Hello Greg, [Cc += dt maintainers] On Tue, Apr 06, 2021 at 09:13:04AM +0200, Greg Kroah-Hartman wrote: > On Mon, Apr 05, 2021 at 11:44:46PM +0200, Sebastian Reichel wrote: > > 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@xxxxxxxxxxxxx> > > > > > > > > > > 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@xxxxxxxxxxxxxx/ > > > > > > 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. I was one who expressed concerns in the earlier rounds that dt isn't the right place for this. dt is about hardware description, but choosing a good value for the dma buffer size is driver tuning and depends on the individual requirements. (latency, throughput, memory consumption, robustness under system load). I can even imagine use cases where the settings should be changed dynamically, which cannot (easily) be done using dt. While I see your point that a driver specific sysfs property is unusual/strange/whatever every downside you mentioned also applies to a dt property (or a custom ioctl). Among the solutions I can imagine, my preference order is: - automatic tuning - sysfs property - further discussion - dt property - custom ioctl I wonder if there is a sensible way to implement a automatic tuning. In the use case mentioned in the commit log, Sebastian's need is low latency for a constantly sending microcontroller on the other side. Is it sensible to make the used dma buffers smaller if we have a certain throughput? Or is that too magic and doomed to fail covering most use cases? If that doesn't work, I support Sebastian's approach to do that in a sysfs property. Sebastian, have you evaluated just not using dma for these UARTs? Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | https://www.pengutronix.de/ |
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