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

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Message-ID: <97d8026590edd4911eb03d775b10f14ecc60ba6e.camel@baylibre.com>
Date: Wed, 26 Nov 2025 17:45:25 +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 18:25 +0200, Andy Shevchenko wrote:
> On Wed, Nov 26, 2025 at 01:08:23PM +0100, Francesco Lavra wrote:
> > 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:
> 
> ...
> 
> > > >         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.
> 
> OK. But this is not the case for Tegra234? Or is it and it uses DMA for
> UART?

Yes, that is the case, Tegra234 has just a different DMA controller
(TEGRA186_GPC_DMA), which is used by the UART driver as long as the
relevant device tree node properties are in place.

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