linux-kernel - Re: [PATCHv4] serial: imx: Add DMA buffer configuration via sysfs

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Message-ID: <20210405214446.zhidvtvahcfp4wxa@earth.universe>
Date:   Mon, 5 Apr 2021 23:44:46 +0200
From:   Sebastian Reichel <sebastian.reichel@...labora.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jirislaby@...nel.org>, Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>, Ian Ray <ian.ray@...com>,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
        kernel@...labora.com
Subject: Re: [PATCHv4] serial: imx: Add DMA buffer configuration via sysfs

Hi,

On Fri, Mar 05, 2021 at 01:42:52PM +0100, Sebastian Reichel wrote:
> On Fri, Mar 05, 2021 at 01:06:12PM +0100, Greg Kroah-Hartman wrote:
> > On Fri, Mar 05, 2021 at 12:50:58PM +0100, Sebastian Reichel wrote:
> > > From: Fabien Lahoudere <fabien.lahoudere@...labora.com>
> > > 
> > > In order to optimize serial communication (performance/throughput VS
> > > latency), we may need to tweak DMA period number and size. This adds
> > > sysfs attributes to configure those values before initialising DMA.
> > > The defaults will stay the same as before (16 buffers with a size of
> > > 1024 bytes). Afterwards the values can be read/write with the
> > > following sysfs files:
> > > 
> > > /sys/class/tty/ttymxc*/dma_buffer_size
> > > /sys/class/tty/ttymxc*/dma_buffer_count
> > 
> > Ick no.  Custom sysfs attributes for things like serial ports are crazy.
> > 
> > > This is mainly needed for GEHC CS ONE (arch/arm/boot/dts/imx53-ppd.dts),
> > > which has multiple microcontrollers connected via UART controlling. One
> > > of the UARTs is connected to an on-board microcontroller at 19200 baud,
> > > which constantly pushes critical data (so aging character detect
> > > interrupt will never trigger). This data must be processed at 50-200 Hz,
> > > so UART should return data in less than 5-20ms. With 1024 byte DMA
> > > buffer (and a constant data stream) the read operation instead needs
> > > 1024 byte / 19200 baud = 53.333ms, which is way too long (note: Worst
> > > Case would be remote processor sending data with short pauses <= 7
> > > characters, which would further increase this number). The current
> > > downstream kernel instead configures 24 bytes resulting in 1.25ms,
> > > but that is obviously not sensible for normal UART use cases and cannot
> > > be used as new default.
> > 
> > Why can't this be a device tree attribute? Why does this have to be a
> > sysfs thing that no one will know how to tune and set over time.  This
> > hardware should not force a user to manually tune it to get it to work
> > properly, this isn't the 1990's anymore :(
> > 
> > Please never force a user to choose stuff like this, they never will
> > know what to do.
> 
> This used to be a DT attribute in PATCHv1. It has been moved over to
> sysfs since PATCHv2, since it does not describe the hardware, but
> configuration. Unfortunately lore.kernel.org does not have the full
> thread, but this is the discussion:
> 
> https://lore.kernel.org/linux-serial/20170629182618.jpahpmuq364ldcv2@pengutronix.de/
> 
> From downstream POV this can be done either by adding a DT property
> to the UART node, or by adding a udev rule.
> 
> From my POV there is not a huge difference. In both cases we will
> be bound by an ABI afterwards, in both cases people will usually
> stick to the default value and in both cases people that do deviate
> from the default probably ran into problems and started to look
> for a solution.

ping? It's not very nice to get a rejected in cycles :(

-- Sebastian

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