Date: Tue, 3 Jun 1997 12:41:14 +0200 (MET DST) From: Luigi Rizzo <luigi@labinfo.iet.unipi.it> To: toor@dyson.iquest.net (John S. Dyson) Cc: hackers@freebsd.org Subject: Re: DMA for IDE drives ? Message-ID: <199706031041.MAA25969@labinfo.iet.unipi.it> In-Reply-To: <199705211437.JAA01152@dyson.iquest.net> from "John S. Dyson" at May 21, 97 09:37:22 am
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Following the suggestion from John Dyson, I have downloaded the
documents for the PIIX controller (i82371FB/i82371SB) which is used
on several of our motherboards.
Maybe I am oversimplifying things but it seems that DMA for IDE
could be implemented as follows:
assuming that the conditions to use DMA on IDE apply (i.e a PIIX
controller is detected, a drive supporting DMA is present on the
channel), a read or write operation is required:
in wdstart():
for write operations:
instead of calling the outsw()/outsl, program the controller
as specified in the PIIX manual, bottom of page 97, steps 1, 2,
4 (step 3 should have been already taken care of; or the
command to request a DMA write transfer is different from a
regular write command ?)
for read operations:
before returning, program the PIIX controller executing steps
1,2,4 as above;
in wdintr()
clear the stop bit in the dma-ide command register of the PIIX
(step 7 page 98 of the docs)
additionally, for read ops, do not issue the insw/insl instructions
[One difficulty which might arise is that there is one DMA for each ide
channel. So, presumably, we cannot overlap read/write ops on separate
drives; I don't even know if we can do it now, btw.]
Is it that simple, or there are other details I am not considering ?
Thanks
Luigi
-----------------------------+--------------------------------------
Luigi Rizzo | Dip. di Ingegneria dell'Informazione
email: luigi@iet.unipi.it | Universita' di Pisa
tel: +39-50-568533 | via Diotisalvi 2, 56126 PISA (Italy)
fax: +39-50-568522 | http://www.iet.unipi.it/~luigi/
_____________________________|______________________________________
for write operations:
> John
>
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