Re: [BUG] slab debug vs. L1 alignement

From: Benjamin Herrenschmidt (benh_at_kernel.crashing.org)
Date: 08/16/03

  • Next message: Christoph Hellwig: "[PATCH] mark devfs obsolete" 
    To: Kai Makisara <Kai.Makisara@kolumbus.fi>
Date:	16 Aug 2003 13:36:30 +0200

    > ->
    > A character device (like st) doing direct i/o from user buffer to/from a
    > SCSI device does not currently have any alignment restrictions. I think
    > restricted alignment can't be required from a user of an ordinary
    > character device. This must then be handled by the driver. The solution is
    > to use bounce buffers in the driver if the alignment does not meet the
    > lower level requirements. This leads to surprises with performance if the
    > user buffer alignment does not satisfy the requirements (e.g., malloc()
    > may or may not return properly aligned blocks). These surprises should be
    > avoided as far as the hardware allows.

    THe low level driver can't do the bounce buffer thing, it has to be
    done at higher layers.

    > If an architecture has restrictions, they must, of course, be taken into
    > account. However, this should not punish architectures that don't have the
    > restrictions. Specifying that DMA buffers must be cache-line aligned would
    > be too strict. A separate alignment constraint for DMA in general and for
    > a device in specific would be a better alternative (a device may have
    > tighter restrictions than an architecture). The same applies to buffer
    > sizes. This would mean adding two more masks for each device (like the
    > current DMA address mask for a device).

    That won't help for buffers coming from higher layers that don't know
    the device they'll end up to

    Ben.
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  • Next message: Christoph Hellwig: "[PATCH] mark devfs obsolete"

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