Hi Daniel,
On Wed, Sep 04, 2019 at 10:11:11AM +0200, Daniel Vetter wrote:
On Wed, Sep 4, 2019 at 8:53 AM Thomas Zimmermann tzimmermann@suse.de wrote:
Hi
Am 04.09.19 um 08:27 schrieb Feng Tang:
Thank you for testing. But don't get too excited, because the patch simulates a bug that was present in the original mgag200 code. A significant number of frames are simply skipped. That is apparently the reason why it's faster.
Thanks for the detailed info, so the original code skips time-consuming work inside atomic context on purpose. Is there any space to optmise it? If 2 scheduled update worker are handled at almost same time, can one be skipped?
To my knowledge, there's only one instance of the worker. Re-scheduling the worker before a previous instance started, will not create a second instance. The worker's instance will complete all pending updates. So in some way, skipping workers already happens.
So I think that the most often fbcon update from atomic context is the blinking cursor. If you disable that one you should be back to the old performance level I think, since just writing to dmesg is from process context, so shouldn't change.
Hmm, then for the old driver, it should also do the most update in non-atomic context?
One other thing is, I profiled that updating a 3MB shadow buffer needs 20 ms, which transfer to 150 MB/s bandwidth. Could it be related with the cache setting of DRM shadow buffer? say the orginal code use a cachable buffer?
https://unix.stackexchange.com/questions/3759/how-to-stop-cursor-from-blinki...
Bunch of tricks, but tbh I haven't tested them.
Thomas has suggested to disable curson by echo 0 > /sys/devices/virtual/graphics/fbcon/cursor_blink
We tried that way, and no change for the performance data.
Thanks, Feng
In any case, I still strongly advice you don't print anything to dmesg or fbcon while benchmarking, because dmesg/printf are anything but fast, especially if a gpu driver is involved. There's some efforts to make the dmesg/printk side less painful (untangling the console_lock from printk), but fundamentally printing to the gpu from the kernel through dmesg/fbcon won't be cheap. It's just not something we optimize beyond "make sure it works for emergencies". -Daniel
Best regards Thomas
Thanks, Feng
Best regards Thomas
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