[Gllug] weird CD / modem problemm

Nix nix at esperi.demon.co.uk
Sat Apr 13 11:20:16 UTC 2002

On 13 Apr 2002, Mike Brodbelt mused:
> On Sat, 2002-04-13 at 01:05, Vincent AE Scott wrote:
>> for whatever bizzare reason, reading data off the ide dvd drive fsck's
>> up my modem/serial port.  does anyone have the slightest fsck'ing clue
>> what might cause such a monumental bodge up for my all singing and
>> dancing multimedia monster?
> It could conceivably be a DMA conflict. I heard of one occasion where a
> machine was continually dying - careful examination revealed a DMA
> conflict between the NIC and the hard drive, which meant that
> occasionally raw ethernet frames got written to random areas of the
> disk.... 16550 UARTS can do DMA, so it's worth checking.

They *can*?!

In this case, I'd say convincing the 16550 UART to stop doing DMA is the
right move. PIO serial isn't bad: PIO disks are bad.

It could equally well be that the machine is simply slow enough that the
interrupt load of doing other stuff (like IDE DVD) leads to dropped
packets: serial ports aren't exactly at the top of the interrupt
hierarchy by default, but (especially with 16450 UARTS and below, the
unbuffered ones) you *need* them to be at the top.

irqtune can fix this: it's made my 486-50 quite capable of handling 56K
transfer through a 16450 UART.

`Unless they've moved it since I last checked, travelling between
 England and America does not involve crossing the equator.'
   --- pir

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