Re: Safety of IRQ during i/o

[Marcin Dalecki <dalecki@evision.ag>]

Bartlomiej Zolnierkiewicz wrote:
> On Wed, 24 Jul 2002, Pete Zaitcev wrote:
> 
> 
>>>[...]
>>>I would think that this would be safe when using DMA, and likely to be
>>>safe for PIO and more recent chipsets, but I wouldn't want to actually
>>>tell anyone that.
>>
>>A little story from OLS. I have a 486/75 laptop, which can only
>>do PIO. It always was losing characters evern on 9600 baud on its
>>serial port, and I thought it was simply broken for five years.
> 
> 
> :-)
> 
> 
>>A guy who did a security talk showed me that doing hdparm -u
>>fixes the problem. Apparently, the lappy has a non-buffering UART.
>>
>>So, it seems that hdparm -u is a very useful thing for obsotele
>>boxes. If you do DMA, you probably do not care.
> 
> 
> Yup, for PIO unmask (if possible) is a must.

It's even for DMA a good thing, since the IRQ handler in question can
reenter the RQ handler. The invention of the not unmasking
behaviour in Linux is the result of some not entierly ATA-2 compliant
devices long long time ago gone. Basically XT disks on PC. They did have 
the habbit of splewing IRQs too early for command ACK.