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

[Francesco Lavra <flavra@baylibre.com>]

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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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