Re: [RFC PATCH v1 00/25] printk: new implementation

[Petr Mladek]

On Fri 2020-01-24 17:09:29, Eugeniu Rosca wrote:
> Hi Geert,
> 
> On Wed, Jan 22, 2020 at 08:48:12PM +0100, Geert Uytterhoeven wrote:
> > On Wed, Jan 22, 2020 at 5:59 PM Eugeniu Rosca <erosca@xxxxxxxxxxxxxx> wrote:
> > > On Wed, Jan 22, 2020 at 08:31:44AM +0100, Geert Uytterhoeven wrote:
> > > > On Wed, Jan 22, 2020 at 3:34 AM Eugeniu Rosca <erosca@xxxxxxxxxxxxxx> wrote:
> > > > > So, what's specific to R-Car3, based on my testing, is that the issue
> > > > > can only be reproduced if the printk storm originates on CPU0
> 
> Slight amendment the above statement. Below results are got on R-Car
> H3ULCB running renesas-drivers-2020-01-14-v5.5-rc6 (cX stands for CPUx,
> whitespace stands for clean audio, '!' stands for distorted audio):
> 
>                   printk @:
>                   c0 c1 c2 c3 c4 c5 c6 c7
> speaker-test @ c0 !
>                c1 !  !
>                c2 !     !
>                c3 !        !
>                c4 !           !
>                c5 !              !
>                c6 !                 !
>                c7 !                    !
> 
> One can see two patterns in the chart. The audio has glitches whenever:
>  - printk and the audio application run on the same CPU, or:
>  - printk runs on CPU0

The proper longterm solution seems to be offloading printk console
handling to a kthread so that it can be bound to a particular CPU
and does not block audio.

Anyway, there is a question whether you really need to send all messages
via the serial console. It might make sense to filter less important
messages using "loglevel=" or "quiet" kernel parameters. The full
log can be read later from userspace (dmesg, syslog, /dev/kmsg).
Filtering can get disabled when debugging non-booting kernel.
In this case, a distorted audio is the least problem.

Best Regards,
Petr