On Tue Oct 31, 2023 at 12:22 PM CET, Russell King (Oracle) wrote: > On Tue, Oct 31, 2023 at 12:04:11PM +0100, Théo Lebrun wrote: > > On Tue Oct 31, 2023 at 11:11 AM CET, Russell King (Oracle) wrote: > > > There is no point in supporting 5 or 6 bits for console usage. Think > > > about it. What values are going to be sent over the console? It'll be > > > ASCII, which requires at _least_ 7-bit. 6-bit would turn alpha > > > characters into control characters, punctuation and numbers. 5-bit > > > would be all control characters. > > > > > > So there's no point trying to do anything with 5 or 6 bits per byte, > > > and I decided we might as well take that as an error (or maybe a > > > case that the hardware has not been setup) and default to 8 bits per > > > byte. > > > > I see your point. Two things come to mind: > > > > - I added this parsing of 5/6 bits to be symmetrical with > > pl011_set_termios that handles 5/6 properly. Should pl011_set_termios > > be modified then? > > Why should it? Note that I said above about _console_ usage which is > what you were referring to - the early code that sets up the console > by either reading the current settings (so that we can transparently > use the UART when its handed over already setup by a boot loader). > > This is completely different to what happens once the kernel is running. > Userspace might very well have a reason to set 5 or 6 bits if it wants > to communicate with a device that uses those sizes. > > However, such a device won't be a console for the reasons I outlined > above (it will truncate the ASCII characters turning console messages > into garbage.) I'm not sure I get it. (1) We assume it is a console so it's ASCII so no reason to set to 5 or 6 bits per word. But (2) there might be a reason to set the UART to 5 or 6 bits, the userspace decides. How do the two interact? Say we boot to Linux, userspace configures to 6 bits because reasons and we reset. At second probe we see a config of 6 bits per word but assume that can't be logical, even though it is. What makes us suppose at probe that it must be a console? I won't die on a hill for this topic; we'll go the way you prefer! Regards, -- Théo Lebrun, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
