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SubjectRE: questions regarding possible violation of AHCI spec in AHCI driver
Hi, Tejun,

Here is new patch based on your suggestion (against 2.6.37 release)

--- libahci.c.orig 2011-01-11 10:46:56.623991326 -0800
+++ libahci.c 2011-01-11 10:52:19.634036938 -0800
@@ -542,6 +542,15 @@
void __iomem *port_mmio = ahci_port_base(ap);
u32 tmp;
+ u8 status;
+ status = readl(port_mmio + PORT_TFDATA);
+ if (status & (ATA_BUSY | ATA_DRQ) ||
+ ahci_scr_read(&ap->link, SCR_STATUS, &tmp) ||
+ (tmp & 0x0f) != 0x03) {
+ printk(KERN_DEBUG "START bit was not set in %s\n", __func__);
+ return;
+ }

/* start DMA */
tmp = readl(port_mmio + PORT_CMD);
I will test host controller made by other vendors and report issues soon.


From: Tejun Heo [] On Behalf Of Tejun Heo []
Sent: Tuesday, January 11, 2011 10:23 AM
To: Jian Peng
Cc: Robert Hancock;;; ide
Subject: Re: questions regarding possible violation of AHCI spec in AHCI driver

On Tue, Jan 11, 2011 at 10:09:08AM -0800, Jian Peng wrote:
> Happy Holiday! I want to revisit this issue and hopefully get
> consensus on it asap.

Happy new year!

> Here is the sequence that will cause problem if BSY|DRQ and SSTS.DET
> was not checked in start_engine:
> 1. SUD bit was set in ahci_power_up() to start communication between
> host and device
> 2. START bit was set in ahci_start_engine() to prepare for data
> transfer (should check condition and do not set START bit here
> per spec)
> By the time, host did not receive first FIS so BSY|DRQ was not
> cleared
> 3. inside ahci_hardreset(), call ahci_stop_engine() first, host
> controller will take time to clean up internal pipeline and
> transit to idle state
> 4. toggle SCTL.DET to reset interface, now COMRESET was not sent
> since internal state machine stuck at #2 and #3 (per> spec),
> BSY|DRQ bit was not cleared
> 5. call ahci_start_engine() again and try to read ID from device but
> interface is busy since BSY bit was not cleared, failed here
> At the end of section 10.1 of AHCI spec (rev 1.3), it states
> Software shall not set PxCMD.ST to '1' until it is determined that a
> functional device is present on the port as determined by
> PxTFD.STS.BSY = '0', PxTFD.STS.DRQ = '0', and PxSSTS.DET = 3h.
> It is likely used to prevent host controller from jumping into wrong
> state before first FIS was received.

I still have to say it's pretty silly of the controller to stall

> Please review this issue, and let me know how to resolve it by
> either adopting my previous patch, or creating a new patch.

You haven't really brought up any new issue so my comment stays the

>> I'm not objecting to the change but you guys probably want to fix
>> the controller in following revisions. If we're gonna make the
>> change, I'd like to go with the previous version without the vendor
>> check. What is the timeframe for the controller release? Would it
>> be enough to merge the change during 2.6.38-rc1? After baking it
>> for some time in 2.6.38, we can propagate the change back through
>> -stable.

So, yeah, let's proceed with the patch. Please make the following
changes and resubmit.

* Please don't do readl() on the same line as variable declaration and
drop the unnecessary & 0xff.

* Hmmm... please consider adding a KERN_DEBUG message when
ahci_start_port() exits without starting the port. Just in case it
causes problems on other controllers.

Thank you.


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