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    SubjectRe: cdrom: use kmalloced buffers instead of buffers on stack
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    On Tue, 22 Apr 2008 07:48:58 +0200
    tsbogend@alpha.franken.de (Thomas Bogendoerfer) wrote:

    > On Mon, Apr 21, 2008 at 10:01:26PM -0400, Jeff Garzik wrote:
    > > > cdrom: use kmalloced buffers instead of buffers on stack
    > > >
    > > > If cdrom commands are issued to a scsi drive in most cases the buffer
    > > > will be
    > > > filled via dma. This leads to bad stack corruption on non coherent
    > > > platforms,
    > > > because the buffers are neither cache line aligned nor is the size a
    > > > multiple
    > > > of the cache line size. Using kmalloced buffers avoids this.
    > > >
    > > > Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
    > > > Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
    > > >---
    > > > drivers/cdrom/cdrom.c | 274
    > > > ++++++++++++++++++++++++++++++++-----------------
    > > > 1 files changed, 181 insertions(+), 93 deletions(-)
    > >
    > > Eh... AFAICS this is only really useful in two of the cases converted.
    > >
    > > For all the other cases (<= 32 bytes), it is _far_ less complex, far
    > > less code to simply communicate the additional alignment requirements to
    > > the compiler.
    > >
    > > What about __attribute__ __aligned__? Was that tried?
    >
    > I used that while narrowing down the bug. But not only the alignment is
    > important, but also size needs to be a multiple of the cache line size.
    > Which means it needs to be 128 bytes for most SGI machines. That
    > and the following in DMA-mapping.txt
    >
    > "This rule also means that you may use neither kernel image addresses
    > (items in data/text/bss segments), nor module image addresses, nor
    > stack addresses for DMA."
    >
    > let me choose the kmalloc() solution.

    Can we advertise such architecture's dma restrictions? For example, if
    we can update dma_pad_mask and dma_alignment in request_queue,
    blk_rq_map_kern uses a proper bounce buffer for such
    architectures. Then we can avoid putting extra complexity in uppper
    drivers such as cdrom.c


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