Re: [PATCH] scsi: sd: add runtime pm to open / release

Martin Kepplinger
2020-07-29T15:53:39Z

On 29.07.20 17:40, Alan Stern wrote:
> On Wed, Jul 29, 2020 at 07:53:52AM -0700, James Bottomley wrote:
>> On Wed, 2020-07-29 at 07:46 -0700, James Bottomley wrote:
>>> On Wed, 2020-07-29 at 10:32 -0400, Alan Stern wrote:
>>>> On Wed, Jul 29, 2020 at 04:12:22PM +0200, Martin Kepplinger wrote:
>>>>> On 28.07.20 22:02, Alan Stern wrote:
>>>>>> On Tue, Jul 28, 2020 at 09:02:44AM +0200, Martin Kepplinger
>>>>>> wrote:
>>>>>>> Hi Alan,
>>>>>>>
>>>>>>> Any API cleanup is of course welcome. I just wanted to remind
>>>>>>> you that the underlying problem: broken block device runtime
>>>>>>> pm. Your initial proposed fix "almost" did it and mounting
>>>>>>> works but during file access, it still just looks like a
>>>>>>> runtime_resume is missing somewhere.
>>>>>>
>>>>>> Well, I have tested that proposed fix several times, and on my
>>>>>> system it's working perfectly. When I stop accessing a drive
>>>>>> it autosuspends, and when I access it again it gets resumed and
>>>>>> works -- as you would expect.
>>>>>
>>>>> that's weird. when I mount, everything looks good, "sda1". But as
>>>>> soon as I cd to the mountpoint and do "ls" (on another SD card
>>>>> "ls" works but actual file reading leads to the exact same
>>>>> errors), I get:
>>>>>
>>>>> [ 77.474632] sd 0:0:0:0: [sda] tag#0 UNKNOWN(0x2003) Result:
>>>>> hostbyte=0x00 driverbyte=0x08 cmd_age=0s
>>>>> [ 77.474647] sd 0:0:0:0: [sda] tag#0 Sense Key : 0x6 [current]
>>>>> [ 77.474655] sd 0:0:0:0: [sda] tag#0 ASC=0x28 ASCQ=0x0
>>>>> [ 77.474667] sd 0:0:0:0: [sda] tag#0 CDB: opcode=0x28 28 00 00
>>>>> 00 60 40 00 00 01 00
>>>>
>>>> This error report comes from the SCSI layer, not the block layer.
>>>
>>> That sense code means "NOT READY TO READY CHANGE, MEDIUM MAY HAVE
>>> CHANGED" so it sounds like it something we should be
>>> ignoring. Usually this signals a problem, like you changed the
>>> medium manually (ejected the CD). But in this case you can tell us
>>> to expect this by setting
>>>
>>> sdev->expecting_cc_ua
>>>
>>> And we'll retry. I think you need to set this on all resumed
>>> devices.
>>
>> Actually, it's not quite that easy, we filter out this ASC/ASCQ
>> combination from the check because we should never ignore medium might
>> have changed events on running devices. We could ignore it if we had a
>> flag to say the power has been yanked (perhaps an additional sdev flag
>> you set on resume) but we would still miss the case where you really
>> had powered off the drive and then changed the media ... if you can
>> regard this as the user's problem, then we might have a solution.
>
> Indeed, I was going to make the same point.
>
> The only reasonable conclusion is that suspending these SD card readers
> isn't safe unless they don't contain a card -- or maybe just if the
> device file isn't open or mounted.
>
> Although support for this sort of thing could be added to the kernel,
> for now it's best to rely on userspace doing the right thing. The
> kernel doesn't even know which devices suffer from this problem.
>
>

well, userspace can do something like "automatically unmount if not
used" but that is *way* more inefficient than if the kernel would
support this is some way or another, for SD cards.