[PATCH] mmc: failure of block read wait for long time

[Ghorai, Sukumar]

Adrian,

> -----Original Message-----
> From: Adrian Hunter [mailto:adrian.hunter at nokia.com]
> Sent: Monday, September 20, 2010 1:24 PM
> To: Ghorai, Sukumar
> Cc: linux-mmc at vger.kernel.org; linux-arm-kernel at lists.infradead.org;
> Adrian Hunter
> Subject: Re: [PATCH] mmc: failure of block read wait for long time
> 
> On 14/09/10 08:15, ext Ghorai, Sukumar wrote:
> > Adrian,
> >
> > [..snip..]
> >>>>> [Ghorai] Adrian,
> >>>>> Yes this works and reduced the retry by 1/4 (2048 to 512 times for
> 1MB
> >>>> data read) form the original code;
> >>>>> Initially it was retrying for each page(512 bytes) after multi-block
> >>>> read fail; but this solution is retying for each segment(2048 bytes);
> >>>>> 1. Now say block layrer reading 1MB and failed for the 1st segment.
> So
> >>>> it will still retry for 1MB/2048-bytes, i.e. 512 times.
> >>>>> 2. So do you think any good reason to retry again and again?
> >>>> If you have 1MB that is not readable, it sounds like the card is
> broken.
> >>>> Why are so many reads failing?  Has the card been removed?
> >>>>
> >>>> You might very rarely see ECC errors in a small number of sectors,
> >>>> but more than that sounds like something else is broken.
> >>>
> >>> [Ghorai] yes, one example is we remove the card when reading data,
> >>
> >> Well, that is a different case.  Once the card has gone, the block
> driver
> >> can (and will once the remove method is called) error out all I/O
> >> requests without sending them to MMC.  That doesn't happen until there
> >> is a card detect interrupt and a resulting rescan.
> >
> > [Ghorai] here we are discussing two problem,
> > 1. If IO failed how to stop retry; because of -
> > 	a. internal card error
> > 	b. issue in Filesystem, driver, or host controller issue
> > 	c. or cards removed.
> >
> > 2. And 2nd how to sync block-layer IO, if card removed,
> > 	a. case 1: when card removed interrupt support by the platform
> > 	b. case 2: when card removed interrupt does not support by the
> platform?
> >
> >>
> >> A possible solution is to put a flag on mmc_card to indicate card_gone
> >> that gets set as soon as the drivers card detect interrupt shows there
> >> is no card (hoping that we are not getting any bouncing on card detect)
> >> and then have mmc_wait_for_req() simple return -ENODEV immediately if
> >> the card_gone flag is set.  Finally, if the mmc block driver sees
> >> a -ENODEV error, it should also check the card_gone flag (via a new
> >> core function) and if the card is gone, do not retry - and perhaps
> >> even error out the rest of the I/O request queue as well.
> >
> > [Ghorai] your idea address the 2.a case, but not 2.b, 1.a, 1.b
> 
> The card removal case can be extended to use the bus ops detect method
> when there is no card detect irq.  I will send a RFC patch.
> 
> With respect to 1.a:
>   - If the card has an internal error, then it is broken.  The user
>   should remove the card and use a better one.  I do not see how reducing
>   retry delays really helps the user very much.  Arguably if the card
>   becomes unresponsive, the MMC core could provide a facility to
>   reinitialise the card, but that is yet another issue.
> 
> With respect to 1.b:
>   - The file system cannot cause the block driver to have I/O errors.
>   - If there are errors in the driver they should be fixed.
>   - If there are hardware problems with the host controller, then
>   it is up to the host controller driver to deal with them e.g.
>   by resetting the controller.  I don't see what this has to do with
>   the block driver.
> 
> You leave out the important case of ECC errors.  I am concerned about
> this because of the possibility that it happens inside a file system
> journal e.g. EXT4 journal.  Perhaps the journal may be recovered if the
> error only affects the last transaction, but perhaps not if it destroys
> other transactions - which could happen if the approach you suggest
> is taken.
> 
[Ghorai] Thanks lot for your descriptive answer.
1. Can you answer this? 2.b. case 2: when card removed interrupt does not support by the platform?

2. Why block layer handling for inter-leave data? Can you give example diver who is returning interleave data? And how to tell application that buffer having interleave data?

> >
> > And the solution I was proposing to return the status of IO failure as
> soon as possible to above layer; and handle the card removed interrupt
> separately or any other issue in h/w or s/w or combination of both. Or
> just think again when platform don't have the card remove interrupt.
> >
> > So my patch addresses the 1st part
> 
> It is absolutely unacceptable to return I/O errors to the upper layers
> for segments that do not have errors.
> 
> > And for the 2nd part we can submit the patch anytime.
> >
> >>
> >> I can suggest a patch if you want but I am on vacation next week so
> >> it will have to wait a couple of weeks.
> >>
> >>> And moreover we should not give the interleave data to apps, as we
> don't
> >> have option to tell application for the valid data.
> >>>
> > [..snip..]
> > http://comments.gmane.org/gmane.linux.kernel.mmc/2714
> >
> >>>
> >
> >