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@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 :( -- Sebastian
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