On Wed, 16 Dec 2009, Grant Grundler wrote: > +linux-usb > > On Wed, Dec 16, 2009 at 8:51 AM, Clemens Fruhwirth > <clemens@xxxxxxxxxxxxx> wrote: > > On Thu, Oct 15, 2009 at 1:29 AM, Grant Grundler <grundler@xxxxxxxxxx> wrote: > >> On Wed, Oct 14, 2009 at 2:13 PM, Clemens Fruhwirth > >> <clemens@xxxxxxxxxxxxx> wrote: > >>> Short: Attached is an incomplete patch that allows user space to force > >>> the capacity of an scsi device to a specific value. Any not over > >>> my-dead-body objections with that? > >> > >> No major objection - just the usual nits: > >> Why not make /sys/block/sdX/size writeable? > >>   (instead of adding an ioctl()) > > > > Oh, I like that. Sorry it took me so long to reply. It turned out that > > this is a much better idea. The value that is printed when reading the > > size attribute is to the value that I set in my patch indirectly > > through set_capacity. So, making size writable replaces that totally. > > > > Patch attached. Isn't just changing a block device's size at random times a bad thing to do? The size gets used a for a bunch of different things (checking partition boundaries, validating I/O block ranges, and so on). > >> After reading the wiki page[1], my impression is that the block size > >> gets reported wrong and thus the total capacity is miscalculated for > >> 1GB-4GB SD cards with 1024 or 2048 byte blocks. Two questions around this: > >> a) Does the host need to know the correct block size for correct operation? > > > > From experiments it does not look like that's the case. This is > > surprising though, as one would suspect the card not to  function > > properly otherwise. Frankly speaking, I have not looked down the > > calling chain, especially how usb-storage treats these blocks, but I'd > > say not at all. Seems like the USB reader, I have, just doesn't get > > the capacity right, by not considering BLOCK_LEN > 512. A lot of older card readers can't handle cards larger than 1 or 2 GB (I forget which). Or they see that the card has a larger block size and then don't report it to the host. > >> b) Do the devices report which SD Card Association spec they are compliant > >>  with in the USB headers? No. Cards don't report anything but block data; only the reader provides meta-information. And it doesn't necessarily know anything about the card -- in fact, the card might not even be inserted when the reader is queried. > > Autofixing this doesn't seem possible to me (except for the partition > > trick) as the capacity = BLOCK_LEN * BLOCKS. If BLOCK_LEN isn't > > detected correctly, we can't distinguish a 1GB from 2GB card. > > Agreed. That's why I'm thinking getting BLOCK_LEN right is more important > than hacking the resulting partition size. I'd be surprised if this conversation > hasn't already occurred on linux-usb mailing list. It has. Even if you managed to fix the block size, there's a good chance the reader wouldn't work with large cards. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
