Messages in this thread |  | | | Date | Sun, 16 Mar 2008 14:06:22 -0700 (PDT) | | From | Linus Torvalds <> | | Subject | Re: Linux 2.6.25-rc4 |
| |
On Sun, 16 Mar 2008, Alan Cox wrote: > > > A _lot_ of chips require you to clear the DRQ by taking the data they > > have. > > Almost none and mostly very old ones.
Well, I admittedly haven't been involved in IDE in about a decade, so yeah, maybe it's simply no longer true. That said, if the second bisection was accurate (which is admittedly a rasonably big "if"), it really looks like it would be related to the fact that we used to empty the data buffer before handling errors for REQ_TYPE_ATA_CMD commands.
> Not in my experience having maintained a lot of ATA drivers for a very > long time. In fact the changes for draining the DRQ went into libata only > very recently because it was only when we had a distro sized userbase > with PATA devices that it became apparent that a few corner case problems > remained.
.. but as you noticed, it's almost never wrong to drain (the only chipset it's marked for is some broken pdc202xx one), and it definitely *is* wrong not to drain.
Also, one reason you'd not necessarily even see this is that with DMA, this is a non-issue (since the hardware DMA engine will be doing all the draining), so in order to ever see this you have to still use PIO _and_ you have to see IDE command errors in the first place _and_ you have to have a device that actually keeps DRQ enabled even at an error.
All of which are hopefully fairly rare by now (and getting rarer, at last for the PIO one).
I also wouldn't be entirely surprised if the DRQ behavior may even be command-specific, with the regular data path for read/write quite possibly being different from the special commands that go through some internal drive firmware logic paths.
So I could well imagine (for example) that when a drive raises an IO error due to a read or write fault, the DRQ line will be cleared by the drive, but that special commands might have some firmware-directed separate FIFO that needs draining.
Linus
|  |