Re: [PATCH 0/8] soc: samsung: Add USIv2 driver

[Krzysztof Kozlowski]

On 28/11/2021 04:15, David Virag wrote:
> On Sun, 2021-11-28 at 00:32 +0200, Sam Protsenko wrote:
>> USIv2 IP-core provides selectable serial protocol (UART, SPI or
>> High-Speed I2C); only one can be chosen at a time. This series
>> implements USIv2 driver, which allows one to select particular USI
>> function in device tree, and also performs USI block initialization.
>>
>> With that driver implemented, it's not needed to do USI
>> initialization
>> in protocol drivers anymore, so that code is removed from the serial
>> driver.
>>
> 
> I think the downstream way of doing this (USI node reg being on the
> SW_CONF register itself rather than an offset from uart/i2c/spi, the
> USI driver only controlling the SW_CONF, and the uart/i2c/spi drivers
> controlling their USI_CON and USI_OPTION regs) is cleaner, better, and
> easier to adapt to USIv1 too.
> 
> For example: I'm sure this is the case on USIv2 devices too, but on
> Exynos7885, different devices have USI modes configured differently.
> For example a Samsung Galaxy A8 (2018) has all the USI blocks
> configured as SPI while a Samsung Galaxy M20 has the first USI
> configured as dual HSI2C, the second as HSI2C on the first 2 pins and
> the third as HSI2C on the last 2 pins. With this way of doing
> everything on USIv2 we'd need 3 disabled USIv2 nodes in the SoC DTSI
> for one USI block, each for every protocol the USI block can do, all
> having a single child for their protocol and each referencing the same
> sysreg (not even sure if that's even supported). Then the board DTS
> could enable the USI node it needs.

It's not supported (one cannot have three same nodes with same unit
addresses), so this would be solved by dropping out unused interfaces,
commenting them out or storing everything under one USI:

usi@0x1abcdef0 {
  serial@.... {
    status = "okay";
  }

  i2c@.... {
    status = "disabled";
  }

  spi@.... {
    status = "disabled";
  }
}

> 
> With the downstream way we could have just one USI node and we could
> add the 3 protocols it can do disabled as seperate or child nodes. This
> way the board DTS only needs to set the appropriate mode setting and
> enable the protocol it needs. I'd say much better than having 3 USI
> nodes for the same USI block.

Then however you need to handle probe ordering and possible probe deferrals.

> 
> Also this way is pretty USIv2 centric. Adding USIv1 support to this
> driver is difficult this way because of the the lack of USI_CON and
> USI_OPTION registers as a whole (so having nowhere to actually set the
> reg of the USI node to, as the only thing USIv1 has is the SW_CONF
> register). 

How is it difficult? Not having a register is easy - noop on given platform.

> In my opinion being able to use the same driver and same
> device tree layout for USIv1 and USIv2 is a definite plus
> 
> The only real drawback of that way is having to add code for USIv2
> inside the UART, HSI2C, and SPI drivers but in my opinion the benefits
> overweigh the drawbacks greatly. We could even make the uart/spi/hsi2c
> drivers call a helper function in the USI driver to set their USI_CON
> and USI_OPTION registers up so that code would be shared and not
> duplicated. Wether this patch gets applied like this is not my choice
> though, I'll let the people responsible decide
> :-)
> 
> Anyways, soon enough I can write an USIv1 driver after I submit all the
> 7885 stuff I'm working on currently. If you want to, you can add USIv2
> support to that driver, or if an USIv2 driver is already in upstream at
> that point, if it is written in the downstream way I can add v1 support
> to that, or if it's like this I'll have to make a whole seperate driver
> with a whole seperate DT structure.
> 
> Best regards,
> David
> 


Best regards,
Krzysztof