Re: [RFC PATCH 3/3] drm/log: Introduce a new boot logger to draw the kmsg on the screen

[Jocelyn Falempe]

On 01/08/2024 12:51, John Ogness wrote:
> On 2024-08-01, Jocelyn Falempe <jfalempe@xxxxxxxxxx> wrote:
> >   * It uses a circular buffer so the console->write() callback is very
> >     quick, and will never stall.
> >   * Drawing is done asynchronously using a workqueue.
>
> For CON_NBCON, neither of the above points are necessary. You can draw
> directly from the write_thread() callback. See below:
>
> > +static bool drm_log_work_draw(void)
> > +{
> > +	unsigned int len;
> > +	char buf[512];
> > +
> > +	len = drm_log_buf_read(buf, sizeof(buf));
> > +	if (len)
> > +		drm_log_draw_all(buf, len);
> > +	return len != 0;
> > +}
>
> For CON_NBCON, this is essentially your write_thread() callback:
>
> void drm_log_write_thread(struct console *con,
> 			  struct nbcon_write_context *wctxt)
> {
> 	drm_log_draw_all(wctxt->outbuf, wctxt->len);
> }
>
> You cannot implement a write_atomic() callback because the console must
> be able to print directly in NMI context and must not defer. But
> write_atomic() is optional, so you should be fine there.
>
> Disclaimer: Only in PREEMPT_RT patchset at the moment.

Thanks, so that means the circular buffer and workqueue are only 
necessary until write_thread() is merged in mainline. It will be a nice 
simplification.
I think I can also register one console for each drm driver, which will 
simplify drm_log even further. (currently it would mean having a 
circular buffer and work function for each driver which is a bit too much).
Do you know if there is a chance to have write_thread() in 6.12 or 6.13 ?

> John Ogness >

Best regards

--

Jocelyn