[PATCH] ath79: ar8216: make switch register access atomic

[Chuanhong Guo]

Hi!

On Mon, Sep 21, 2020 at 9:24 PM Felix Fietkau <nbd at nbd.name> wrote:
> > +
> > +     split_addr((u32) reg, &r1, &r2, &page);
> > +
> > ++    local_irq_save(flags);
> > +     mutex_lock(&bus->mdio_lock);
> Taking a mutex in an irq-disabled section is basically asking for a
> deadlock. What router is this issue reproduced on?

Any ar934x/qca953x router using built-in switch, based on
Flyspray bug reports. I can reproduce it on ar9341/tp-link-wr841n-v8
As there's an ancient commit about the same issue I guess
it appears on ar724x/ar933x as well.
here's the commit message:
commit 5d77f370d695c9a70f25ffb8367db64efadaaedd
Author: Gabor Juhos <juhosg at openwrt.org>
Date:   Sun May 8 16:32:53 2011 +0000

    ar71xx: ag71xx: make switch register access atomic

    Reading of the PHY registers occasionally returns with bogus values
    under heavy load. This misleads the PHY driver and thus causes false
    link/speed change notifications which leads to performance loss.
[and some dmesg after this]

> What else is running at the time this happens?

I'm using a minimal wr841n-v8 image. (make menuconfig,
select the device, save and build the firmware)
no extra background tasks are running.
swconfig led trigger is used so the kernel is constantly
polling port status from switch in the background.

-- 
Regards,
Chuanhong Guo