Re: [PATCH v2 4/8] dt-bindings: serial: Add support for selecting data transfer mode

[Viken Dadhaniya]

On 2/9/2025 3:49 PM, Krzysztof Kozlowski wrote:
> On 09/02/2025 11:11, Viken Dadhaniya wrote:
> > > > > > > Just one blank line, but anyway, this property should not be in three
> > > > > > > places. Do you really expect that each of serial engines within one
> > > > > > > GeniQUP will be configured differently by TZ?
> > > > > >
> > > > > > Yes, each SE is configured separately and it's quite frequent when
> > > > > > different SEs have different DMA configuration.
> > > > >
> > > > > Well, I checked at sm8550 and sm8650 and each pair of SE - which shares
> > > > > resources - has the same DMAs, so I would not call it frequent. Care to
> > > > > bring an example where same serial engines have different DMAs and
> > > > > different TZ? We do not talk about single QUP.
> > > >
> > > > Well, I don't have access to the latest sm8550 / sm8650 devcfg sources.
> > > > I checked the RB5 ones. As far as I understand out of 14 enabled SEs
> > > > only two are configured for the GSI DMA, others should use FIFO / SE
> > > > DMA. Same applies to the SM8250 MTP devices. Checking the RB1 / RB2
> > > > setup also shows 3 out of 6 SEs being set for GSI.
> > >
> > > I think selecting GSI DMA is only for devices needs high speed streaming
> > > to the
> > > device, like the touch screen, using GSI DMA for random small access is
> > > a non-sense.
> > >
> > > But the thing is, in the TZ world the configuration was static so we had
> > > no choice
> > > of using GSI DMA when configured, but now we have the choice so we could
> > > totally
> > > reconfigure the SE with the transfer type (FIFO, SE DMA or GSI DMA) as
> > > runtime and
> > > drop this attribute.
> > >
> > > So instead of hardcoding this, add a way to dynamically select either of
> > > the 3
> > > transfer types when firmware can be loaded from HLOS.
> > >
> > > Neil
To exactly summarize:
GSI DMA and CPU DMA are mostly same performance unless we have
multiprocessor systems queuing the transfers
together.

GSI DMA to be used when multiprocessor systems (Application 
processor/TZ/modem/ADSP subsystems) has use cases together.
If only single subsystem or processor is using CPU DMA mode should be used.

Hardware guidance and configuration suggest that CPU DMA and FIFO can be 
switched but GSI DMA.

CPI DMA : Doesn't work with multiple subsystems.
FIFO : same as CPU DMA but < 64 bytes (FIFO_SIZE)
GSI_DMA: Work with multiple subsystems

Overall, there will be GSI and non-GSI modes. Dynamic switching is only 
required in PIO mode (FIFO and CPU DMA).

> > Yes, GSI DMA mode is required for specific use cases only.
> >
> > Dynamically switching from GSI mode to non-GSI mode is neither possible
> > nor useful. For each SE, the use case is fixed, and based on the use
> > case, the developer can choose the mode via the device tree property.
>
> No, it cannot. Do not describe downstream as something set in stone.

Sorry, I am not referring to downstream but rather to the general 
process of mode selection.

Please let us know if you need more clarity.

> Best regards,
> Krzysztof