linux-kernel - Re: [PATCH] serial: tegra: remove Kconfig dependency on APB DMA controller

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Message-ID: <562c2b30820d083ff0b5e04ac176ed66c0064363.camel@baylibre.com>
Date: Wed, 26 Nov 2025 13:08:23 +0100
From: Francesco Lavra <flavra@...libre.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby
 <jirislaby@...nel.org>, Kartik Rajput <kkartik@...dia.com>, Geert
 Uytterhoeven <geert+renesas@...der.be>, Wolfram Sang
 <wsa+renesas@...g-engineering.com>,  Robert Marko
 <robert.marko@...tura.hr>, Thierry Bultel
 <thierry.bultel.yh@...renesas.com>, Douglas Anderson
 <dianders@...omium.org>, linux-kernel@...r.kernel.org, 
 linux-serial@...r.kernel.org
Subject: Re: [PATCH] serial: tegra: remove Kconfig dependency on APB DMA
 controller

On Wed, 2025-11-26 at 13:20 +0200, Andy Shevchenko wrote:
> On Wed, Nov 26, 2025 at 10:07:59AM +0100, Francesco Lavra wrote:
> > This driver runs also on SoCs without a Tegra20 APB DMA controller
> > (e.g.
> > Tegra234).
> > Remove the Kconfig dependency on TEGRA20_APB_DMA, and remove reference
> > to
> > the APB DMA controller from the Kconfig help text.
> 
> ...
> 
> >         help
> >           Support for the on-chip UARTs on the NVIDIA Tegra series SOCs
> >           providing /dev/ttyTHS0, 1, 2, 3 and 4 (note, some machines
> > may not
> >           provide all of these ports, depending on how the serial port
> > -         are enabled). This driver uses the APB DMA to achieve higher
> > baudrate
> > -         and better performance.
> > +         are enabled).
> 
> I think this removes a good piece of information. Perhaps rephrase?
> 
>           This driver may use the APB DMA when available to achieve
>           higher baudrate and better performance.

I think this sentence would make it sound like the driver performs better
if the APB DMA controller is available, but in reality the driver just uses
the generic DMA API like most serial drivers, and there is nothing APB-
specific in it. If another DMA controller (e.g. GPC on Tegra234) is
available instead of the APB one, the serial peripheral will be just as
fast.

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