linux-kernel - Re: Help with SATA driver/libata/SCSI subsystem

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Message-ID: <4A0B213F.2060900@garzik.org>
Date:	Wed, 13 May 2009 15:36:31 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	C Z <cziom2523@...mail.com>
CC:	linux-kernel@...r.kernel.org,
	Linux IDE mailing list <linux-ide@...r.kernel.org>
Subject: Re: Help with SATA driver/libata/SCSI subsystem

C Z wrote:
> I am attempting to write a SATA driver under libata for a custom core on a Microblaze Xilinx FPGA. I can not get ata_device_add() in drivers/ata/libata-core.c to return successfully.
> 
> I have tracked the immediate problem to the function scsi_eh_flush_done_q() in drivers/scsi/scsi_error.c. The problem is this function calls "list_for_each_entry_safe(scmd, next, done_q, eh_entry)". Unfortunately, this returns the result scmd = 0xFFFFFFF0. This very strange address is the result of containerof() being called on a NULL pointer. The NULL pointer is the done_q list head. This is clearly wrong.
> 
> I have 2 questions. First, why is libata invoking the error handler after a probe in ata_device_add? This seems very counter intuitive to me. It isn't at all clear why libata needs to do this. Can someone explain the rationale behind this function?
> 
> Second, and more importantly, can someone who is experienced with this subsystem explain in simple terms the call flow that happens in the function ata_device_add? I am having trouble understanding which procedure should have issued a command and set up the done_q list head. Therefore I don't know how to track this problem further. Indeed, it doesn't appear to me that anyone actually issues a command during the probe. I'm guessing it was supposed to have been done somewhere, but where/in what function should I start looking? What was the command supposed to have been?
> 
> I am using an old kernel version 2.6.20, however I don't believe these subsystems have changed substantially since then.
> 
> Any assistance or advice from someone who has dealth with this area of the kernel before is appreciated.

Please cc linux-ide@...r.kernel.org, and turn on word wrapping in your 
email.

Please also use an updated kernel, libata has changed substantially 
since 2.6.20.

Hardware drivers do not invoke ata_device_add() [which does not exist 
anymore], so it sounds like you are doing something really wrong...

	Jeff



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