Re: [Bug 1412] Copy from USB1 CF/SM reader stalls, no actual content is read (only directory structure)

[Jens Axboe]

On Tue, Nov 04 2003, Alan Stern wrote:
> On Tue, 4 Nov 2003, Jens Axboe wrote:
> 
> > On Sat, Nov 01 2003, Alan Stern wrote:
> > > 
> > > There's no need.  The output from that diagnostic is definitive -- this 
> > > isn't a USB problem.  It's got to be a problem higher up, maybe in the 
> > > SCSI layer but more likely in the block layer or the file system code.
> > > 
> > > Oct 31 21:12:09 rousalka kernel: usb-storage: READ_10: read page 160 pagect 8
> > > Oct 31 21:12:09 rousalka kernel: usb-storage: sddr09_read_data: use_sg = 1, sg[0].page = c18c5b40, .offset = 0, sg_address = 00000000
> > > Oct 31 21:12:09 rousalka kernel: usb-storage: Read 8 pages, from PBA 11 (LBA 5) page 0
> > > Oct 31 21:12:09 rousalka kernel: usb-storage: usb_stor_ctrl_transfer: rq=00 rqtype=41 value=0000 index=00 len=12
> > > Oct 31 21:12:09 rousalka kernel: usb-storage: Status code 0; transferred 12/12
> > > Oct 31 21:12:09 rousalka kernel: usb-storage: -- transfer complete
> > > Oct 31 21:12:09 rousalka kernel: usb-storage: usb_stor_bulk_transfer_buf: xfer 4096 bytes
> > > Oct 31 21:12:09 rousalka kernel: usb-storage: Status code 0; transferred 4096/4096
> > > Oct 31 21:12:09 rousalka kernel: usb-storage: -- transfer complete
> > > Oct 31 21:12:09 rousalka kernel: usb-storage: us_copy_to_sgbuf: buffer = f38e6000, len = 4096, content = f7e01e40, *index = 0, *offset = 0, use_sg = 1
> > > Oct 31 21:12:09 rousalka kernel: usb-storage: in loop, ptr = 00000000
> > > 
> > > The line where it says sg_address = 00000000 indicates that something has
> > > asked us to read data from the device and store it at a virtual address
> > > that isn't mapped to memory!  You can see the same thing in the last line,
> > > where ptr = 00000000.  No wonder you get an oops.
> > 
> > I don't think your analysis is correct. The sg entry has a page element
> > assigned, it just looks like someone didn't dma map the sg entry yet. I
> > had a quick peek in sddr09, lots of ugliness in there... The
> > sg_address() crap needs to go. And kfree() on ptr + offset, irk. And I
> > hope you are not dma'ing into these pages. Missing checks for kmalloc()
> > failure. Etc.
> > 
> > Someone needs to read up on DMA-mapping.txt and fix it.  Clean these
> > bits up and I bet it works.
> 
> 
> I think you're partly right but wrong where it matters.
> 
> 	1. No, the sg entry was DMA-mapped previously and then unmapped.
> The problem Nicholas encountered is an oops that occurs later during a
> memcpy, not during a DMA operation.  Apparently the page is not locked in
> memory.

Well what's the difference? Still a driver bug. Where does the dma
mapping happen?

> 	2. Yes, sddr09 is ugly.  I don't mind agreeing with that since I 
> didn't write it :-)

:)

> 	3. What's wrong with sg_adress()?  I notice that bio_data() in 
> include/linux/bio.h is practically the same.

Yeah, bio_data() is not supposed to be used either, but it also has a
comment to that effect. sg_address() is worse, because it's readding
something that 2.5 explicitly removed to encourage proper use of the pci
dma mapping api.

> 	4. What's wrong with kfree() on ptr + offset?  As long as that 
> points to the same address as was kmalloc'ed, it should work fine.

Ah you are right, there's nothing wrong with that. It's just the
implementation that looks odd. Why not do away with that kmalloc() and
virt_to_page(), and just alloc_page() explicitly? Kill the 2.4 ifdefs at
the same time.

> 	5. We are doing DMA into those pages.  What's wrong with that?  
> They were allocated using kmalloc, so there should be no difficulty in 
> mapping and using them.

Where are they mapped? Are you flushing buffers appropriately?

> 	6. There _are_ checks for kmalloc() failing; they just aren't 
> where you would expect to see them.  The check depends on virt_to_page() 
> returning 0 when passed an argument of 0.  Maybe that assumption is wrong.
> Certainly it wouldn't hurt to do the checking properly.

Irk that's even more ugly... And it's wrong, too.

> 	7. DMA-mapping isn't the issue.  The problem is that 
> sddr09_read_data() -- not sddr09_read_map() as you seem to have assumed -- 
> has been passed an sg entry for which page_address() returns 0.

But why does that happen, if ->page is set? That's the mystery, and that
heavily points to a driver bug.

-- 
Jens Axboe

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