On Donnerstag, 19. August 2021 16:16:17 CEST Mark Brown wrote:
On Thu, Aug 19, 2021 at 03:52:55PM +0200, Nicolas Frattaroli wrote:
On Donnerstag, 19. August 2021 14:08:36 CEST Robin Murphy wrote:
- rockchip,no-dmaengine:
- description:
If present, driver will not register a pcm dmaengine, only thedai.
If the dai is part of multi-dais, the property should bepresent.
- type: boolean
That sounds a lot more like a policy decision specific to the Linux driver implementation, than something which really belongs in DT as a description of the platform.
I agree. Should I be refactoring this into a module parameter or something along those lines? I'm unsure of where this goes.
Why is this even required? What is "multi-dais" and why would registering the DMA stuff cause a problem?
After some research, it appears that multi-dais is a special driver that downstream uses to allow multiple sub-DAIs to be combined into one DAI that has all the channels of the sub-DAIs. This does not seem like something that should be done at that level to me, because it seems like it's pushing a sound driver configuration into the realm of hardware description.
In retrospect, I should have stripped this out before submitting it, because I should not be submitting things I don't understand completely. I apologise.
The particular configuration may even vary per-board; an I2S/TDM controller may be connected to an external codec which does not support capture, whereas on another board it may be connected to one that does.
If the external device doesn't support both directions then why does the driver for the I2S controller in the CPU care? The constraint handling code in the core will ensure that nothing tries to start something that isn't supported
I went over the downstream text binding description again and from that it appears that the playback/capture-only capability is something specific to the controller, not to any device connected to it.
The downstream device tree for the rk3568 specifies playback-only for I2S0, a.k.a. the one connected to the HDMI that I can't test because we currently don't have a video clock. Another downstream device tree, specific to what appears to be a robot demo for the px30 SoC, uses this property on i2s1, which tells me that this is not an actual description of the controller hardware but just a description of the application.
While not relevant to the device tree schema, the driver reacts to these properties by setting the opposite directions _minimum_ channel number to 0 (from the default of 2.)
My conclusion from this is that this reeks of nonsense and I will look into what happens when I simply remove these properties and lower the channels_min to 0 in the driver. If it turns out that on some SoC for some I2S/TDM controller instance there is a limitation where specifying that the controller only handles either capture or playback does make sense, we can always add it later.
Thank you for your comments, Nicolas Frattaroli