Re: [RFC PATCH 0/3] UART slave device bus

[Sebastian Reichel <sre@kernel.org>]

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Hi,

On Mon, Aug 22, 2016 at 11:23:26PM +0200, H. Nikolaus Schaller wrote:
> > Am 22.08.2016 um 22:39 schrieb Sebastian Reichel <sre@kernel.org>:
> > 
> > Hi,
> > 
> > On Sun, Aug 21, 2016 at 09:50:57AM +0200, H. Nikolaus Schaller wrote:
> >>> Am 20.08.2016 um 15:34 schrieb One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>:
> >>>> What it is not about are UART/RS232 converters connected through USB or virtual
> >>>> serial ports created for WWAN modems (e.g. /dev/ttyACM, /dev/ttyHSO). Or BT devices
> >>>> connected through USB (even if they also run HCI protocol).
> >>> 
> >>> It actually has to be about both because you will find the exact same
> >>> device wired via USB SSIC/HSIC to a USB UART or via a classic UART. Not is
> >>> it just about embedded boards.
> >> 
> >> Not necessarily.
> >> 
> >> We often have two interface options for exactly the sam sensor chips. They can be connected
> >> either through SPI or I2C. Which means that there is a core driver for the chip and two different
> >> transport glue components (see e.g. iio/accel/bmc150).
> >> 
> >> This does not require I2C to be able to handle SPI or vice versa or provide a common API.
> > 
> > I don't understand this comparison. I2C and SPI are different
> > protocols,
> 
> Yes, they are different on protocol level, but on both you transfer blocks of data from/to a slave device
> which usually can be addressed. And for some chips they are just two slightly alternative serial interfaces.
> 
> > while native UART and USB-connected UART are both UART.
> 
> I see what you mean, but kernel divides between directly connected UART and USB-connected UART.
> 
> drivers/usb/serial/ vs. drivers/tty/serial/
> 
> to implement two different groups of UARTs. Although on user space level they are harmonized again.
> This is why I compare with i2c and spi. But each such comparison is not perfect.
> 
> Anyways, to me it looks as if everybody wants to make the solution work for usb-uarts as well
> (although I still would like to see a real world use-case).
> 
> > 
> >> And most Bluetooth devices I know have either UART or a direct
> >> USB interface. So in the USB case there is no need to connect
> >> it through some USB-UART bridge and treat it as an UART at all.
> > 
> > I think having support for USB-UART dongles is useful for
> > driver development and testing on non-embedded HW.
> 
> Hm. I assume you mean the Bluetooth situation where both, embedded UART
> connected chips and USB dongles are available.

No. I mean I have some serial device, which is connected to the
embedded UART, but I also have a standalone version. For driver
development I can just use my standalone serial device, connect
it to an USB-UART and develop the driver on non embedded HW.
Then I can use the same driver on my embedded platform and it
works, since it uses the same API.

For e.g. I2C this works perfectly fine. I already did this with
the I2C interface exposed on my notebook's VGA port.

> I am not a specialist for such things, but  I think you have three
> options to connect bluetooth:
> 
> a) SoC-UART <-> BT-Chip-UART-port
> b) USB-UART (FT232, PL2303 etc.) <-> BT-Chip-UART-port
> c) USB <-> BT-Chip-USB-port (not UART involved at all)
>
> Case c) IMHO means you anyways need a special USB driver for the BT-Chip connected
> through USB and plugging it into a non-embedded USB port does not automatically
> show it as a tty interface. So you can't use it for testing the UART drivers.
> 
> BTW: the Wi2Wi W2CBW003 chip comes in two firmware variants: one for UART and
> one for USB. So they are also not exchangeable.

Yes, let's ignore option c). I'm talking about UART only. If the
chip has native USB support, then that's a different driver. Note,
that for more complex drivers it may become possible to use the same
high-level driver via regmap at some point. Not sure if this kind of
HW exists, though.

> Variant b) is IMHO of no practical relevance (but I may be wrong)
> because it would mean to add some costly FT232 or PL2302 chip
> where a different firmware variant works with direct USB
> connection.

Well for some chips there is not native USB support. But my scenario
was about development. Let's say I have a serial-chip and I want to
develop a driver for it. It would be nice if I can develop the
driver with a USB-UART and then use it on my embedded system.

There are usb-serial devices, which could benefit from support
btw. I would find it really useful, if the Dangerous Prototype's
Bus Pirate would expose native /dev/i2c and /dev/spi and it's
based on FT232.

> So to me it looks as if you need to develop different low-level
> drivers anyways.

No. You say, that option b) is irrelevant and assume, that every
serial chip also has native USB support.

-- Sebastian

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