[alsa-devel] MIDI on ice1724 - real-time kernel problem

Pavel Hofman pavel.hofman at insite.cz
Thu May 8 21:35:51 CEST 2008

Takashi Iwai wrote:
> At Sat, 03 May 2008 21:12:02 +0200,
> Pavel Hofman wrote:
>> Takashi Iwai wrote:
>>> At Wed, 30 Apr 2008 22:07:42 +0200,
>>> Pavel Hofman wrote:
>>>> Takashi Iwai wrote:
>>>>> At Fri, 25 Apr 2008 09:06:19 +0200,
>>>>> Pavel Hofman wrote:
>>>> The code works fine on standard kernel.
>>>> But in the RT kernel of Ubuntu 7.10 (2.6.22-14-rt, 1000HZ compared to
>>>> 250HZ of the -generic kernel), MIDI interrupts get thrown continuously,
>>>> hogging CPU with the IRQXX "process".
>>>> When playing 44.1kHz audio with buffer size of 32768,
>>>> snd_vt1724_interrupt is called every 185ms. With the buffer size of 
>>>> 4096, it is every 23ms. So far so good.
>>>> But when I try to output some midi, the routine gets called every 24us
>>>> (i.e. 1000 times faster). Comparing the time difference between my
>>>> several debug printk's in snd_vt1724_interrupt (approx. 5us between each
>>>> printk record a few lines of code apart) and between each call of the
>>>> method (24us), I thought there would be a loop somewhere in the
>>>> interrupt handler calling code. But /proc/interrupts really shows over
>>>> 40k interrupts a second for ICE1724.
>>>> Changing the watermarks up to 0x1f makes no difference.
>>> That looks bad.
>>>> Masking the MIDI interrupts works, snd_vt1724_interrupt is called only
>>>> during VT1724_IRQ_MTPCM interrupts, with the MIDI status bits
>>>> MPU_TX/MPU_RX set when reading/writing with amidi.
>>> Hm, is it TX or RX, or in both cases?
>>> Could you check which one (TX or RX) causes this?
>> Both reading (amidi -p hw:0 -d) and writing (amidi -p hw:0 -S 
>> F0411042110C000000000074FF0411042110C000000) starts the TX interrupt 
>> flood (irq status: 0x20) immediately. Upon opening input as well as 
>> output the method snd_mpu401_do_reset is called - is perhaps the culprit 
>> somewhere there?
> Possibly.  It write 0x00 to data at the beginning in
> snd_mpu401_uart_cmd().  What happens if you remove it?

Unfortunately nothing, no change.

Actually, when does the MPU_TX interrupt get thrown? Upon exhausting 
some output buffer? I understand interrupts for DMA, but mpu uart does 
not use DMA, right?

Thanks a lot for help, I am not giving up :)


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