Re: [PATCH v2 00/16] drm/exynos: Convert driver to drm bridge

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hi Michael,

On 10.11.2020 09:13, Michael Tretter wrote:
> On Mon, 09 Nov 2020 12:15:39 +0900, Inki Dae wrote:
>> 20. 9. 11. 오후 10:53에 Michael Tretter 이(가) 쓴 글:
>>> This is v2 of the series to convert the Exynos MIPI DSI driver into a drm
>>> bridge and make it usable with other drivers. Although the driver is
>>> converted, it still supports the component framework API to stay compliant
>>> with the Exynos DRM driver.
>>>
>>> The Exynos MIPI DSI Phy is also found on the i.MX8M Mini. However, on the
>>> i.MX8M Mini, the bridge is driven by an LCDIF display controller instead of
>>> the Exynos Decon. The driver for the LCDIF does not use the component
>>> framework, but uses drm bridges.
>>>
>>> I don't have any Exynos SoC to actually test the series. I build a dummy to
>>> test the bridge with a component driver, to make sure that at least the
>>> initialization is working. Furthermore, tested the driver as a bridge with a
>>> few additional unfinished patches on the i.MX8M Mini EVK. However, somebody
>>> should verify that the driver is still working on Exynos hardware.
>>>
>>> I also changed the order of the patches to first make the driver more platform
>>> independent (patches 2 to 8), then convert to a drm bridge driver (patches 10
>> Just a fundamental question, A MIPI-DSI(Display Serial Interface) bus device
>> would be one of an encoder type of devices not bridge such as DSI to LVDS
>> and LVDS to DSI bridge devices, and also image enhancer and image compressor
>> in case of Exynos.
> I don't understand, why the MIPI-DSI bus device would be an encoder type and
> DSI to LVDS or MIPI-DSI to HDMI would be bridges. For example, the device tree
> documentation for the DSIM states that the DSIM receives RGB video as input
> and produces MIPI-DSI as output. Thus, the DSIM is basically a parallel RGB to
> MIPI-DSI bridge and the encoder is the LCD controller that encodes the video
> data as parallel RGB.
>
> On the i.MX8MM, the LCDIF is already the encoder. On Exynos, the series
> implements the encoder in the platform glue, but in the end the encoder can
> probably be moved to the DECON.

This is probably the historical decision. That time when Exynos DSI 
driver has been implemented, support for DRM bridges wasn't ready enough 
to use to for such purpose.

Frankly, I'm still not convinced that the current DRM bridge framework 
provides everything needed to reimplement the Exynos DSI driver with all 
its features. There are a lots of corner cases and order-specific bits 
in turning on/off the display pipeline, which don't map nicely to the 
bridge pre_enable (called in post-order) and enable (called in 
pre-order) callbacks. Especially if you consider that there might be 
another bridge before and after.

I think that Andrzej Hajda already pointed those drawbacks of the 
current design. Last week I've spent some significant amount of time 
playing with exynos dsi code to check how to match its operations 
(especially the runtime power management) to this design with the 
current boards (Arndale with additional DSI->LVDS bridge and panel, 
Trats2 with DSI panel and TM2e with MIC 'in-bridge' and DSI panel), but 
without a success.

>> Why do you want to convert such MIPI-DSI driver to bridge type of driver?
>> Seems not sensible. The reason would be just to share MIPI-DSI phy driver
>> for Exynos with i.MX8M Mini?
> Yes, the reason is that the driver should be shared between Exynos and
> i.MX8MM. It is the same IP and I don't see a reason why we should introduce
> another driver for the same IP if the driver works for both SoCs.

I think that the easiest way to share this driver between Exynos and iMX 
would be to extract the low-level code (the code that plays with 
hardware registers) to the common plane, while keeping the separate DRM 
glue, device component and platform device parts. This way the Exynos 
will continue to use it in the current form, while iMX can start using 
it as DRM bridge. A bit similar approach is used with exynos_dp driver 
and analogix_dp bridge shared with other SoCs.

I hope that later, when the bridge related issue are resolved, the 
Exynos can be converted too, so the encoder creation moved to FIMD and 
Decon.

Best regards

-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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