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. thanks, greg k-h
