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

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Date:   Fri, 5 Mar 2021 13:06:12 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sebastian Reichel <sebastian.reichel@...labora.com>
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

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.

thanks,

greg k-h