libpayload: console: Allow output drivers to print whole strings at once

Hardware / Coreboot - Julius Werner [chromium.org] - 22 December 2014 14:44 UTC

The console output driver framework in libpayload is currently built on the putchar primitive, meaning that every driver's function gets called one character at a time. This becomes an issue when we add drivers that could output multiple characters at a time, but have a high constant overhead per invocation (such as the planned GDB stub, which needs to wrap a special frame around output strings and wait for an acknowledgement from the server).

This patch adds a new 'write' function pointer to the console_output_driver structure as an alternative to 'putchar'. Output drivers need to provide at least one of the two ('write' is preferred if available). The CBMEM console driver is ported as a proof of concept (since it's our most performace-critical driver and should in theory benefit the most from less function pointer invocations, although it's probably still negligible compared to the big sprawling mess that is printf()).

Even with this fix, the problem remains that printf() was written with the putchar primitive in mind. Even though normal text already contains an optimization to allow multiple characters at a time, almost all formatting directives cause their output (including things like padding whitespace) to be putchar()ed one character at a time. Therefore, this patch reworks parts of the output code (especially number printing) to all but remove that inefficiency (directives still invoke an extra write() call, but at least not one per character). Since I'm touching printf() core code anyway, I also tried to salvage what I could from that weird, broken "return negative on error" code path (not that any of our current output drivers can trigger it anyway).

A final consequence of this patch is that the responsibility to prepend line feeds with carriage returns is moved into the output driver implementations. Doing this only makes sense for drivers with explicit cursor position control (i.e. serial or video), and things like the CBMEM console that appears like a normal file to the system really have no business containing carriage returns (we don't want people to accidentally associate us with Windows, now, do we?).

BUG=chrome-os-partner:18390 TEST=Made sure video and CBMEM console still look good, tried printf() with as many weird edge-case strings as I could find and compared serial output as well as sprintf() return value.

Original-Change-Id: Ie05ae489332a0103461620f5348774b6d4afd91a

2fe505b libpayload: console: Allow output drivers to print whole strings at once
payloads/libpayload/drivers/cbmem_console.c | 10 +-
payloads/libpayload/drivers/serial/8250.c | 2 +
payloads/libpayload/drivers/serial/tegra.c | 2 +
payloads/libpayload/drivers/video/video.c | 1 +
payloads/libpayload/include/libpayload.h | 4 +-
payloads/libpayload/libc/console.c | 29 +--
payloads/libpayload/libc/printf.c | 267 ++++++++++++---------------
7 files changed, 150 insertions(+), 165 deletions(-)

Upstream: review.coreboot.org

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