Re: [PATCH 2/9] macintosh/via-macii: Poll the device most likely to respond

From: Guenter Roeck <linux@roeck-us.net>
To: Finn Thain <fthain@telegraphics.com.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Joshua Thompson <funaho@jurai.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	linux-m68k@lists.linux-m68k.org,
	Laurent Vivier <lvivier@redhat.com>,
	Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/9] macintosh/via-macii: Poll the device most likely to respond
Date: Sun, 9 Aug 2020 18:16:59 -0700	[thread overview]
Message-ID: <e3252495-2d5e-269f-6fa2-f46bbf609a53@roeck-us.net> (raw)
In-Reply-To: <alpine.LNX.2.23.453.2008100844450.8@nippy.intranet>

Hi,

On 8/9/20 3:58 PM, Finn Thain wrote:
> On Sun, 9 Aug 2020, Guenter Roeck wrote:
> 
>> Hi,
>>
>> On Sun, Jun 28, 2020 at 02:23:12PM +1000, Finn Thain wrote:
>>> Poll the most recently polled device by default, rather than the lowest
>>> device address that happens to be enabled in autopoll_devs. This improves
>>> input latency. Re-use macii_queue_poll() rather than duplicate that logic.
>>> This eliminates a static struct and function.
>>>
>>> Fixes: d95fd5fce88f0 ("m68k: Mac II ADB fixes") # v5.0+
>>> Tested-by: Stan Johnson <userm57@yahoo.com>
>>> Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
>>
>> With this patch applied, the qemu "q800" emulation no longer works and 
>> is stuck in early boot. Any idea why that might be the case, and/or how 
>> to debug it ?
>>
> 
> The problem you're seeing was mentioned in the cover letter,
> https://lore.kernel.org/linux-m68k/cover.1593318192.git.fthain@telegraphics.com.au/
> 
> Since this series was merged, Linus' tree is no longer compatible with 
> long-standing QEMU bugs.
> 
> The best way to fix this is to upgrade QEMU (latest is 5.1.0-rc3). Or use 
> the serial console instead of the framebuffer console.
> 

I have no problem with that. Actually, I had checked the qemu commit log,
 but somehow I had missed missed the commits there.

> I regret the inconvenience but the alternative was worse: adding code to 
> Linux to get compatibility with QEMU bugs (which were added to QEMU due to 
> Linux bugs).
> 
> My main concern is correct operation on actual hardware, as always. But 
> some QEMU developers are working on support for operating systems besides 
> Linux.
> 
> Therefore, I believe that both QEMU and Linux should aim for compatibility 
> with actual hardware and not bug compatibility with each other.
> 
I absolutely agree.

I repeated the test on the mainline kernel with qemu v5.1-rc3, and it works.
I also made sure that older versions of Linux still work with the qemu
v5.1.0-rc3. So everything is good, and sorry for the noise.

Thanks,
Guenter