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

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Message-Id: <E6228980-6EC9-49BB-84FD-9F13C2DC4612@goldelico.com>
Date:   Tue, 23 Aug 2016 09:28:40 +0200
From:   "H. Nikolaus Schaller" <hns@...delico.com>
To:     Sebastian Reichel <sre@...nel.org>
Cc:     One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        Oleksij Rempel <linux@...pel-privat.de>,
        Rob Herring <robh@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Marcel Holtmann <marcel@...tmann.org>,
        Jiri Slaby <jslaby@...e.com>, Pavel Machek <pavel@....cz>,
        Peter Hurley <peter@...leysoftware.com>,
        NeilBrown <neil@...wn.name>, Arnd Bergmann <arnd@...db.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        "open list:BLUETOOTH DRIVERS" <linux-bluetooth@...r.kernel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/3] UART slave device bus

Hi,

> Am 23.08.2016 um 00:42 schrieb Sebastian Reichel <sre@...nel.org>:
> 
>> 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.

Exactly.

> 
>> 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

Yes it would be nice, but is this a thing with significant practical relevance?

Usually you have to write drivers for a complete device where the slave
chip is already wired up to a SoC-UART.

Sometimes you can get a bare chip where you can connect to an
USB-UART. But someone has to design that piece of special hardware
for you. If you are really lucky there is an evaluation board.

And in that case I would use a RasPi or BeagleBone and tie up directly
to some SoC-UART instead of using an intermediate USB-UART adapter.
Because it is more close to timing relations to the final SoC based design.

> 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.

Oh, that is an interesting device I didn't know yet.

> 
>> 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.

I just assume that b) is rarely used because there are alternatives.
Although it would be a nice option.

Anyways, while following the discussion this is not the most important
facet of the overall topic.

BR,
Nikolaus



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