DRM i2c module or bridge ?

[Archit Taneja]

On 11/12/2015 07:20 PM, Emil Velikov wrote:
> On 12 November 2015 at 13:18, Thierry Reding <thierry.reding at gmail.com> wrote:
>> On Thu, Nov 12, 2015 at 12:48:51PM +0000, Emil Velikov wrote:
>>> Hello Thierry, all,
>>>
>>> Inspired by a recent discussion I was started wondering - where is the
>>> cut between DRM i2c modules (most of which encoders/transmitters) and
>>> bridge drivers (again some of which i2c encoders) ? Does anyone has
>>> some pointers on the topic ?
>>
>> DRM bridge is a superset of I2C encoders, so everything that I2C encoder
>> drivers do they should be able to do with DRM bridges, and more. There
>> isn't a strict guideline here, but I think there's general agreement
>> that new drivers should be using the DRM bridge framework. The primary
>> reason is that bridges integrate seamlessly with the driver model, that
>> is, the drivers that implement them are regular drivers that register
>> with the corresponding bus and get bound to a device, whereas the I2C
>> encoder infrastructure is mostly about manually instantiating devices.
>>
>> For existing drivers I guess they could all be converted, but doing so
>> may require a bit of work. They also tend to work as-is, so finding
>> volunteers to do the conversion is probably going to be difficult given
>> the lack of motivation.
>>
>> Thierry
>>
>>> Based on the above I did a very quick search for third party IP
>>> modules in the DRM subsystem:
>>>
>>> * i915
>>> dvo_ch7017.c
>>> dvo_ch7xxx.c
>>> dvo_ivch.c
>>> dvo_ns2501.c
>>> dvo_sil164.c
>>> dvo_tfp410.c
>>
>> It looks like these use some framework that's custom to the i915 driver
>> but could otherwise easily be DRM bridges.
>>
>>> * gma500
>>> tc35876x-dsi-lvds.c
>>
>> This seems to be some sort of hybrid bridge and panel driver.
>>
>>> * sti
>>> sti_hdmi_tx3g0c55phy.c
>>> sti_hdmi_tx3g4c28phy.c
>>
>> These seem to implement some sort of PHY interface, but from a quick
>> look moving these to the PHY framework seems overkill. They seem no good
>> fit for DRM bridge because they are not separate devices, but rather the
>> SoC generation specific bits of the STi HDMI driver.
>>
>>> (and for posterity)
>>> * i2c
>>> adv7511.c
>>> ch7006_drv.c
>>> sil164_drv.c
>>> tda998x_drv.c
>>>
>>>
>>> * bridge
>>> dw_hdmi.c
>>> nxp-ptn3460.c
>>> parade-ps8622.c
>>>
>>>
>>> By the looks of it, we can move rework (some of?) the above into
>>> i2c/bridge modules and in other cases (sil164) just use the existing
>>> one ? I'm neither volunteering nor suggesting people must work of
>>> these, merely curious.
>>
>> My take on this is that it's probably best to keep the above in their
>> current form. If they need to be shared across multiple hardware setups
>> it might make sense to convert them to DRM bridge drivers.
>>
>> For new drivers it's probably best to make them bridge drivers from the
>> start.
>>
> Thanks for the comprehensive reply Thierry. Pretty sure there are
> other people wondering about these - this should straighten things
> out.

Please refer to the following thread for a similar discussion:

http://lists.freedesktop.org/archives/dri-devel/2015-July/087097.html

>
> Just a small note: considering that most desktop GPUs are moving (have
> moved ?) away from third party encoders/transmitters I doubt we'll be
> seeing any movements on that front.

We still have a requirement for such encoders in the SoC world. A SoC
may provide a particular kind of encoder output, but we might need to
convert that into another type of encoded output. There are multiple
reasons why we might want to do this (SoC limitations, support old
encoded formats like LVDS, weird requirements on some boards).

There is also a trend of re-use of the same third party encoder IPs
across multiple SoCs. Having bridges for such IPs is helpful too.

Archit

>
> Cheers,
> Emil
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>

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