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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Apr 05, 2021 at 11:44:46PM +0200, Sebastian Reichel wrote:
> 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@collabora.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 :(

I recommend working with the DT people here, as custom sysfs attributes
for things like this that are really just describing the hardware is
crazy.

thanks,

greg k-h