linux-kernel - Re: [PATCH 1/2] dt-bindings: serial: Add bindings for nvidia,tegra264-utc

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Message-ID: <5j3fhrauu7j4vdal4yi34yj5egxe42i3aj4ezuc5oblhz6nfca@ebewyvonecj5>
Date: Wed, 29 Jan 2025 15:52:39 +0100
From: Thierry Reding <thierry.reding@...il.com>
To: Kartik Rajput <kkartik@...dia.com>
Cc: "krzk@...nel.org" <krzk@...nel.org>, Jon Hunter <jonathanh@...dia.com>, 
	"robh@...nel.org" <robh@...nel.org>, "robert.marko@...tura.hr" <robert.marko@...tura.hr>, 
	"arnd@...nel.org" <arnd@...nel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"conor+dt@...nel.org" <conor+dt@...nel.org>, "geert+renesas@...der.be" <geert+renesas@...der.be>, 
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>, "jirislaby@...nel.org" <jirislaby@...nel.org>, 
	"krzk+dt@...nel.org" <krzk+dt@...nel.org>, "hvilleneuve@...onoff.com" <hvilleneuve@...onoff.com>, 
	"schnelle@...ux.ibm.com" <schnelle@...ux.ibm.com>, "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>, 
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>, 
	"andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>, "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH 1/2] dt-bindings: serial: Add bindings for
 nvidia,tegra264-utc

On Wed, Jan 29, 2025 at 07:30:55AM +0000, Kartik Rajput wrote:
> Thanks for reviewing the patch Krzysztof!
> 
> On Tue, 2025-01-28 at 08:52 +0100, Krzysztof Kozlowski wrote:
> > External email: Use caution opening links or attachments
> > 
> > 
> > On Tue, Jan 28, 2025 at 12:16:32PM +0530, Kartik Rajput wrote:
[...]
> > > +  nvidia,utc-fifo-threshold:
> > > +    $ref: /schemas/types.yaml#/definitions/uint32
> > > +    description:
> > > +      This property specifies the UTC TX and RX client FIFO
> > > threshold in
> > > +      terms of occupancy.
> > > +
> > > +      This property should have the same value as the burst size
> > > (number
> > > +      of characters read by the Tegra UTC hardware at a time from
> > > each
> > > +      client) which is configured by the bootloader.
> > 
> > Title says this is a client, so quite confusing. Anyway, why is this
> > board specific?
> 
> The client FIFO threshold should match the burst size configured in the
> UTC controller by bootloader. This value could change depending on what
> bootloader has programmed. Hence, this is moved to the device-tree.
> 
> > 
> > Also, missing constraints, missing units. Why common serial
> > properties
> > are not applicable?
> > 
> 
> I do see current-speed defined in serial-peripheral-props.yaml, that
> can be used here. I also see "rx-threshold" and "tx-threshold"
> properties defined in serial.yaml, maybe those can be utilized here. I
> will update this in v2.

I suppose "rx-threshold" and "tx-threshold" could be used instead of the
custom "nvidia,utc-fifo-threshold" property. It looks like the hardware
has separate values for the threshold in both directions, so this would
give us a more accurate description (though from the current state of
affairs it looks like both are always going to be the same).

I'm not so sure about "current-speed", though. There's no concept of
speed for the UTC, right? It's effectively backed by a physical UART
that will run at a certain speed, but given that it will multiplex data
from a variety of sources, "current-speed" will not be accurate in many
cases.

Thierry

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