Re: [PATCH] mmc_spi: do propper retry managment in the block layer

[Pierre Ossman]

On Tue, 14 Apr 2009 16:27:26 +0200
Wolfgang MÃes <wolfgang.mues@xxxxxxxxxxxx> wrote:

> Hello Pierre,
> 
> Am Samstag, 11. April 2009 schrieb Pierre Ossman:
> > NAK. Writes cannot be retried safely as upper layers rely on the fact
> > that writes fail in a linear manner (a stupid assumption IMO, but
> > that's the way things are).
> 
> Hmmm.... so this patch has to be improved. I think the (original) code in 
> mmc_blk_issue_rq() is somewhat questionable. As far as I understand the code, 
> in the case of an error, brq.data.bytes_xfered is not examined, so the code 
> starts over from the very first sector of the request, and rereads all 
> sectors (one after the another).
> 

That would be a bug, yes.

> So if there is a non-recoverable write error, this code must stop and do not 
> continue to write, right?

Yes.

> But if there is a read error, it is OK and advisable to try to read the rest 
> of the sectors?

We're working on the assumption that reads do not modify any state in
the card, so those should be safe to retry any number of times. :)

> If there is a write error in the middle of a request, is it OK to do
> __blk_end_request(req, 0, brq.data.bytes_xfered);
> and try to retry from this point, and stop if there is a non-recoverable write 
> error?

Afraid not. The reason is that bytes_xfered is not guaranteed to be
exact. It gives you a lower bound, but not an upper one.

> > > +		/* Invalid response. This is most likely a transmission
> > > +		 * error from card to host.
> > > +		 */
> > > +		case -EINVAL:
> >
> > EINVAL is actually "host controller driver/hardware does not support
> > this type of request".
> 
> "Hardware" is including the hardware of the SD card. This should NEVER happen, 
> because block.c issues only valid requests, which are translated to mandatory 
> commands for the mmc/sd card.
> 
> So if there is an EINVAL result from the lower layers, it comes from 
> transmission errors (card has not understand the command, or host has not 
> understand the response).
> 
> If EINVAL is coming from the HOST CONTROLER, a retry will not harm anyone.
> 
> I have seen EINVAL results due to spikes on the SD lines.
> 
> Please correct me if I am wrong.
> 

You're wrong in that EINVAL should not be sent for any kind of runtime
hardware errors like that. But I might have missed some instances during
reviews, so there might be drivers that give that error message for
things they shouldn't.

From include/mmc/core.h:

 * EINVAL       Request cannot be performed because of restrictions
 *              in hardware and/or the driver

I can't really see a case where EINVAL should be a transient condition,
so there shouldn't be any point retrying a request that gives that
error.

Rgds
-- 
     -- Pierre Ossman

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