Subject: Re: [BUG] slab debug vs. L1 alignement
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Kai Makisara <Kai.Makisara@kolumbus.fi>
Cc: Manfred Spraul <manfred@colorfullife.com>,
       linux-kernel mailing list <linux-kernel@vger.kernel.org>
In-Reply-To: <Pine.LNX.4.56.0308161359460.1703@kai.makisara.local>
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> ->
> 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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