only option, and would be fine at this point.
>>>
>>> Cheers,
>>> Angelo
>>>
>>>>> We could alternatively match on the DSI controller's dsi->host->dev instead of 
>>>>> the SoC root compatible.
>>>>>
>>>>> Neil
>>>>
>>>
>>
>